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DBI(3)                User Contributed Perl Documentation               DBI(3)

NAME
       DBI - Database independent interface for Perl

SYNOPSIS
         use DBI;

         @driver_names = DBI->available_drivers;
         %drivers      = DBI->installed_drivers;
         @data_sources = DBI->data_sources($driver_name, \%attr);

         $dbh = DBI->connect($data_source, $username, $auth, \%attr);

         $rv  = $dbh->do($statement);
         $rv  = $dbh->do($statement, \%attr);
         $rv  = $dbh->do($statement, \%attr, @bind_values);

         $ary_ref  = $dbh->selectall_arrayref($statement);
         $hash_ref = $dbh->selectall_hashref($statement, $key_field);

         $ary_ref  = $dbh->selectcol_arrayref($statement);
         $ary_ref  = $dbh->selectcol_arrayref($statement, \%attr);

         @row_ary  = $dbh->selectrow_array($statement);
         $ary_ref  = $dbh->selectrow_arrayref($statement);
         $hash_ref = $dbh->selectrow_hashref($statement);

         $sth = $dbh->prepare($statement);
         $sth = $dbh->prepare_cached($statement);

         $rc = $sth->bind_param($p_num, $bind_value);
         $rc = $sth->bind_param($p_num, $bind_value, $bind_type);
         $rc = $sth->bind_param($p_num, $bind_value, \%attr);

         $rv = $sth->execute;
         $rv = $sth->execute(@bind_values);
         $rv = $sth->execute_array(\%attr, ...);

         $rc = $sth->bind_col($col_num, \$col_variable);
         $rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);

         @row_ary  = $sth->fetchrow_array;
         $ary_ref  = $sth->fetchrow_arrayref;
         $hash_ref = $sth->fetchrow_hashref;

         $ary_ref  = $sth->fetchall_arrayref;
         $ary_ref  = $sth->fetchall_arrayref( $slice, $max_rows );

         $hash_ref = $sth->fetchall_hashref( $key_field );

         $rv  = $sth->rows;

         $rc  = $dbh->begin_work;
         $rc  = $dbh->commit;
         $rc  = $dbh->rollback;

         $quoted_string = $dbh->quote($string);

         $rc  = $h->err;
         $str = $h->errstr;
         $rv  = $h->state;

         $rc  = $dbh->disconnect;

       The synopsis above only lists the major methods and parameters.

   GETTING HELP
       If you have questions about DBI, or DBD driver modules, you can get
       help from the dbi-users@perl.org mailing list.  You don't have to
       subscribe to the list in order to post, though I'd recommend it. You
       can get help on subscribing and using the list by emailing
       dbi-users-help@perl.org.

       I don't recommend the DBI cpanforum (at
       http://www.cpanforum.com/dist/DBI) because relatively few people read
       it compared with dbi-users@perl.org.

       To help you make the best use of the dbi-users mailing list, and any
       other lists or forums you may use, I strongly recommend that you read
       "How To Ask Questions The Smart Way" by Eric Raymond:
       http://www.catb.org/~esr/faqs/smart-questions.html
       <http://www.catb.org/~esr/faqs/smart-questions.html>.

       If you think you've found a bug then please also read "How to Report
       Bugs Effectively" by Simon Tatham:
       <http://www.chiark.greenend.org.uk/~sgtatham/bugs.html>.

       The DBI home page at <http://dbi.perl.org/> and the DBI FAQ at
       http://faq.dbi-support.com/ <http://faq.dbi-support.com/> are always
       worth a visit.  They include links to other resources.

       Before asking any questions, reread this document, consult the archives
       and read the DBI FAQ. The archives are listed at the end of this
       document and on the DBI home page.

       This document often uses terms like references, objects, methods.  If
       you're not familiar with those terms then it would be a good idea to
       read at least the following perl manuals first: perlreftut, perldsc,
       perllol, and perlboot.

       Please note that Tim Bunce does not maintain the mailing lists or the
       web page (generous volunteers do that).  So please don't send mail
       directly to him; he just doesn't have the time to answer questions
       personally. The dbi-users mailing list has lots of experienced people
       who should be able to help you if you need it. If you do email Tim he's
       very likely to just forward it to the mailing list.

   NOTES
       This is the DBI specification that corresponds to the DBI version 1.609
       ($Revision: 12812 $).

       The DBI is evolving at a steady pace, so it's good to check that you
       have the latest copy.

       The significant user-visible changes in each release are documented in
       the DBI::Changes module so you can read them by executing "perldoc
       DBI::Changes".

       Some DBI changes require changes in the drivers, but the drivers can
       take some time to catch up. Newer versions of the DBI have added
       features that may not yet be supported by the drivers you use.  Talk to
       the authors of your drivers if you need a new feature that's not yet
       supported.

       Features added after DBI 1.21 (February 2002) are marked in the text
       with the version number of the DBI release they first appeared in.

       Extensions to the DBI API often use the "DBIx::*" namespace.  See
       "Naming Conventions and Name Space". DBI extension modules can be found
       at <http://search.cpan.org/search?mode=module&query=DBIx>.  And all
       modules related to the DBI can be found at
       <http://search.cpan.org/search?query=DBI&mode=all>.

DESCRIPTION
       The DBI is a database access module for the Perl programming language.
       It defines a set of methods, variables, and conventions that provide a
       consistent database interface, independent of the actual database being
       used.

       It is important to remember that the DBI is just an interface.  The DBI
       is a layer of "glue" between an application and one or more database
       driver modules.  It is the driver modules which do most of the real
       work. The DBI provides a standard interface and framework for the
       drivers to operate within.

   Architecture of a DBI Application
                    |<- Scope of DBI ->|
                         .-.   .--------------.   .-------------.
         .-------.       | |---| XYZ Driver   |---| XYZ Engine  |
         | Perl  |       | |   `--------------'   `-------------'
         | script|  |A|  |D|   .--------------.   .-------------.
         | using |--|P|--|B|---|Oracle Driver |---|Oracle Engine|
         | DBI   |  |I|  |I|   `--------------'   `-------------'
         | API   |       | |...
         |methods|       | |... Other drivers
         `-------'       | |...
                         `-'

       The API, or Application Programming Interface, defines the call
       interface and variables for Perl scripts to use. The API is implemented
       by the Perl DBI extension.

       The DBI "dispatches" the method calls to the appropriate driver for
       actual execution.  The DBI is also responsible for the dynamic loading
       of drivers, error checking and handling, providing default
       implementations for methods, and many other non-database specific
       duties.

       Each driver contains implementations of the DBI methods using the
       private interface functions of the corresponding database engine.  Only
       authors of sophisticated/multi-database applications or generic library
       functions need be concerned with drivers.

   Notation and Conventions
       The following conventions are used in this document:

         $dbh    Database handle object
         $sth    Statement handle object
         $drh    Driver handle object (rarely seen or used in applications)
         $h      Any of the handle types above ($dbh, $sth, or $drh)
         $rc     General Return Code  (boolean: true=ok, false=error)
         $rv     General Return Value (typically an integer)
         @ary    List of values returned from the database, typically a row of data
         $rows   Number of rows processed (if available, else -1)
         $fh     A filehandle
         undef   NULL values are represented by undefined values in Perl
         \%attr  Reference to a hash of attribute values passed to methods

       Note that Perl will automatically destroy database and statement handle
       objects if all references to them are deleted.

   Outline Usage
       To use DBI, first you need to load the DBI module:

         use DBI;
         use strict;

       (The "use strict;" isn't required but is strongly recommended.)

       Then you need to "connect" to your data source and get a handle for
       that connection:

         $dbh = DBI->connect($dsn, $user, $password,
                             { RaiseError => 1, AutoCommit => 0 });

       Since connecting can be expensive, you generally just connect at the
       start of your program and disconnect at the end.

       Explicitly defining the required "AutoCommit" behaviour is strongly
       recommended and may become mandatory in a later version.  This
       determines whether changes are automatically committed to the database
       when executed, or need to be explicitly committed later.

       The DBI allows an application to "prepare" statements for later
       execution.  A prepared statement is identified by a statement handle
       held in a Perl variable.  We'll call the Perl variable $sth in our
       examples.

       The typical method call sequence for a "SELECT" statement is:

         prepare,
           execute, fetch, fetch, ...
           execute, fetch, fetch, ...
           execute, fetch, fetch, ...

       for example:

         $sth = $dbh->prepare("SELECT foo, bar FROM table WHERE baz=?");

         $sth->execute( $baz );

         while ( @row = $sth->fetchrow_array ) {
           print "@row\n";
         }

       The typical method call sequence for a non-"SELECT" statement is:

         prepare,
           execute,
           execute,
           execute.

       for example:

         $sth = $dbh->prepare("INSERT INTO table(foo,bar,baz) VALUES (?,?,?)");

         while(<CSV>) {
           chomp;
           my ($foo,$bar,$baz) = split /,/;
               $sth->execute( $foo, $bar, $baz );
         }

       The "do()" method can be used for non repeated non-"SELECT" statement
       (or with drivers that don't support placeholders):

         $rows_affected = $dbh->do("UPDATE your_table SET foo = foo + 1");

       To commit your changes to the database (when "AutoCommit" is off):

         $dbh->commit;  # or call $dbh->rollback; to undo changes

       Finally, when you have finished working with the data source, you
       should "disconnect" from it:

         $dbh->disconnect;

   General Interface Rules & Caveats
       The DBI does not have a concept of a "current session". Every session
       has a handle object (i.e., a $dbh) returned from the "connect" method.
       That handle object is used to invoke database related methods.

       Most data is returned to the Perl script as strings. (Null values are
       returned as "undef".)  This allows arbitrary precision numeric data to
       be handled without loss of accuracy.  Beware that Perl may not preserve
       the same accuracy when the string is used as a number.

       Dates and times are returned as character strings in the current
       default format of the corresponding database engine.  Time zone effects
       are database/driver dependent.

       Perl supports binary data in Perl strings, and the DBI will pass binary
       data to and from the driver without change. It is up to the driver
       implementors to decide how they wish to handle such binary data.

       Perl supports two kinds of strings: unicode (utf8 internally) and non-
       unicode (defaults to iso-8859-1 if forced to assume an encoding).
       Drivers should accept both kinds of strings and, if required, convert
       them to the character set of the database being used. Similarly, when
       fetching from the database character data that isn't iso-8859-1 the
       driver should convert it into utf8.

       Multiple SQL statements may not be combined in a single statement
       handle ($sth), although some databases and drivers do support this
       (notably Sybase and SQL Server).

       Non-sequential record reads are not supported in this version of the
       DBI.  In other words, records can only be fetched in the order that the
       database returned them, and once fetched they are forgotten.

       Positioned updates and deletes are not directly supported by the DBI.
       See the description of the "CursorName" attribute for an alternative.

       Individual driver implementors are free to provide any private
       functions and/or handle attributes that they feel are useful.  Private
       driver functions can be invoked using the DBI "func()" method.  Private
       driver attributes are accessed just like standard attributes.

       Many methods have an optional "\%attr" parameter which can be used to
       pass information to the driver implementing the method. Except where
       specifically documented, the "\%attr" parameter can only be used to
       pass driver specific hints. In general, you can ignore "\%attr"
       parameters or pass it as "undef".

   Naming Conventions and Name Space
       The DBI package and all packages below it ("DBI::*") are reserved for
       use by the DBI. Extensions and related modules use the "DBIx::"
       namespace (see http://www.perl.com/CPAN/modules/by-module/DBIx/
       <http://www.perl.com/CPAN/modules/by-module/DBIx/>).  Package names
       beginning with "DBD::" are reserved for use by DBI database drivers.
       All environment variables used by the DBI or by individual DBDs begin
       with ""DBI_"" or ""DBD_"".

       The letter case used for attribute names is significant and plays an
       important part in the portability of DBI scripts.  The case of the
       attribute name is used to signify who defined the meaning of that name
       and its values.

         Case of name  Has a meaning defined by
         ------------  ------------------------
         UPPER_CASE    Standards, e.g.,  X/Open, ISO SQL92 etc (portable)
         MixedCase     DBI API (portable), underscores are not used.
         lower_case    Driver or database engine specific (non-portable)

       It is of the utmost importance that Driver developers only use
       lowercase attribute names when defining private attributes. Private
       attribute names must be prefixed with the driver name or suitable
       abbreviation (e.g., ""ora_"" for Oracle, ""ing_"" for Ingres, etc).

   SQL - A Query Language
       Most DBI drivers require applications to use a dialect of SQL
       (Structured Query Language) to interact with the database engine.  The
       "Standards Reference Information" section provides links to useful
       information about SQL.

       The DBI itself does not mandate or require any particular language to
       be used; it is language independent. In ODBC terms, the DBI is in
       "pass-thru" mode, although individual drivers might not be. The only
       requirement is that queries and other statements must be expressed as a
       single string of characters passed as the first argument to the
       "prepare" or "do" methods.

       For an interesting diversion on the real history of RDBMS and SQL, from
       the people who made it happen, see:

         http://ftp.digital.com/pub/DEC/SRC/technical-notes/SRC-1997-018-html/sqlr95.html

       Follow the "Full Contents" then "Intergalactic dataspeak" links for the
       SQL history.

   Placeholders and Bind Values
       Some drivers support placeholders and bind values.  Placeholders, also
       called parameter markers, are used to indicate values in a database
       statement that will be supplied later, before the prepared statement is
       executed.  For example, an application might use the following to
       insert a row of data into the SALES table:

         INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)

       or the following, to select the description for a product:

         SELECT description FROM products WHERE product_code = ?

       The "?" characters are the placeholders.  The association of actual
       values with placeholders is known as binding, and the values are
       referred to as bind values.  Note that the "?" is not enclosed in
       quotation marks, even when the placeholder represents a string.

       Some drivers also allow placeholders like ":"name and ":"N (e.g., ":1",
       ":2", and so on) in addition to "?", but their use is not portable.

       If the ":"N form of placeholder is supported by the driver you're
       using, then you should be able to use either "bind_param" or "execute"
       to bind values. Check your driver documentation.

       With most drivers, placeholders can't be used for any element of a
       statement that would prevent the database server from validating the
       statement and creating a query execution plan for it. For example:

         "SELECT name, age FROM ?"         # wrong (will probably fail)
         "SELECT name, ?   FROM people"    # wrong (but may not 'fail')

       Also, placeholders can only represent single scalar values.  For
       example, the following statement won't work as expected for more than
       one value:

         "SELECT name, age FROM people WHERE name IN (?)"    # wrong
         "SELECT name, age FROM people WHERE name IN (?,?)"  # two names

       When using placeholders with the SQL "LIKE" qualifier, you must
       remember that the placeholder substitutes for the whole string.  So you
       should use ""... LIKE ? ..."" and include any wildcard characters in
       the value that you bind to the placeholder.

       NULL Values

       Undefined values, or "undef", are used to indicate NULL values.  You
       can insert and update columns with a NULL value as you would a non-NULL
       value.  These examples insert and update the column "age" with a NULL
       value:

         $sth = $dbh->prepare(qq{
           INSERT INTO people (fullname, age) VALUES (?, ?)
         });
         $sth->execute("Joe Bloggs", undef);

         $sth = $dbh->prepare(qq{
           UPDATE people SET age = ? WHERE fullname = ?
         });
         $sth->execute(undef, "Joe Bloggs");

       However, care must be taken when trying to use NULL values in a "WHERE"
       clause.  Consider:

         SELECT fullname FROM people WHERE age = ?

       Binding an "undef" (NULL) to the placeholder will not select rows which
       have a NULL "age"!  At least for database engines that conform to the
       SQL standard.  Refer to the SQL manual for your database engine or any
       SQL book for the reasons for this.  To explicitly select NULLs you have
       to say ""WHERE age IS NULL"".

       A common issue is to have a code fragment handle a value that could be
       either "defined" or "undef" (non-NULL or NULL) at runtime.  A simple
       technique is to prepare the appropriate statement as needed, and
       substitute the placeholder for non-NULL cases:

         $sql_clause = defined $age? "age = ?" : "age IS NULL";
         $sth = $dbh->prepare(qq{
           SELECT fullname FROM people WHERE $sql_clause
         });
         $sth->execute(defined $age ? $age : ());

       The following technique illustrates qualifying a "WHERE" clause with
       several columns, whose associated values ("defined" or "undef") are in
       a hash %h:

         for my $col ("age", "phone", "email") {
           if (defined $h{$col}) {
             push @sql_qual, "$col = ?";
             push @sql_bind, $h{$col};
           }
           else {
             push @sql_qual, "$col IS NULL";
           }
         }
         $sql_clause = join(" AND ", @sql_qual);
         $sth = $dbh->prepare(qq{
             SELECT fullname FROM people WHERE $sql_clause
         });
         $sth->execute(@sql_bind);

       The techniques above call prepare for the SQL statement with each call
       to execute.  Because calls to prepare() can be expensive, performance
       can suffer when an application iterates many times over statements like
       the above.

       A better solution is a single "WHERE" clause that supports both NULL
       and non-NULL comparisons.  Its SQL statement would need to be prepared
       only once for all cases, thus improving performance.  Several examples
       of "WHERE" clauses that support this are presented below.  But each
       example lacks portability, robustness, or simplicity.  Whether an
       example is supported on your database engine depends on what SQL
       extensions it provides, and where it supports the "?"  placeholder in a
       statement.

         0)  age = ?
         1)  NVL(age, xx) = NVL(?, xx)
         2)  ISNULL(age, xx) = ISNULL(?, xx)
         3)  DECODE(age, ?, 1, 0) = 1
         4)  age = ? OR (age IS NULL AND ? IS NULL)
         5)  age = ? OR (age IS NULL AND SP_ISNULL(?) = 1)
         6)  age = ? OR (age IS NULL AND ? = 1)

       Statements formed with the above "WHERE" clauses require execute
       statements as follows.  The arguments are required, whether their
       values are "defined" or "undef".

         0,1,2,3)  $sth->execute($age);
         4,5)      $sth->execute($age, $age);
         6)        $sth->execute($age, defined($age) ? 0 : 1);

       Example 0 should not work (as mentioned earlier), but may work on a few
       database engines anyway (e.g. Sybase).  Example 0 is part of examples
       4, 5, and 6, so if example 0 works, these other examples may work, even
       if the engine does not properly support the right hand side of the "OR"
       expression.

       Examples 1 and 2 are not robust: they require that you provide a valid
       column value xx (e.g. '~') which is not present in any row.  That means
       you must have some notion of what data won't be stored in the column,
       and expect clients to adhere to that.

       Example 5 requires that you provide a stored procedure (SP_ISNULL in
       this example) that acts as a function: it checks whether a value is
       null, and returns 1 if it is, or 0 if not.

       Example 6, the least simple, is probably the most portable, i.e., it
       should work with with most, if not all, database engines.

       Here is a table that indicates which examples above are known to work
       on various database engines:

                          -----Examples------
                          0  1  2  3  4  5  6
                          -  -  -  -  -  -  -
         Oracle 9         N  Y  N  Y  Y  ?  Y
         Informix IDS 9   N  N  N  Y  N  Y  Y
         MS SQL           N  N  Y  N  Y  ?  Y
         Sybase           Y  N  N  N  N  N  Y
         AnyData,DBM,CSV  Y  N  N  N  Y  Y* Y
         SQLite 3.3       N  N  N  N  Y  N  N

       * Works only because Example 0 works.

       DBI provides a sample perl script that will test the examples above on
       your database engine and tell you which ones work.  It is located in
       the ex/ subdirectory of the DBI source distribution, or here:
       <http://svn.perl.org/modules/dbi/trunk/ex/perl_dbi_nulls_test.pl>
       Please use the script to help us fill-in and maintain this table.

       Performance

       Without using placeholders, the insert statement shown previously would
       have to contain the literal values to be inserted and would have to be
       re-prepared and re-executed for each row. With placeholders, the insert
       statement only needs to be prepared once. The bind values for each row
       can be given to the "execute" method each time it's called. By avoiding
       the need to re-prepare the statement for each row, the application
       typically runs many times faster. Here's an example:

         my $sth = $dbh->prepare(q{
           INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
         }) or die $dbh->errstr;
         while (<>) {
             chomp;
             my ($product_code, $qty, $price) = split /,/;
             $sth->execute($product_code, $qty, $price) or die $dbh->errstr;
         }
         $dbh->commit or die $dbh->errstr;

       See "execute" and "bind_param" for more details.

       The "q{...}" style quoting used in this example avoids clashing with
       quotes that may be used in the SQL statement. Use the double-quote like
       "qq{...}" operator if you want to interpolate variables into the
       string.  See "Quote and Quote-like Operators" in perlop for more
       details.

       See also the "bind_columns" method, which is used to associate Perl
       variables with the output columns of a "SELECT" statement.

THE DBI PACKAGE AND CLASS
       In this section, we cover the DBI class methods, utility functions, and
       the dynamic attributes associated with generic DBI handles.

   DBI Constants
       Constants representing the values of the SQL standard types can be
       imported individually by name, or all together by importing the special
       ":sql_types" tag.

       The names and values of all the defined SQL standard types can be
       produced like this:

         foreach (@{ $DBI::EXPORT_TAGS{sql_types} }) {
           printf "%s=%d\n", $_, &{"DBI::$_"};
         }

       These constants are defined by SQL/CLI, ODBC or both.  "SQL_BIGINT" is
       (currently) omitted, because SQL/CLI and ODBC provide conflicting
       codes.

       See the "type_info", "type_info_all", and "bind_param" methods for
       possible uses.

       Note that just because the DBI defines a named constant for a given
       data type doesn't mean that drivers will support that data type.

   DBI Class Methods
       The following methods are provided by the DBI class:

       "parse_dsn"

         ($scheme, $driver, $attr_string, $attr_hash, $driver_dsn) = DBI->parse_dsn($dsn)
             or die "Can't parse DBI DSN '$dsn'";

       Breaks apart a DBI Data Source Name (DSN) and returns the individual
       parts. If $dsn doesn't contain a valid DSN then parse_dsn() returns an
       empty list.

       $scheme is the first part of the DSN and is currently always 'dbi'.
       $driver is the driver name, possibly defaulted to $ENV{DBI_DRIVER}, and
       may be undefined.  $attr_string is the contents of the optional
       attribute string, which may be undefined.  If $attr_string is not empty
       then $attr_hash is a reference to a hash containing the parsed
       attribute names and values.  $driver_dsn is the last part of the DBI
       DSN string. For example:

         ($scheme, $driver, $attr_string, $attr_hash, $driver_dsn)
             = DBI->parse_dsn("DBI:MyDriver(RaiseError=>1):db=test;port=42");
         $scheme      = 'dbi';
         $driver      = 'MyDriver';
         $attr_string = 'RaiseError=>1';
         $attr_hash   = { 'RaiseError' => '1' };
         $driver_dsn  = 'db=test;port=42';

       The parse_dsn() method was added in DBI 1.43.

       "connect"

         $dbh = DBI->connect($data_source, $username, $password)
                   or die $DBI::errstr;
         $dbh = DBI->connect($data_source, $username, $password, \%attr)
                   or die $DBI::errstr;

       Establishes a database connection, or session, to the requested
       $data_source.  Returns a database handle object if the connection
       succeeds. Use "$dbh->disconnect" to terminate the connection.

       If the connect fails (see below), it returns "undef" and sets both
       $DBI::err and $DBI::errstr. (It does not explicitly set $!.) You should
       generally test the return status of "connect" and "print $DBI::errstr"
       if it has failed.

       Multiple simultaneous connections to multiple databases through
       multiple drivers can be made via the DBI. Simply make one "connect"
       call for each database and keep a copy of each returned database
       handle.

       The $data_source value must begin with ""dbi:"driver_name":"".  The
       driver_name specifies the driver that will be used to make the
       connection. (Letter case is significant.)

       As a convenience, if the $data_source parameter is undefined or empty,
       the DBI will substitute the value of the environment variable
       "DBI_DSN".  If just the driver_name part is empty (i.e., the
       $data_source prefix is ""dbi::""), the environment variable
       "DBI_DRIVER" is used. If neither variable is set, then "connect" dies.

       Examples of $data_source values are:

         dbi:DriverName:database_name
         dbi:DriverName:database_name@hostname:port
         dbi:DriverName:database=database_name;host=hostname;port=port

       There is no standard for the text following the driver name. Each
       driver is free to use whatever syntax it wants. The only requirement
       the DBI makes is that all the information is supplied in a single
       string.  You must consult the documentation for the drivers you are
       using for a description of the syntax they require.

       It is recommended that drivers support the ODBC style, shown in the
       last example above. It is also recommended that that they support the
       three common names '"host"', '"port"', and '"database"' (plus '"db"' as
       an alias for "database"). This simplifies automatic construction of
       basic DSNs: "dbi:$driver:database=$db;host=$host;port=$port".  Drivers
       should aim to 'do something reasonable' when given a DSN in this form,
       but if any part is meaningless for that driver (such as 'port' for
       Informix) it should generate an error if that part is not empty.

       If the environment variable "DBI_AUTOPROXY" is defined (and the driver
       in $data_source is not ""Proxy"") then the connect request will
       automatically be changed to:

         $ENV{DBI_AUTOPROXY};dsn=$data_source

       "DBI_AUTOPROXY" is typically set as
       ""dbi:Proxy:hostname=...;port=..."".  If $ENV{DBI_AUTOPROXY} doesn't
       begin with '"dbi:"' then "dbi:Proxy:" will be prepended to it first.
       See the DBD::Proxy documentation for more details.

       If $username or $password are undefined (rather than just empty), then
       the DBI will substitute the values of the "DBI_USER" and "DBI_PASS"
       environment variables, respectively.  The DBI will warn if the
       environment variables are not defined.  However, the everyday use of
       these environment variables is not recommended for security reasons.
       The mechanism is primarily intended to simplify testing.  See below for
       alternative way to specify the username and password.

       "DBI->connect" automatically installs the driver if it has not been
       installed yet. Driver installation either returns a valid driver
       handle, or it dies with an error message that includes the string
       ""install_driver"" and the underlying problem. So "DBI->connect" will
       die on a driver installation failure and will only return "undef" on a
       connect failure, in which case $DBI::errstr will hold the error
       message.  Use "eval { ... }" if you need to catch the
       ""install_driver"" error.

       The $data_source argument (with the ""dbi:...:"" prefix removed) and
       the $username and $password arguments are then passed to the driver for
       processing. The DBI does not define any interpretation for the contents
       of these fields.  The driver is free to interpret the $data_source,
       $username, and $password fields in any way, and supply whatever
       defaults are appropriate for the engine being accessed.  (Oracle, for
       example, uses the ORACLE_SID and TWO_TASK environment variables if no
       $data_source is specified.)

       The "AutoCommit" and "PrintError" attributes for each connection
       default to "on". (See "AutoCommit" and "PrintError" for more
       information.)  However, it is strongly recommended that you explicitly
       define "AutoCommit" rather than rely on the default. The "PrintWarn"
       attribute defaults to on if $^W is true, i.e., perl is running with
       warnings enabled.

       The "\%attr" parameter can be used to alter the default settings of
       "PrintError", "RaiseError", "AutoCommit", and other attributes. For
       example:

         $dbh = DBI->connect($data_source, $user, $pass, {
               PrintError => 0,
               AutoCommit => 0
         });

       The username and password can also be specified using the attributes
       "Username" and "Password", in which case they take precedence over the
       $username and $password parameters.

       You can also define connection attribute values within the $data_source
       parameter. For example:

         dbi:DriverName(PrintWarn=>1,PrintError=>0,Taint=>1):...

       Individual attributes values specified in this way take precedence over
       any conflicting values specified via the "\%attr" parameter to
       "connect".

       The "dbi_connect_method" attribute can be used to specify which driver
       method should be called to establish the connection. The only useful
       values are 'connect', 'connect_cached', or some specialized case like
       'Apache::DBI::connect' (which is automatically the default when running
       within Apache).

       Where possible, each session ($dbh) is independent from the
       transactions in other sessions. This is useful when you need to hold
       cursors open across transactions--for example, if you use one session
       for your long lifespan cursors (typically read-only) and another for
       your short update transactions.

       For compatibility with old DBI scripts, the driver can be specified by
       passing its name as the fourth argument to "connect" (instead of
       "\%attr"):

         $dbh = DBI->connect($data_source, $user, $pass, $driver);

       In this "old-style" form of "connect", the $data_source should not
       start with ""dbi:driver_name:"". (If it does, the embedded driver_name
       will be ignored). Also note that in this older form of "connect", the
       "$dbh->{AutoCommit}" attribute is undefined, the "$dbh->{PrintError}"
       attribute is off, and the old "DBI_DBNAME" environment variable is
       checked if "DBI_DSN" is not defined. Beware that this "old-style"
       "connect" will soon be withdrawn in a future version of DBI.

       "connect_cached"

         $dbh = DBI->connect_cached($data_source, $username, $password)
                   or die $DBI::errstr;
         $dbh = DBI->connect_cached($data_source, $username, $password, \%attr)
                   or die $DBI::errstr;

       "connect_cached" is like "connect", except that the database handle
       returned is also stored in a hash associated with the given parameters.
       If another call is made to "connect_cached" with the same parameter
       values, then the corresponding cached $dbh will be returned if it is
       still valid.  The cached database handle is replaced with a new
       connection if it has been disconnected or if the "ping" method fails.

       That the behaviour of this method differs in several respects from the
       behaviour of persistent connections implemented by Apache::DBI.
       However, if Apache::DBI is loaded then "connect_cached" will use it.

       Caching connections can be useful in some applications, but it can also
       cause problems, such as too many connections, and so should be used
       with care. In particular, avoid changing the attributes of a database
       handle created via connect_cached() because it will affect other code
       that may be using the same handle.

       Where multiple separate parts of a program are using connect_cached()
       to connect to the same database with the same (initial) attributes it
       is a good idea to add a private attribute to the connect_cached() call
       to effectively limit the scope of the caching. For example:

         DBI->connect_cached(..., { private_foo_cachekey => "Bar", ... });

       Handles returned from that connect_cached() call will only be returned
       by other connect_cached() call elsewhere in the code if those other
       calls also pass in the same attribute values, including the private
       one.  (I've used "private_foo_cachekey" here as an example, you can use
       any attribute name with a "private_" prefix.)

       Taking that one step further, you can limit a particular
       connect_cached() call to return handles unique to that one place in the
       code by setting the private attribute to a unique value for that place:

         DBI->connect_cached(..., { private_foo_cachekey => __FILE__.__LINE__, ... });

       By using a private attribute you still get connection caching for the
       individual calls to connect_cached() but, by making separate database
       connections for separate parts of the code, the database handles are
       isolated from any attribute changes made to other handles.

       The cache can be accessed (and cleared) via the "CachedKids" attribute:

         my $CachedKids_hashref = $dbh->{Driver}->{CachedKids};
         %$CachedKids_hashref = () if $CachedKids_hashref;

       "available_drivers"

         @ary = DBI->available_drivers;
         @ary = DBI->available_drivers($quiet);

       Returns a list of all available drivers by searching for "DBD::*"
       modules through the directories in @INC. By default, a warning is given
       if some drivers are hidden by others of the same name in earlier
       directories. Passing a true value for $quiet will inhibit the warning.

       "installed_drivers"

         %drivers = DBI->installed_drivers();

       Returns a list of driver name and driver handle pairs for all drivers
       'installed' (loaded) into the current process.  The driver name does
       not include the 'DBD::' prefix.

       To get a list of all drivers available in your perl installation you
       can use "available_drivers".

       Added in DBI 1.49.

       "installed_versions"

         DBI->installed_versions;
         @ary  = DBI->installed_versions;
         %hash = DBI->installed_versions;

       Calls available_drivers() and attempts to load each of them in turn
       using install_driver().  For each load that succeeds the driver name
       and version number are added to a hash. When running under
       DBI::PurePerl drivers which appear not be pure-perl are ignored.

       When called in array context the list of successfully loaded drivers is
       returned (without the 'DBD::' prefix).

       When called in scalar context a reference to the hash is returned and
       the hash will also contain other entries for the "DBI" version, "OS"
       name, etc.

       When called in a void context the installed_versions() method will
       print out a formatted list of the hash contents, one per line.

       Due to the potentially high memory cost and unknown risks of loading in
       an unknown number of drivers that just happen to be installed on the
       system, this method is not recommended for general use.  Use
       available_drivers() instead.

       The installed_versions() method is primarily intended as a quick way to
       see from the command line what's installed. For example:

         perl -MDBI -e 'DBI->installed_versions'

       The installed_versions() method was added in DBI 1.38.

       "data_sources"

         @ary = DBI->data_sources($driver);
         @ary = DBI->data_sources($driver, \%attr);

       Returns a list of data sources (databases) available via the named
       driver.  If $driver is empty or "undef", then the value of the
       "DBI_DRIVER" environment variable is used.

       The driver will be loaded if it hasn't been already. Note that if the
       driver loading fails then data_sources() dies with an error message
       that includes the string ""install_driver"" and the underlying problem.

       Data sources are returned in a form suitable for passing to the
       "connect" method (that is, they will include the ""dbi:$driver:""
       prefix).

       Note that many drivers have no way of knowing what data sources might
       be available for it. These drivers return an empty or incomplete list
       or may require driver-specific attributes.

       There is also a data_sources() method defined for database handles.

       "trace"

         DBI->trace($trace_setting)
         DBI->trace($trace_setting, $trace_filename)
         DBI->trace($trace_setting, $trace_filehandle)
         $trace_setting = DBI->trace;

       The "DBI->trace" method sets the global default trace settings and
       returns the previous trace settings. It can also be used to change
       where the trace output is sent.

       There's a similar method, "$h->trace", which sets the trace settings
       for the specific handle it's called on.

       See the "TRACING" section for full details about the DBI's powerful
       tracing facilities.

       "visit_handles"

         DBI->visit_handles( $coderef );
         DBI->visit_handles( $coderef, $info );

       Where $coderef is a reference to a subroutine and $info is an arbitrary
       value which, if undefined, defaults to a reference to an empty hash.
       Returns $info.

       For each installed driver handle, if any, $coderef is invoked as:

         $coderef->($driver_handle, $info);

       If the execution of $coderef returns a true value then
       "visit_child_handles" is called on that child handle and passed the
       returned value as $info.

       For example:

         my $info = $dbh->{Driver}->visit_child_handles(sub {
             my ($h, $info) = @_;
             ++$info->{ $h->{Type} }; # count types of handles (dr/db/st)
             return $info; # visit kids
         });

       See also "visit_child_handles".

   DBI Utility Functions
       In addition to the DBI methods listed in the previous section, the DBI
       package also provides several utility functions.

       These can be imported into your code by listing them in the "use"
       statement. For example:

         use DBI qw(neat data_diff);

       Alternatively, all these utility functions (except hash) can be
       imported using the ":utils" import tag. For example:

         use DBI qw(:utils);

       "data_string_desc"

         $description = data_string_desc($string);

       Returns an informal description of the string. For example:

         UTF8 off, ASCII, 42 characters 42 bytes
         UTF8 off, non-ASCII, 42 characters 42 bytes
         UTF8 on, non-ASCII, 4 characters 6 bytes
         UTF8 on but INVALID encoding, non-ASCII, 4 characters 6 bytes
         UTF8 off, undef

       The initial "UTF8" on/off refers to Perl's internal SvUTF8 flag.  If
       $string has the SvUTF8 flag set but the sequence of bytes it contains
       are not a valid UTF-8 encoding then data_string_desc() will report
       "UTF8 on but INVALID encoding".

       The "ASCII" vs "non-ASCII" portion shows "ASCII" if all the characters
       in the string are ASCII (have code points <= 127).

       The data_string_desc() function was added in DBI 1.46.

       "data_string_diff"

         $diff = data_string_diff($a, $b);

       Returns an informal description of the first character difference
       between the strings. If both $a and $b contain the same sequence of
       characters then data_string_diff() returns an empty string.  For
       example:

        Params a & b     Result
        ------------     ------
        'aaa', 'aaa'     ''
        'aaa', 'abc'     'Strings differ at index 2: a[2]=a, b[2]=b'
        'aaa', undef     'String b is undef, string a has 3 characters'
        'aaa', 'aa'      'String b truncated after 2 characters'

       Unicode characters are reported in "\x{XXXX}" format. Unicode code
       points in the range U+0800 to U+08FF are unassigned and most likely to
       occur due to double-encoding. Characters in this range are reported as
       "\x{08XX}='C'" where "C" is the corresponding latin-1 character.

       The data_string_diff() function only considers logical characters and
       not the underlying encoding. See "data_diff" for an alternative.

       The data_string_diff() function was added in DBI 1.46.

       "data_diff"

         $diff = data_diff($a, $b);
         $diff = data_diff($a, $b, $logical);

       Returns an informal description of the difference between two strings.
       It calls "data_string_desc" and "data_string_diff" and returns the
       combined results as a multi-line string.

       For example, "data_diff("abc", "ab\x{263a}")" will return:

         a: UTF8 off, ASCII, 3 characters 3 bytes
         b: UTF8 on, non-ASCII, 3 characters 5 bytes
         Strings differ at index 2: a[2]=c, b[2]=\x{263A}

       If $a and $b are identical in both the characters they contain and
       their physical encoding then data_diff() returns an empty string.  If
       $logical is true then physical encoding differences are ignored (but
       are still reported if there is a difference in the characters).

       The data_diff() function was added in DBI 1.46.

       "neat"

         $str = neat($value);
         $str = neat($value, $maxlen);

       Return a string containing a neat (and tidy) representation of the
       supplied value.

       Strings will be quoted, although internal quotes will not be escaped.
       Values known to be numeric will be unquoted. Undefined (NULL) values
       will be shown as "undef" (without quotes).

       If the string is flagged internally as utf8 then double quotes will be
       used, otherwise single quotes are used and unprintable characters will
       be replaced by dot (.).

       For result strings longer than $maxlen the result string will be
       truncated to "$maxlen-4" and ""...'"" will be appended.  If $maxlen is
       0 or "undef", it defaults to $DBI::neat_maxlen which, in turn, defaults
       to 400.

       This function is designed to format values for human consumption.  It
       is used internally by the DBI for "trace" output. It should typically
       not be used for formatting values for database use.  (See also
       "quote".)

       "neat_list"

         $str = neat_list(\@listref, $maxlen, $field_sep);

       Calls "neat" on each element of the list and returns a string
       containing the results joined with $field_sep. $field_sep defaults to
       ", ".

       "looks_like_number"

         @bool = looks_like_number(@array);

       Returns true for each element that looks like a number.  Returns false
       for each element that does not look like a number.  Returns "undef" for
       each element that is undefined or empty.

       "hash"

         $hash_value = DBI::hash($buffer, $type);

       Return a 32-bit integer 'hash' value corresponding to the contents of
       $buffer.  The $type parameter selects which kind of hash algorithm
       should be used.

       For the technically curious, type 0 (which is the default if $type
       isn't specified) is based on the Perl 5.1 hash except that the value is
       forced to be negative (for obscure historical reasons).  Type 1 is the
       better "Fowler / Noll / Vo" (FNV) hash. See
       <http://www.isthe.com/chongo/tech/comp/fnv/> for more information.
       Both types are implemented in C and are very fast.

       This function doesn't have much to do with databases, except that it
       can be handy to store hash values in a database.

   DBI Dynamic Attributes
       Dynamic attributes are always associated with the last handle used
       (that handle is represented by $h in the descriptions below).

       Where an attribute is equivalent to a method call, then refer to the
       method call for all related documentation.

       Warning: these attributes are provided as a convenience but they do
       have limitations. Specifically, they have a short lifespan: because
       they are associated with the last handle used, they should only be used
       immediately after calling the method that "sets" them.  If in any
       doubt, use the corresponding method call.

       $DBI::err

       Equivalent to "$h->err".

       $DBI::errstr

       Equivalent to "$h->errstr".

       $DBI::state

       Equivalent to "$h->state".

       $DBI::rows

       Equivalent to "$h->rows". Please refer to the documentation for the
       "rows" method.

       $DBI::lasth

       Returns the DBI object handle used for the most recent DBI method call.
       If the last DBI method call was a DESTROY then $DBI::lasth will return
       the handle of the parent of the destroyed handle, if there is one.

METHODS COMMON TO ALL HANDLES
       The following methods can be used by all types of DBI handles.

       "err"

         $rv = $h->err;

       Returns the native database engine error code from the last driver
       method called. The code is typically an integer but you should not
       assume that.

       The DBI resets $h->err to undef before almost all DBI method calls, so
       the value only has a short lifespan. Also, for most drivers, the
       statement handles share the same error variable as the parent database
       handle, so calling a method on one handle may reset the error on the
       related handles.

       (Methods which don't reset err before being called include err() and
       errstr(), obviously, state(), rows(), func(), trace(), trace_msg(),
       ping(), and the tied hash attribute FETCH() and STORE() methods.)

       If you need to test for specific error conditions and have your program
       be portable to different database engines, then you'll need to
       determine what the corresponding error codes are for all those engines
       and test for all of them.

       The DBI uses the value of $DBI::stderr as the "err" value for internal
       errors.  Drivers should also do likewise.  The default value for
       $DBI::stderr is 2000000000.

       A driver may return 0 from err() to indicate a warning condition after
       a method call. Similarly, a driver may return an empty string to
       indicate a 'success with information' condition. In both these cases
       the value is false but not undef. The errstr() and state() methods may
       be used to retrieve extra information in these cases.

       See "set_err" for more information.

       "errstr"

         $str = $h->errstr;

       Returns the native database engine error message from the last DBI
       method called. This has the same lifespan issues as the "err" method
       described above.

       The returned string may contain multiple messages separated by newline
       characters.

       The errstr() method should not be used to test for errors, use err()
       for that, because drivers may return 'success with information' or
       warning messages via errstr() for methods that have not 'failed'.

       See "set_err" for more information.

       "state"

         $str = $h->state;

       Returns a state code in the standard SQLSTATE five character format.
       Note that the specific success code 00000 is translated to any empty
       string (false). If the driver does not support SQLSTATE (and most
       don't), then state() will return "S1000" (General Error) for all
       errors.

       The driver is free to return any value via "state", e.g., warning
       codes, even if it has not declared an error by returning a true value
       via the "err" method described above.

       The state() method should not be used to test for errors, use err() for
       that, because drivers may return a 'success with information' or
       warning state code via state() for methods that have not 'failed'.

       "set_err"

         $rv = $h->set_err($err, $errstr);
         $rv = $h->set_err($err, $errstr, $state);
         $rv = $h->set_err($err, $errstr, $state, $method);
         $rv = $h->set_err($err, $errstr, $state, $method, $rv);

       Set the "err", "errstr", and "state" values for the handle.  This
       method is typically only used by DBI drivers and DBI subclasses.

       If the "HandleSetErr" attribute holds a reference to a subroutine it is
       called first. The subroutine can alter the $err, $errstr, $state, and
       $method values. See "HandleSetErr" for full details.  If the subroutine
       returns a true value then the handle "err", "errstr", and "state"
       values are not altered and set_err() returns an empty list (it normally
       returns $rv which defaults to undef, see below).

       Setting "err" to a true value indicates an error and will trigger the
       normal DBI error handling mechanisms, such as "RaiseError" and
       "HandleError", if they are enabled, when execution returns from the DBI
       back to the application.

       Setting "err" to "" indicates an 'information' state, and setting it to
       "0" indicates a 'warning' state. Setting "err" to "undef" also sets
       "errstr" to undef, and "state" to "", irrespective of the values of the
       $errstr and $state parameters.

       The $method parameter provides an alternate method name for the
       "RaiseError"/"PrintError"/"PrintWarn" error string instead of the
       fairly unhelpful '"set_err"'.

       The "set_err" method normally returns undef.  The $rv parameter
       provides an alternate return value.

       Some special rules apply if the "err" or "errstr" values for the handle
       are already set...

       If "errstr" is true then: "" [err was %s now %s]"" is appended if $err
       is true and "err" is already true and the new err value differs from
       the original one. Similarly "" [state was %s now %s]"" is appended if
       $state is true and "state" is already true and the new state value
       differs from the original one. Finally ""\n"" and the new $errstr are
       appended if $errstr differs from the existing errstr value. Obviously
       the %s's above are replaced by the corresponding values.

       The handle "err" value is set to $err if: $err is true; or handle "err"
       value is undef; or $err is defined and the length is greater than the
       handle "err" length. The effect is that an 'information' state only
       overrides undef; a 'warning' overrides undef or 'information', and an
       'error' state overrides anything.

       The handle "state" value is set to $state if $state is true and the
       handle "err" value was set (by the rules above).

       Support for warning and information states was added in DBI 1.41.

       "trace"

         $h->trace($trace_settings);
         $h->trace($trace_settings, $trace_filename);
         $trace_settings = $h->trace;

       The trace() method is used to alter the trace settings for a handle
       (and any future children of that handle).  It can also be used to
       change where the trace output is sent.

       There's a similar method, "DBI->trace", which sets the global default
       trace settings.

       See the "TRACING" section for full details about the DBI's powerful
       tracing facilities.

       "trace_msg"

         $h->trace_msg($message_text);
         $h->trace_msg($message_text, $min_level);

       Writes $message_text to the trace file if the trace level is greater
       than or equal to $min_level (which defaults to 1).  Can also be called
       as "DBI->trace_msg($msg)".

       See "TRACING" for more details.

       "func"

         $h->func(@func_arguments, $func_name) or die ...;

       The "func" method can be used to call private non-standard and non-
       portable methods implemented by the driver. Note that the function name
       is given as the last argument.

       It's also important to note that the func() method does not clear a
       previous error ($DBI::err etc.) and it does not trigger automatic error
       detection (RaiseError etc.) so you must check the return status and/or
       $h->err to detect errors.

       (This method is not directly related to calling stored procedures.
       Calling stored procedures is currently not defined by the DBI.  Some
       drivers, such as DBD::Oracle, support it in non-portable ways.  See
       driver documentation for more details.)

       See also install_method() in DBI::DBD for how you can avoid needing to
       use func() and gain direct access to driver-private methods.

       "can"

         $is_implemented = $h->can($method_name);

       Returns true if $method_name is implemented by the driver or a default
       method is provided by the DBI.  It returns false where a driver hasn't
       implemented a method and the default method is provided by the DBI is
       just an empty stub.

       "parse_trace_flags"

         $trace_settings_integer = $h->parse_trace_flags($trace_settings);

       Parses a string containing trace settings and returns the corresponding
       integer value used internally by the DBI and drivers.

       The $trace_settings argument is a string containing a trace level
       between 0 and 15 and/or trace flag names separated by vertical bar
       (""|"") or comma ("","") characters. For example: "SQL|3|foo".

       It uses the parse_trace_flag() method, described below, to process the
       individual trace flag names.

       The parse_trace_flags() method was added in DBI 1.42.

       "parse_trace_flag"

         $bit_flag = $h->parse_trace_flag($trace_flag_name);

       Returns the bit flag corresponding to the trace flag name in
       $trace_flag_name.  Drivers are expected to override this method and
       check if $trace_flag_name is a driver specific trace flags and, if not,
       then call the DBI's default parse_trace_flag().

       The parse_trace_flag() method was added in DBI 1.42.

       "private_attribute_info"

         $hash_ref = $h->private_attribute_info();

       Returns a reference to a hash whose keys are the names of driver-
       private attributes available for the kind of handle (driver, database,
       statement) that the method was called on.

       For example, the return value when called with a DBD::Sybase $dbh could
       look like this:

         {
             syb_dynamic_supported => undef,
             syb_oc_version => undef,
             syb_server_version => undef,
             syb_server_version_string => undef,
         }

       and when called with a DBD::Sybase $sth they could look like this:

         {
             syb_types => undef,
             syb_proc_status => undef,
             syb_result_type => undef,
         }

       The values should be undef. Meanings may be assigned to particular
       values in future.

       "swap_inner_handle"

         $rc = $h1->swap_inner_handle( $h2 );
         $rc = $h1->swap_inner_handle( $h2, $allow_reparent );

       Brain transplants for handles. You don't need to know about this unless
       you want to become a handle surgeon.

       A DBI handle is a reference to a tied hash. A tied hash has an inner
       hash that actually holds the contents.  The swap_inner_handle() method
       swaps the inner hashes between two handles. The $h1 and $h2 handles
       still point to the same tied hashes, but what those hashes are tied to
       has been swapped.  In effect $h1 becomes $h2 and vice-versa. This is
       powerful stuff, expect problems. Use with care.

       As a small safety measure, the two handles, $h1 and $h2, have to share
       the same parent unless $allow_reparent is true.

       The swap_inner_handle() method was added in DBI 1.44.

       Here's a quick kind of 'diagram' as a worked example to help think
       about what's happening:

           Original state:
                   dbh1o -> dbh1i
                   sthAo -> sthAi(dbh1i)
                   dbh2o -> dbh2i

           swap_inner_handle dbh1o with dbh2o:
                   dbh2o -> dbh1i
                   sthAo -> sthAi(dbh1i)
                   dbh1o -> dbh2i

           create new sth from dbh1o:
                   dbh2o -> dbh1i
                   sthAo -> sthAi(dbh1i)
                   dbh1o -> dbh2i
                   sthBo -> sthBi(dbh2i)

           swap_inner_handle sthAo with sthBo:
                   dbh2o -> dbh1i
                   sthBo -> sthAi(dbh1i)
                   dbh1o -> dbh2i
                   sthAo -> sthBi(dbh2i)

       "visit_child_handles"

         $h->visit_child_handles( $coderef );
         $h->visit_child_handles( $coderef, $info );

       Where $coderef is a reference to a subroutine and $info is an arbitrary
       value which, if undefined, defaults to a reference to an empty hash.
       Returns $info.

       For each child handle of $h, if any, $coderef is invoked as:

         $coderef->($child_handle, $info);

       If the execution of $coderef returns a true value then
       "visit_child_handles" is called on that child handle and passed the
       returned value as $info.

       For example:

         # count database connections with names (DSN) matching a pattern
         my $connections = 0;
         $dbh->{Driver}->visit_child_handles(sub {
             my ($h, $info) = @_;
             ++$connections if $h->{Name} =~ /foo/;
             return 0; # don't visit kids
         })

       See also "visit_handles".

ATTRIBUTES COMMON TO ALL HANDLES
       These attributes are common to all types of DBI handles.

       Some attributes are inherited by child handles. That is, the value of
       an inherited attribute in a newly created statement handle is the same
       as the value in the parent database handle. Changes to attributes in
       the new statement handle do not affect the parent database handle and
       changes to the database handle do not affect existing statement
       handles, only future ones.

       Attempting to set or get the value of an unknown attribute generates a
       warning, except for private driver specific attributes (which all have
       names starting with a lowercase letter).

       Example:

         $h->{AttributeName} = ...;    # set/write
         ... = $h->{AttributeName};    # get/read

       "Warn" (boolean, inherited)

       The "Warn" attribute enables useful warnings for certain bad practices.
       It is enabled by default and should only be disabled in rare
       circumstances.  Since warnings are generated using the Perl "warn"
       function, they can be intercepted using the Perl $SIG{__WARN__} hook.

       The "Warn" attribute is not related to the "PrintWarn" attribute.

       "Active" (boolean, read-only)

       The "Active" attribute is true if the handle object is "active". This
       is rarely used in applications. The exact meaning of active is somewhat
       vague at the moment. For a database handle it typically means that the
       handle is connected to a database ("$dbh->disconnect" sets "Active"
       off).  For a statement handle it typically means that the handle is a
       "SELECT" that may have more data to fetch. (Fetching all the data or
       calling "$sth->finish" sets "Active" off.)

       "Executed" (boolean)

       The "Executed" attribute is true if the handle object has been
       "executed".  Currently only the $dbh do() method and the $sth
       execute(), execute_array(), and execute_for_fetch() methods set the
       "Executed" attribute.

       When it's set on a handle it is also set on the parent handle at the
       same time. So calling execute() on a $sth also sets the "Executed"
       attribute on the parent $dbh.

       The "Executed" attribute for a database handle is cleared by the
       commit() and rollback() methods (even if they fail). The "Executed"
       attribute of a statement handle is not cleared by the DBI under any
       circumstances and so acts as a permanent record of whether the
       statement handle was ever used.

       The "Executed" attribute was added in DBI 1.41.

       "Kids" (integer, read-only)

       For a driver handle, "Kids" is the number of currently existing
       database handles that were created from that driver handle.  For a
       database handle, "Kids" is the number of currently existing statement
       handles that were created from that database handle.  For a statement
       handle, the value is zero.

       "ActiveKids" (integer, read-only)

       Like "Kids", but only counting those that are "Active" (as above).

       "CachedKids" (hash ref)

       For a database handle, "CachedKids" returns a reference to the cache
       (hash) of statement handles created by the "prepare_cached" method.
       For a driver handle, returns a reference to the cache (hash) of
       database handles created by the "connect_cached" method.

       "Type" (scalar, read-only)

       The "Type" attribute identifies the type of a DBI handle.  Returns "dr"
       for driver handles, "db" for database handles and "st" for statement
       handles.

       "ChildHandles" (array ref)

       The ChildHandles attribute contains a reference to an array of all the
       handles created by this handle which are still accessible.  The
       contents of the array are weak-refs and will become undef when the
       handle goes out of scope.

       "ChildHandles" returns undef if your perl version does not support weak
       references (check the Scalar::Util module).  The referenced array
       returned should be treated as read-only.

       For example, to enumerate all driver handles, database handles and
       statement handles:

           sub show_child_handles {
               my ($h, $level) = @_;
               printf "%sh %s %s\n", $h->{Type}, "\t" x $level, $h;
               show_child_handles($_, $level + 1)
                   for (grep { defined } @{$h->{ChildHandles}});
           }

           my %drivers = DBI->installed_drivers();
           show_child_handles($_, 0) for (values %drivers);

       "CompatMode" (boolean, inherited)

       The "CompatMode" attribute is used by emulation layers (such as
       Oraperl) to enable compatible behaviour in the underlying driver (e.g.,
       DBD::Oracle) for this handle. Not normally set by application code.

       It also has the effect of disabling the 'quick FETCH' of attribute
       values from the handles attribute cache. So all attribute values are
       handled by the drivers own FETCH method. This makes them slightly
       slower but is useful for special-purpose drivers like DBD::Multiplex.

       "InactiveDestroy" (boolean)

       The default value, false, means a handle will be fully destroyed as
       normal when the last reference to it is removed, just as you'd expect.

       If set true then the handle will be treated by the DESTROY as if it was
       no longer Active, and so the database engine related effects of
       DESTROYing a handle will be skipped.

       Think of the name as meaning 'treat the handle as not-Active in the
       DESTROY method'.

       For a database handle, this attribute does not disable an explicit call
       to the disconnect method, only the implicit call from DESTROY that
       happens if the handle is still marked as "Active".

       This attribute is specifically designed for use in Unix applications
       that "fork" child processes. Either the parent or the child process,
       but not both, should set "InactiveDestroy" true on all their shared
       handles.  (Note that some databases, including Oracle, don't support
       passing a database connection across a fork.)

       To help tracing applications using fork the process id is shown in the
       trace log whenever a DBI or handle trace() method is called.  The
       process id also shown for every method call if the DBI trace level (not
       handle trace level) is set high enough to show the trace from the DBI's
       method dispatcher, e.g. >= 9.

       "PrintWarn" (boolean, inherited)

       The "PrintWarn" attribute controls the printing of warnings recorded by
       the driver.  When set to a true value the DBI will check method calls
       to see if a warning condition has been set. If so, the DBI will
       effectively do a "warn("$class $method warning: $DBI::errstr")" where
       $class is the driver class and $method is the name of the method which
       failed. E.g.,

         DBD::Oracle::db execute warning: ... warning text here ...

       By default, "DBI->connect" sets "PrintWarn" "on" if $^W is true, i.e.,
       perl is running with warnings enabled.

       If desired, the warnings can be caught and processed using a
       $SIG{__WARN__} handler or modules like CGI::Carp and CGI::ErrorWrap.

       See also "set_err" for how warnings are recorded and "HandleSetErr" for
       how to influence it.

       Fetching the full details of warnings can require an extra round-trip
       to the database server for some drivers. In which case the driver may
       opt to only fetch the full details of warnings if the "PrintWarn"
       attribute is true. If "PrintWarn" is false then these drivers should
       still indicate the fact that there were warnings by setting the warning
       string to, for example: "3 warnings".

       "PrintError" (boolean, inherited)

       The "PrintError" attribute can be used to force errors to generate
       warnings (using "warn") in addition to returning error codes in the
       normal way.  When set "on", any method which results in an error
       occurring will cause the DBI to effectively do a "warn("$class $method
       failed: $DBI::errstr")" where $class is the driver class and $method is
       the name of the method which failed. E.g.,

         DBD::Oracle::db prepare failed: ... error text here ...

       By default, "DBI->connect" sets "PrintError" "on".

       If desired, the warnings can be caught and processed using a
       $SIG{__WARN__} handler or modules like CGI::Carp and CGI::ErrorWrap.

       "RaiseError" (boolean, inherited)

       The "RaiseError" attribute can be used to force errors to raise
       exceptions rather than simply return error codes in the normal way. It
       is "off" by default.  When set "on", any method which results in an
       error will cause the DBI to effectively do a "die("$class $method
       failed: $DBI::errstr")", where $class is the driver class and $method
       is the name of the method that failed. E.g.,

         DBD::Oracle::db prepare failed: ... error text here ...

       If you turn "RaiseError" on then you'd normally turn "PrintError" off.
       If "PrintError" is also on, then the "PrintError" is done first
       (naturally).

       Typically "RaiseError" is used in conjunction with "eval { ... }" to
       catch the exception that's been thrown and followed by an "if ($@) {
       ... }" block to handle the caught exception.  For example:

         eval {
           ...
           $sth->execute();
           ...
         };
         if ($@) {
           # $sth->err and $DBI::err will be true if error was from DBI
           warn $@; # print the error
           ... # do whatever you need to deal with the error
         }

       In that eval block the $DBI::lasth variable can be useful for diagnosis
       and reporting if you can't be sure which handle triggered the error.
       For example, $DBI::lasth->{Type} and $DBI::lasth->{Statement}.

       See also "Transactions".

       If you want to temporarily turn "RaiseError" off (inside a library
       function that is likely to fail, for example), the recommended way is
       like this:

         {
           local $h->{RaiseError};  # localize and turn off for this block
           ...
         }

       The original value will automatically and reliably be restored by Perl,
       regardless of how the block is exited.  The same logic applies to other
       attributes, including "PrintError".

       "HandleError" (code ref, inherited)

       The "HandleError" attribute can be used to provide your own alternative
       behaviour in case of errors. If set to a reference to a subroutine then
       that subroutine is called when an error is detected (at the same point
       that "RaiseError" and "PrintError" are handled).

       The subroutine is called with three parameters: the error message
       string that "RaiseError" and "PrintError" would use, the DBI handle
       being used, and the first value being returned by the method that
       failed (typically undef).

       If the subroutine returns a false value then the "RaiseError" and/or
       "PrintError" attributes are checked and acted upon as normal.

       For example, to "die" with a full stack trace for any error:

         use Carp;
         $h->{HandleError} = sub { confess(shift) };

       Or to turn errors into exceptions:

         use Exception; # or your own favourite exception module
         $h->{HandleError} = sub { Exception->new('DBI')->raise($_[0]) };

       It is possible to 'stack' multiple HandleError handlers by using
       closures:

         sub your_subroutine {
           my $previous_handler = $h->{HandleError};
           $h->{HandleError} = sub {
             return 1 if $previous_handler and &$previous_handler(@_);
             ... your code here ...
           };
         }

       Using a "my" inside a subroutine to store the previous "HandleError"
       value is important.  See perlsub and perlref for more information about
       closures.

       It is possible for "HandleError" to alter the error message that will
       be used by "RaiseError" and "PrintError" if it returns false.  It can
       do that by altering the value of $_[0]. This example appends a stack
       trace to all errors and, unlike the previous example using
       Carp::confess, this will work "PrintError" as well as "RaiseError":

         $h->{HandleError} = sub { $_[0]=Carp::longmess($_[0]); 0; };

       It is also possible for "HandleError" to hide an error, to a limited
       degree, by using "set_err" to reset $DBI::err and $DBI::errstr, and
       altering the return value of the failed method. For example:

         $h->{HandleError} = sub {
           return 0 unless $_[0] =~ /^\S+ fetchrow_arrayref failed:/;
           return 0 unless $_[1]->err == 1234; # the error to 'hide'
           $h->set_err(undef,undef);   # turn off the error
           $_[2] = [ ... ];    # supply alternative return value
           return 1;
         };

       This only works for methods which return a single value and is hard to
       make reliable (avoiding infinite loops, for example) and so isn't
       recommended for general use!  If you find a good use for it then please
       let me know.

       "HandleSetErr" (code ref, inherited)

       The "HandleSetErr" attribute can be used to intercept the setting of
       handle "err", "errstr", and "state" values.  If set to a reference to a
       subroutine then that subroutine is called whenever set_err() is called,
       typically by the driver or a subclass.

       The subroutine is called with five arguments, the first five that were
       passed to set_err(): the handle, the "err", "errstr", and "state"
       values being set, and the method name. These can be altered by changing
       the values in the @_ array. The return value affects set_err()
       behaviour, see "set_err" for details.

       It is possible to 'stack' multiple HandleSetErr handlers by using
       closures. See "HandleError" for an example.

       The "HandleSetErr" and "HandleError" subroutines differ in subtle but
       significant ways. HandleError is only invoked at the point where the
       DBI is about to return to the application with "err" set true.  It's
       not invoked by the failure of a method that's been called by another
       DBI method.  HandleSetErr, on the other hand, is called whenever
       set_err() is called with a defined "err" value, even if false.  So it's
       not just for errors, despite the name, but also warn and info states.
       The set_err() method, and thus HandleSetErr, may be called multiple
       times within a method and is usually invoked from deep within driver
       code.

       In theory a driver can use the return value from HandleSetErr via
       set_err() to decide whether to continue or not. If set_err() returns an
       empty list, indicating that the HandleSetErr code has 'handled' the
       'error', the driver could then continue instead of failing (if that's a
       reasonable thing to do).  This isn't excepted to be common and any such
       cases should be clearly marked in the driver documentation and
       discussed on the dbi-dev mailing list.

       The "HandleSetErr" attribute was added in DBI 1.41.

       "ErrCount" (unsigned integer)

       The "ErrCount" attribute is incremented whenever the set_err() method
       records an error. It isn't incremented by warnings or information
       states. It is not reset by the DBI at any time.

       The "ErrCount" attribute was added in DBI 1.41. Older drivers may not
       have been updated to use set_err() to record errors and so this
       attribute may not be incremented when using them.

       "ShowErrorStatement" (boolean, inherited)

       The "ShowErrorStatement" attribute can be used to cause the relevant
       Statement text to be appended to the error messages generated by the
       "RaiseError", "PrintError", and "PrintWarn" attributes.  Only applies
       to errors on statement handles plus the prepare(), do(), and the
       various "select*()" database handle methods.  (The exact format of the
       appended text is subject to change.)

       If "$h->{ParamValues}" returns a hash reference of parameter
       (placeholder) values then those are formatted and appended to the end
       of the Statement text in the error message.

       "TraceLevel" (integer, inherited)

       The "TraceLevel" attribute can be used as an alternative to the "trace"
       method to set the DBI trace level and trace flags for a specific
       handle.  See "TRACING" for more details.

       The "TraceLevel" attribute is especially useful combined with "local"
       to alter the trace settings for just a single block of code.

       "FetchHashKeyName" (string, inherited)

       The "FetchHashKeyName" attribute is used to specify whether the
       fetchrow_hashref() method should perform case conversion on the field
       names used for the hash keys. For historical reasons it defaults to
       '"NAME"' but it is recommended to set it to '"NAME_lc"' (convert to
       lower case) or '"NAME_uc"' (convert to upper case) according to your
       preference.  It can only be set for driver and database handles.  For
       statement handles the value is frozen when prepare() is called.

       "ChopBlanks" (boolean, inherited)

       The "ChopBlanks" attribute can be used to control the trimming of
       trailing space characters from fixed width character (CHAR) fields. No
       other field types are affected, even where field values have trailing
       spaces.

       The default is false (although it is possible that the default may
       change).  Applications that need specific behaviour should set the
       attribute as needed.

       Drivers are not required to support this attribute, but any driver
       which does not support it must arrange to return "undef" as the
       attribute value.

       "LongReadLen" (unsigned integer, inherited)

       The "LongReadLen" attribute may be used to control the maximum length
       of 'long' type fields (LONG, BLOB, CLOB, MEMO, etc.) which the driver
       will read from the database automatically when it fetches each row of
       data.

       The "LongReadLen" attribute only relates to fetching and reading long
       values; it is not involved in inserting or updating them.

       A value of 0 means not to automatically fetch any long data.  Drivers
       may return undef or an empty string for long fields when "LongReadLen"
       is 0.

       The default is typically 0 (zero) or 80 bytes but may vary between
       drivers.  Applications fetching long fields should set this value to
       slightly larger than the longest long field value to be fetched.

       Some databases return some long types encoded as pairs of hex digits.
       For these types, "LongReadLen" relates to the underlying data length
       and not the doubled-up length of the encoded string.

       Changing the value of "LongReadLen" for a statement handle after it has
       been "prepare"'d will typically have no effect, so it's common to set
       "LongReadLen" on the $dbh before calling "prepare".

       For most drivers the value used here has a direct effect on the memory
       used by the statement handle while it's active, so don't be too
       generous. If you can't be sure what value to use you could execute an
       extra select statement to determine the longest value.  For example:

         $dbh->{LongReadLen} = $dbh->selectrow_array(qq{
             SELECT MAX(OCTET_LENGTH(long_column_name))
             FROM table WHERE ...
         });
         $sth = $dbh->prepare(qq{
             SELECT long_column_name, ... FROM table WHERE ...
         });

       You may need to take extra care if the table can be modified between
       the first select and the second being executed. You may also need to
       use a different function if OCTET_LENGTH() does not work for long types
       in your database. For example, for Sybase use DATALENGTH() and for
       Oracle use LENGTHB().

       See also "LongTruncOk" for information on truncation of long types.

       "LongTruncOk" (boolean, inherited)

       The "LongTruncOk" attribute may be used to control the effect of
       fetching a long field value which has been truncated (typically because
       it's longer than the value of the "LongReadLen" attribute).

       By default, "LongTruncOk" is false and so fetching a long value that
       needs to be truncated will cause the fetch to fail.  (Applications
       should always be sure to check for errors after a fetch loop in case an
       error, such as a divide by zero or long field truncation, caused the
       fetch to terminate prematurely.)

       If a fetch fails due to a long field truncation when "LongTruncOk" is
       false, many drivers will allow you to continue fetching further rows.

       See also "LongReadLen".

       "TaintIn" (boolean, inherited)

       If the "TaintIn" attribute is set to a true value and Perl is running
       in taint mode (e.g., started with the "-T" option), then all the
       arguments to most DBI method calls are checked for being tainted. This
       may change.

       The attribute defaults to off, even if Perl is in taint mode.  See
       perlsec for more about taint mode.  If Perl is not running in taint
       mode, this attribute has no effect.

       When fetching data that you trust you can turn off the TaintIn
       attribute, for that statement handle, for the duration of the fetch
       loop.

       The "TaintIn" attribute was added in DBI 1.31.

       "TaintOut" (boolean, inherited)

       If the "TaintOut" attribute is set to a true value and Perl is running
       in taint mode (e.g., started with the "-T" option), then most data
       fetched from the database is considered tainted. This may change.

       The attribute defaults to off, even if Perl is in taint mode.  See
       perlsec for more about taint mode.  If Perl is not running in taint
       mode, this attribute has no effect.

       When fetching data that you trust you can turn off the TaintOut
       attribute, for that statement handle, for the duration of the fetch
       loop.

       Currently only fetched data is tainted. It is possible that the results
       of other DBI method calls, and the value of fetched attributes, may
       also be tainted in future versions. That change may well break your
       applications unless you take great care now. If you use DBI Taint mode,
       please report your experience and any suggestions for changes.

       The "TaintOut" attribute was added in DBI 1.31.

       "Taint" (boolean, inherited)

       The "Taint" attribute is a shortcut for "TaintIn" and "TaintOut" (it is
       also present for backwards compatibility).

       Setting this attribute sets both "TaintIn" and "TaintOut", and
       retrieving it returns a true value if and only if "TaintIn" and
       "TaintOut" are both set to true values.

       "Profile" (inherited)

       The "Profile" attribute enables the collection and reporting of method
       call timing statistics.  See the DBI::Profile module documentation for
       much more detail.

       The "Profile" attribute was added in DBI 1.24.

       "ReadOnly" (boolean, inherited)

       An application can set the "ReadOnly" attribute of a handle to a true
       value to indicate that it will not be attempting to make any changes
       using that handle or any children of it.

       Note that the exact definition of 'read only' is rather fuzzy.  For
       more details see the documentation for the driver you're using.

       If the driver can make the handle truly read-only then it should
       (unless doing so would have unpleasant side effect, like changing the
       consistency level from per-statement to per-session).  Otherwise the
       attribute is simply advisory.

       A driver can set the "ReadOnly" attribute itself to indicate that the
       data it is connected to cannot be changed for some reason.

       Library modules and proxy drivers can use the attribute to influence
       their behavior.  For example, the DBD::Gofer driver considers the
       "ReadOnly" attribute when making a decision about whether to retry an
       operation that failed.

       The attribute should be set to 1 or 0 (or undef). Other values are
       reserved.

       "private_your_module_name_*"

       The DBI provides a way to store extra information in a DBI handle as
       "private" attributes. The DBI will allow you to store and retrieve any
       attribute which has a name starting with ""private_"".

       It is strongly recommended that you use just one private attribute
       (e.g., use a hash ref) and give it a long and unambiguous name that
       includes the module or application name that the attribute relates to
       (e.g., ""private_YourFullModuleName_thingy"").

       Because of the way the Perl tie mechanism works you cannot reliably use
       the "||=" operator directly to initialise the attribute, like this:

         my $foo = $dbh->{private_yourmodname_foo} ||= { ... }; # WRONG

       you should use a two step approach like this:

         my $foo = $dbh->{private_yourmodname_foo};
         $foo ||= $dbh->{private_yourmodname_foo} = { ... };

       This attribute is primarily of interest to people sub-classing DBI, or
       for applications to piggy-back extra information onto DBI handles.

DBI DATABASE HANDLE OBJECTS
       This section covers the methods and attributes associated with database
       handles.

   Database Handle Methods
       The following methods are specified for DBI database handles:

       "clone"

         $new_dbh = $dbh->clone();
         $new_dbh = $dbh->clone(\%attr);

       The "clone" method duplicates the $dbh connection by connecting with
       the same parameters ($dsn, $user, $password) as originally used.

       The attributes for the cloned connect are the same as those used for
       the original connect, with some other attribute merged over them
       depending on the \%attr parameter.

       If \%attr is given then the attributes it contains are merged into the
       original attributes and override any with the same names.  Effectively
       the same as doing:

         %attribues_used = ( %original_attributes, %attr );

       If \%attr is not given then it defaults to a hash containing all the
       attributes in the attribute cache of $dbh excluding any non-code
       references, plus the main boolean attributes (RaiseError, PrintError,
       AutoCommit, etc.). This behaviour is subject to change.

       The clone method can be used even if the database handle is
       disconnected.

       The "clone" method was added in DBI 1.33. It is very new and likely to
       change.

       "data_sources"

         @ary = $dbh->data_sources();
         @ary = $dbh->data_sources(\%attr);

       Returns a list of data sources (databases) available via the $dbh
       driver's data_sources() method, plus any extra data sources that the
       driver can discover via the connected $dbh. Typically the extra data
       sources are other databases managed by the same server process that the
       $dbh is connected to.

       Data sources are returned in a form suitable for passing to the
       "connect" method (that is, they will include the ""dbi:$driver:""
       prefix).

       The data_sources() method, for a $dbh, was added in DBI 1.38.

       "do"

         $rows = $dbh->do($statement)           or die $dbh->errstr;
         $rows = $dbh->do($statement, \%attr)   or die $dbh->errstr;
         $rows = $dbh->do($statement, \%attr, @bind_values) or die ...

       Prepare and execute a single statement. Returns the number of rows
       affected or "undef" on error. A return value of "-1" means the number
       of rows is not known, not applicable, or not available.

       This method is typically most useful for non-"SELECT" statements that
       either cannot be prepared in advance (due to a limitation of the
       driver) or do not need to be executed repeatedly. It should not be used
       for "SELECT" statements because it does not return a statement handle
       (so you can't fetch any data).

       The default "do" method is logically similar to:

         sub do {
             my($dbh, $statement, $attr, @bind_values) = @_;
             my $sth = $dbh->prepare($statement, $attr) or return undef;
             $sth->execute(@bind_values) or return undef;
             my $rows = $sth->rows;
             ($rows == 0) ? "0E0" : $rows; # always return true if no error
         }

       For example:

         my $rows_deleted = $dbh->do(q{
             DELETE FROM table
             WHERE status = ?
         }, undef, 'DONE') or die $dbh->errstr;

       Using placeholders and @bind_values with the "do" method can be useful
       because it avoids the need to correctly quote any variables in the
       $statement. But if you'll be executing the statement many times then
       it's more efficient to "prepare" it once and call "execute" many times
       instead.

       The "q{...}" style quoting used in this example avoids clashing with
       quotes that may be used in the SQL statement. Use the double-quote-like
       "qq{...}" operator if you want to interpolate variables into the
       string.  See "Quote and Quote-like Operators" in perlop for more
       details.

       "last_insert_id"

         $rv = $dbh->last_insert_id($catalog, $schema, $table, $field);
         $rv = $dbh->last_insert_id($catalog, $schema, $table, $field, \%attr);

       Returns a value 'identifying' the row just inserted, if possible.
       Typically this would be a value assigned by the database server to a
       column with an auto_increment or serial type.  Returns undef if the
       driver does not support the method or can't determine the value.

       The $catalog, $schema, $table, and $field parameters may be required
       for some drivers (see below).  If you don't know the parameter values
       and your driver does not need them, then use "undef" for each.

       There are several caveats to be aware of with this method if you want
       to use it for portable applications:

       * For some drivers the value may only available immediately after the
       insert statement has executed (e.g., mysql, Informix).

       * For some drivers the $catalog, $schema, $table, and $field parameters
       are required, for others they are ignored (e.g., mysql).

       * Drivers may return an indeterminate value if no insert has been
       performed yet.

       * For some drivers the value may only be available if placeholders have
       not been used (e.g., Sybase, MS SQL). In this case the value returned
       would be from the last non-placeholder insert statement.

       * Some drivers may need driver-specific hints about how to get the
       value. For example, being told the name of the database 'sequence'
       object that holds the value. Any such hints are passed as driver-
       specific attributes in the \%attr parameter.

       * If the underlying database offers nothing better, then some drivers
       may attempt to implement this method by executing ""select max($field)
       from $table"". Drivers using any approach like this should issue a
       warning if "AutoCommit" is true because it is generally unsafe -
       another process may have modified the table between your insert and the
       select. For situations where you know it is safe, such as when you have
       locked the table, you can silence the warning by passing "Warn" => 0 in
       \%attr.

       * If no insert has been performed yet, or the last insert failed, then
       the value is implementation defined.

       Given all the caveats above, it's clear that this method must be used
       with care.

       The "last_insert_id" method was added in DBI 1.38.

       "selectrow_array"

         @row_ary = $dbh->selectrow_array($statement);
         @row_ary = $dbh->selectrow_array($statement, \%attr);
         @row_ary = $dbh->selectrow_array($statement, \%attr, @bind_values);

       This utility method combines "prepare", "execute" and "fetchrow_array"
       into a single call. If called in a list context, it returns the first
       row of data from the statement.  The $statement parameter can be a
       previously prepared statement handle, in which case the "prepare" is
       skipped.

       If any method fails, and "RaiseError" is not set, "selectrow_array"
       will return an empty list.

       If called in a scalar context for a statement handle that has more than
       one column, it is undefined whether the driver will return the value of
       the first column or the last. So don't do that.  Also, in a scalar
       context, an "undef" is returned if there are no more rows or if an
       error occurred. That "undef" can't be distinguished from an "undef"
       returned because the first field value was NULL.  For these reasons you
       should exercise some caution if you use "selectrow_array" in a scalar
       context, or just don't do that.

       "selectrow_arrayref"

         $ary_ref = $dbh->selectrow_arrayref($statement);
         $ary_ref = $dbh->selectrow_arrayref($statement, \%attr);
         $ary_ref = $dbh->selectrow_arrayref($statement, \%attr, @bind_values);

       This utility method combines "prepare", "execute" and
       "fetchrow_arrayref" into a single call. It returns the first row of
       data from the statement.  The $statement parameter can be a previously
       prepared statement handle, in which case the "prepare" is skipped.

       If any method fails, and "RaiseError" is not set, "selectrow_array"
       will return undef.

       "selectrow_hashref"

         $hash_ref = $dbh->selectrow_hashref($statement);
         $hash_ref = $dbh->selectrow_hashref($statement, \%attr);
         $hash_ref = $dbh->selectrow_hashref($statement, \%attr, @bind_values);

       This utility method combines "prepare", "execute" and
       "fetchrow_hashref" into a single call. It returns the first row of data
       from the statement.  The $statement parameter can be a previously
       prepared statement handle, in which case the "prepare" is skipped.

       If any method fails, and "RaiseError" is not set, "selectrow_hashref"
       will return undef.

       "selectall_arrayref"

         $ary_ref = $dbh->selectall_arrayref($statement);
         $ary_ref = $dbh->selectall_arrayref($statement, \%attr);
         $ary_ref = $dbh->selectall_arrayref($statement, \%attr, @bind_values);

       This utility method combines "prepare", "execute" and
       "fetchall_arrayref" into a single call. It returns a reference to an
       array containing a reference to an array (or hash, see below) for each
       row of data fetched.

       The $statement parameter can be a previously prepared statement handle,
       in which case the "prepare" is skipped. This is recommended if the
       statement is going to be executed many times.

       If "RaiseError" is not set and any method except "fetchall_arrayref"
       fails then "selectall_arrayref" will return "undef"; if
       "fetchall_arrayref" fails then it will return with whatever data has
       been fetched thus far. You should check "$sth->err" afterwards (or use
       the "RaiseError" attribute) to discover if the data is complete or was
       truncated due to an error.

       The "fetchall_arrayref" method called by "selectall_arrayref" supports
       a $max_rows parameter. You can specify a value for $max_rows by
       including a '"MaxRows"' attribute in \%attr. In which case finish() is
       called for you after fetchall_arrayref() returns.

       The "fetchall_arrayref" method called by "selectall_arrayref" also
       supports a $slice parameter. You can specify a value for $slice by
       including a '"Slice"' or '"Columns"' attribute in \%attr. The only
       difference between the two is that if "Slice" is not defined and
       "Columns" is an array ref, then the array is assumed to contain column
       index values (which count from 1), rather than perl array index values.
       In which case the array is copied and each value decremented before
       passing to "/fetchall_arrayref".

       You may often want to fetch an array of rows where each row is stored
       as a hash. That can be done simple using:

         my $emps = $dbh->selectall_arrayref(
             "SELECT ename FROM emp ORDER BY ename",
             { Slice => {} }
         );
         foreach my $emp ( @$emps ) {
             print "Employee: $emp->{ename}\n";
         }

       Or, to fetch into an array instead of an array ref:

         @result = @{ $dbh->selectall_arrayref($sql, { Slice => {} }) };

       See "fetchall_arrayref" method for more details.

       "selectall_hashref"

         $hash_ref = $dbh->selectall_hashref($statement, $key_field);
         $hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr);
         $hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr, @bind_values);

       This utility method combines "prepare", "execute" and
       "fetchall_hashref" into a single call. It returns a reference to a hash
       containing one entry, at most, for each row, as returned by
       fetchall_hashref().

       The $statement parameter can be a previously prepared statement handle,
       in which case the "prepare" is skipped.  This is recommended if the
       statement is going to be executed many times.

       The $key_field parameter defines which column, or columns, are used as
       keys in the returned hash. It can either be the name of a single field,
       or a reference to an array containing multiple field names. Using
       multiple names yields a tree of nested hashes.

       If a row has the same key as an earlier row then it replaces the
       earlier row.

       If any method except "fetchrow_hashref" fails, and "RaiseError" is not
       set, "selectall_hashref" will return "undef".  If "fetchrow_hashref"
       fails and "RaiseError" is not set, then it will return with whatever
       data it has fetched thus far. $DBI::err should be checked to catch
       that.

       See fetchall_hashref() for more details.

       "selectcol_arrayref"

         $ary_ref = $dbh->selectcol_arrayref($statement);
         $ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
         $ary_ref = $dbh->selectcol_arrayref($statement, \%attr, @bind_values);

       This utility method combines "prepare", "execute", and fetching one
       column from all the rows, into a single call. It returns a reference to
       an array containing the values of the first column from each row.

       The $statement parameter can be a previously prepared statement handle,
       in which case the "prepare" is skipped. This is recommended if the
       statement is going to be executed many times.

       If any method except "fetch" fails, and "RaiseError" is not set,
       "selectcol_arrayref" will return "undef".  If "fetch" fails and
       "RaiseError" is not set, then it will return with whatever data it has
       fetched thus far. $DBI::err should be checked to catch that.

       The "selectcol_arrayref" method defaults to pushing a single column
       value (the first) from each row into the result array. However, it can
       also push another column, or even multiple columns per row, into the
       result array. This behaviour can be specified via a '"Columns"'
       attribute which must be a ref to an array containing the column number
       or numbers to use. For example:

         # get array of id and name pairs:
         my $ary_ref = $dbh->selectcol_arrayref("select id, name from table", { Columns=>[1,2] });
         my %hash = @$ary_ref; # build hash from key-value pairs so $hash{$id} => name

       You can specify a maximum number of rows to fetch by including a
       '"MaxRows"' attribute in \%attr.

       "prepare"

         $sth = $dbh->prepare($statement)          or die $dbh->errstr;
         $sth = $dbh->prepare($statement, \%attr)  or die $dbh->errstr;

       Prepares a statement for later execution by the database engine and
       returns a reference to a statement handle object.

       The returned statement handle can be used to get attributes of the
       statement and invoke the "execute" method. See "Statement Handle
       Methods".

       Drivers for engines without the concept of preparing a statement will
       typically just store the statement in the returned handle and process
       it when "$sth->execute" is called. Such drivers are unlikely to give
       much useful information about the statement, such as
       "$sth->{NUM_OF_FIELDS}", until after "$sth->execute" has been called.
       Portable applications should take this into account.

       In general, DBI drivers do not parse the contents of the statement
       (other than simply counting any "Placeholders"). The statement is
       passed directly to the database engine, sometimes known as pass-thru
       mode. This has advantages and disadvantages. On the plus side, you can
       access all the functionality of the engine being used. On the downside,
       you're limited if you're using a simple engine, and you need to take
       extra care if writing applications intended to be portable between
       engines.

       Portable applications should not assume that a new statement can be
       prepared and/or executed while still fetching results from a previous
       statement.

       Some command-line SQL tools use statement terminators, like a
       semicolon, to indicate the end of a statement. Such terminators should
       not normally be used with the DBI.

       "prepare_cached"

         $sth = $dbh->prepare_cached($statement)
         $sth = $dbh->prepare_cached($statement, \%attr)
         $sth = $dbh->prepare_cached($statement, \%attr, $if_active)

       Like "prepare" except that the statement handle returned will be stored
       in a hash associated with the $dbh. If another call is made to
       "prepare_cached" with the same $statement and %attr parameter values,
       then the corresponding cached $sth will be returned without contacting
       the database server.

       The $if_active parameter lets you adjust the behaviour if an already
       cached statement handle is still Active.  There are several
       alternatives:

       0: A warning will be generated, and finish() will be called on the
       statement handle before it is returned.  This is the default behaviour
       if $if_active is not passed.
       1: finish() will be called on the statement handle, but the warning is
       suppressed.
       2: Disables any checking.
       3: The existing active statement handle will be removed from the cache
       and a new statement handle prepared and cached in its place. This is
       the safest option because it doesn't affect the state of the old
       handle, it just removes it from the cache. [Added in DBI 1.40]

       Here are some examples of "prepare_cached":

         sub insert_hash {
           my ($table, $field_values) = @_;
           # sort to keep field order, and thus sql, stable for prepare_cached
           my @fields = sort keys %$field_values;
           my @values = @{$field_values}{@fields};
           my $sql = sprintf "insert into %s (%s) values (%s)",
               $table, join(",", @fields), join(",", ("?")x@fields);
           my $sth = $dbh->prepare_cached($sql);
           return $sth->execute(@values);
         }

         sub search_hash {
           my ($table, $field_values) = @_;
           # sort to keep field order, and thus sql, stable for prepare_cached
           my @fields = sort keys %$field_values;
           my @values = @{$field_values}{@fields};
           my $qualifier = "";
           $qualifier = "where ".join(" and ", map { "$_=?" } @fields) if @fields;
           $sth = $dbh->prepare_cached("SELECT * FROM $table $qualifier");
           return $dbh->selectall_arrayref($sth, {}, @values);
         }

       Caveat emptor: This caching can be useful in some applications, but it
       can also cause problems and should be used with care. Here is a
       contrived case where caching would cause a significant problem:

         my $sth = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
         $sth->execute(...);
         while (my $data = $sth->fetchrow_hashref) {

           # later, in some other code called within the loop...
           my $sth2 = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
           $sth2->execute(...);
           while (my $data2 = $sth2->fetchrow_arrayref) {
             do_stuff(...);
           }
         }

       In this example, since both handles are preparing the exact same
       statement, $sth2 will not be its own statement handle, but a duplicate
       of $sth returned from the cache. The results will certainly not be what
       you expect.  Typically the the inner fetch loop will work normally,
       fetching all the records and terminating when there are no more, but
       now $sth is the same as $sth2 the outer fetch loop will also terminate.

       You'll know if you run into this problem because prepare_cached() will
       generate a warning by default (when $if_active is false).

       The cache used by prepare_cached() is keyed by both the statement and
       any attributes so you can also avoid this issue by doing something
       like:

         $sth = $dbh->prepare_cached("...", { dbi_dummy => __FILE__.__LINE__ });

       which will ensure that prepare_cached only returns statements cached by
       that line of code in that source file.

       If you'd like the cache to managed intelligently, you can tie the
       hashref returned by "CachedKids" to an appropriate caching module, such
       as Tie::Cache::LRU:

         my $cache;
         tie %$cache, 'Tie::Cache::LRU', 500;
         $dbh->{CachedKids} = $cache;

       "commit"

         $rc  = $dbh->commit     or die $dbh->errstr;

       Commit (make permanent) the most recent series of database changes if
       the database supports transactions and AutoCommit is off.

       If "AutoCommit" is on, then calling "commit" will issue a "commit
       ineffective with AutoCommit" warning.

       See also "Transactions" in the "FURTHER INFORMATION" section below.

       "rollback"

         $rc  = $dbh->rollback   or die $dbh->errstr;

       Rollback (undo) the most recent series of uncommitted database changes
       if the database supports transactions and AutoCommit is off.

       If "AutoCommit" is on, then calling "rollback" will issue a "rollback
       ineffective with AutoCommit" warning.

       See also "Transactions" in the "FURTHER INFORMATION" section below.

       "begin_work"

         $rc  = $dbh->begin_work   or die $dbh->errstr;

       Enable transactions (by turning "AutoCommit" off) until the next call
       to "commit" or "rollback". After the next "commit" or "rollback",
       "AutoCommit" will automatically be turned on again.

       If "AutoCommit" is already off when "begin_work" is called then it does
       nothing except return an error. If the driver does not support
       transactions then when "begin_work" attempts to set "AutoCommit" off
       the driver will trigger a fatal error.

       See also "Transactions" in the "FURTHER INFORMATION" section below.

       "disconnect"

         $rc = $dbh->disconnect  or warn $dbh->errstr;

       Disconnects the database from the database handle. "disconnect" is
       typically only used before exiting the program. The handle is of little
       use after disconnecting.

       The transaction behaviour of the "disconnect" method is, sadly,
       undefined.  Some database systems (such as Oracle and Ingres) will
       automatically commit any outstanding changes, but others (such as
       Informix) will rollback any outstanding changes.  Applications not
       using "AutoCommit" should explicitly call "commit" or "rollback" before
       calling "disconnect".

       The database is automatically disconnected by the "DESTROY" method if
       still connected when there are no longer any references to the handle.
       The "DESTROY" method for each driver should implicitly call "rollback"
       to undo any uncommitted changes. This is vital behaviour to ensure that
       incomplete transactions don't get committed simply because Perl calls
       "DESTROY" on every object before exiting. Also, do not rely on the
       order of object destruction during "global destruction", as it is
       undefined.

       Generally, if you want your changes to be committed or rolled back when
       you disconnect, then you should explicitly call "commit" or "rollback"
       before disconnecting.

       If you disconnect from a database while you still have active statement
       handles (e.g., SELECT statement handles that may have more data to
       fetch), you will get a warning. The warning may indicate that a fetch
       loop terminated early, perhaps due to an uncaught error.  To avoid the
       warning call the "finish" method on the active handles.

       "ping"

         $rc = $dbh->ping;

       Attempts to determine, in a reasonably efficient way, if the database
       server is still running and the connection to it is still working.
       Individual drivers should implement this function in the most suitable
       manner for their database engine.

       The current default implementation always returns true without actually
       doing anything. Actually, it returns ""0 but true"" which is true but
       zero. That way you can tell if the return value is genuine or just the
       default. Drivers should override this method with one that does the
       right thing for their type of database.

       Few applications would have direct use for this method. See the
       specialized Apache::DBI module for one example usage.

       "get_info"

         $value = $dbh->get_info( $info_type );

       Returns information about the implementation, i.e. driver and data
       source capabilities, restrictions etc. It returns "undef" for unknown
       or unimplemented information types. For example:

         $database_version  = $dbh->get_info(  18 ); # SQL_DBMS_VER
         $max_select_tables = $dbh->get_info( 106 ); # SQL_MAXIMUM_TABLES_IN_SELECT

       See "Standards Reference Information" for more detailed information
       about the information types and their meanings and possible return
       values.

       The DBI::Const::GetInfoType module exports a %GetInfoType hash that can
       be used to map info type names to numbers. For example:

         $database_version = $dbh->get_info( $GetInfoType{SQL_DBMS_VER} );

       The names are a merging of the ANSI and ODBC standards (which differ in
       some cases). See DBI::Const::GetInfoType for more details.

       Because some DBI methods make use of get_info(), drivers are strongly
       encouraged to support at least the following very minimal set of
       information types to ensure the DBI itself works properly:

        Type  Name                        Example A     Example B
        ----  --------------------------  ------------  ----------------
          17  SQL_DBMS_NAME               'ACCESS'      'Oracle'
          18  SQL_DBMS_VER                '03.50.0000'  '08.01.0721 ...'
          29  SQL_IDENTIFIER_QUOTE_CHAR   '`'           '"'
          41  SQL_CATALOG_NAME_SEPARATOR  '.'           '@'
         114  SQL_CATALOG_LOCATION        1             2

       "table_info"

         $sth = $dbh->table_info( $catalog, $schema, $table, $type );
         $sth = $dbh->table_info( $catalog, $schema, $table, $type, \%attr );

         # then $sth->fetchall_arrayref or $sth->fetchall_hashref etc

       Returns an active statement handle that can be used to fetch
       information about tables and views that exist in the database.

       The arguments $catalog, $schema and $table may accept search patterns
       according to the database/driver, for example: $table = '%FOO%';
       Remember that the underscore character ('"_"') is a search pattern that
       means match any character, so 'FOO_%' is the same as 'FOO%' and
       'FOO_BAR%' will match names like 'FOO1BAR'.

       The value of $type is a comma-separated list of one or more types of
       tables to be returned in the result set. Each value may optionally be
       quoted, e.g.:

         $type = "TABLE";
         $type = "'TABLE','VIEW'";

       In addition the following special cases may also be supported by some
       drivers:

       o   If the value of $catalog is '%' and $schema and $table name are
           empty strings, the result set contains a list of catalog names.
           For example:

             $sth = $dbh->table_info('%', '', '');

       o   If the value of $schema is '%' and $catalog and $table are empty
           strings, the result set contains a list of schema names.

       o   If the value of $type is '%' and $catalog, $schema, and $table are
           all empty strings, the result set contains a list of table types.

       If your driver doesn't support one or more of the selection filter
       parameters then you may get back more than you asked for and can do the
       filtering yourself.

       This method can be expensive, and can return a large amount of data.
       (For example, small Oracle installation returns over 2000 rows.)  So
       it's a good idea to use the filters to limit the data as much as
       possible.

       The statement handle returned has at least the following fields in the
       order show below. Other fields, after these, may also be present.

       TABLE_CAT: Table catalog identifier. This field is NULL ("undef") if
       not applicable to the data source, which is usually the case. This
       field is empty if not applicable to the table.

       TABLE_SCHEM: The name of the schema containing the TABLE_NAME value.
       This field is NULL ("undef") if not applicable to data source, and
       empty if not applicable to the table.

       TABLE_NAME: Name of the table (or view, synonym, etc).

       TABLE_TYPE: One of the following: "TABLE", "VIEW", "SYSTEM TABLE",
       "GLOBAL TEMPORARY", "LOCAL TEMPORARY", "ALIAS", "SYNONYM" or a type
       identifier that is specific to the data source.

       REMARKS: A description of the table. May be NULL ("undef").

       Note that "table_info" might not return records for all tables.
       Applications can use any valid table regardless of whether it's
       returned by "table_info".

       See also "tables", "Catalog Methods" and "Standards Reference
       Information".

       "column_info"

         $sth = $dbh->column_info( $catalog, $schema, $table, $column );

         # then $sth->fetchall_arrayref or $sth->fetchall_hashref etc

       Returns an active statement handle that can be used to fetch
       information about columns in specified tables.

       The arguments $schema, $table and $column may accept search patterns
       according to the database/driver, for example: $table = '%FOO%';

       Note: The support for the selection criteria is driver specific. If the
       driver doesn't support one or more of them then you may get back more
       than you asked for and can do the filtering yourself.

       If the arguments don't match any tables then you'll still get a
       statement handle, it'll just return no rows.

       The statement handle returned has at least the following fields in the
       order shown below. Other fields, after these, may also be present.

       TABLE_CAT: The catalog identifier.  This field is NULL ("undef") if not
       applicable to the data source, which is often the case.  This field is
       empty if not applicable to the table.

       TABLE_SCHEM: The schema identifier.  This field is NULL ("undef") if
       not applicable to the data source, and empty if not applicable to the
       table.

       TABLE_NAME: The table identifier.  Note: A driver may provide column
       metadata not only for base tables, but also for derived objects like
       SYNONYMS etc.

       COLUMN_NAME: The column identifier.

       DATA_TYPE: The concise data type code.

       TYPE_NAME: A data source dependent data type name.

       COLUMN_SIZE: The column size.  This is the maximum length in characters
       for character data types, the number of digits or bits for numeric data
       types or the length in the representation of temporal types.  See the
       relevant specifications for detailed information.

       BUFFER_LENGTH: The length in bytes of transferred data.

       DECIMAL_DIGITS: The total number of significant digits to the right of
       the decimal point.

       NUM_PREC_RADIX: The radix for numeric precision.  The value is 10 or 2
       for numeric data types and NULL ("undef") if not applicable.

       NULLABLE: Indicates if a column can accept NULLs.  The following values
       are defined:

         SQL_NO_NULLS          0
         SQL_NULLABLE          1
         SQL_NULLABLE_UNKNOWN  2

       REMARKS: A description of the column.

       COLUMN_DEF: The default value of the column, in a format that can be
       used directly in an SQL statement.

       Note that this may be an expression and not simply the text used for
       the default value in the original CREATE TABLE statement. For example,
       given:

           col1 char(30) default current_user    -- a 'function'
           col2 char(30) default 'string'        -- a string literal

       where "current_user" is the name of a function, the corresponding
       "COLUMN_DEF" values would be:

           Database        col1                     col2
           --------        ----                     ----
           Oracle:         current_user             'string'
           Postgres:       "current_user"()         'string'::text
           MS SQL:         (user_name())            ('string')

       SQL_DATA_TYPE: The SQL data type.

       SQL_DATETIME_SUB: The subtype code for datetime and interval data
       types.

       CHAR_OCTET_LENGTH: The maximum length in bytes of a character or binary
       data type column.

       ORDINAL_POSITION: The column sequence number (starting with 1).

       IS_NULLABLE: Indicates if the column can accept NULLs.  Possible values
       are: 'NO', 'YES' and ''.

       SQL/CLI defines the following additional columns:

         CHAR_SET_CAT
         CHAR_SET_SCHEM
         CHAR_SET_NAME
         COLLATION_CAT
         COLLATION_SCHEM
         COLLATION_NAME
         UDT_CAT
         UDT_SCHEM
         UDT_NAME
         DOMAIN_CAT
         DOMAIN_SCHEM
         DOMAIN_NAME
         SCOPE_CAT
         SCOPE_SCHEM
         SCOPE_NAME
         MAX_CARDINALITY
         DTD_IDENTIFIER
         IS_SELF_REF

       Drivers capable of supplying any of those values should do so in the
       corresponding column and supply undef values for the others.

       Drivers wishing to provide extra database/driver specific information
       should do so in extra columns beyond all those listed above, and use
       lowercase field names with the driver-specific prefix (i.e.,
       'ora_...'). Applications accessing such fields should do so by name and
       not by column number.

       The result set is ordered by TABLE_CAT, TABLE_SCHEM, TABLE_NAME and
       ORDINAL_POSITION.

       Note: There is some overlap with statement handle attributes (in perl)
       and SQLDescribeCol (in ODBC). However, SQLColumns provides more
       metadata.

       See also "Catalog Methods" and "Standards Reference Information".

       "primary_key_info"

         $sth = $dbh->primary_key_info( $catalog, $schema, $table );

         # then $sth->fetchall_arrayref or $sth->fetchall_hashref etc

       Returns an active statement handle that can be used to fetch
       information about columns that make up the primary key for a table.
       The arguments don't accept search patterns (unlike table_info()).

       The statement handle will return one row per column, ordered by
       TABLE_CAT, TABLE_SCHEM, TABLE_NAME, and KEY_SEQ.  If there is no
       primary key then the statement handle will fetch no rows.

       Note: The support for the selection criteria, such as $catalog, is
       driver specific.  If the driver doesn't support catalogs and/or
       schemas, it may ignore these criteria.

       The statement handle returned has at least the following fields in the
       order shown below. Other fields, after these, may also be present.

       TABLE_CAT: The catalog identifier.  This field is NULL ("undef") if not
       applicable to the data source, which is often the case.  This field is
       empty if not applicable to the table.

       TABLE_SCHEM: The schema identifier.  This field is NULL ("undef") if
       not applicable to the data source, and empty if not applicable to the
       table.

       TABLE_NAME: The table identifier.

       COLUMN_NAME: The column identifier.

       KEY_SEQ: The column sequence number (starting with 1).  Note: This
       field is named ORDINAL_POSITION in SQL/CLI.

       PK_NAME: The primary key constraint identifier.  This field is NULL
       ("undef") if not applicable to the data source.

       See also "Catalog Methods" and "Standards Reference Information".

       "primary_key"

         @key_column_names = $dbh->primary_key( $catalog, $schema, $table );

       Simple interface to the primary_key_info() method. Returns a list of
       the column names that comprise the primary key of the specified table.
       The list is in primary key column sequence order.  If there is no
       primary key then an empty list is returned.

       "foreign_key_info"

         $sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
                                      , $fk_catalog, $fk_schema, $fk_table );

         $sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
                                      , $fk_catalog, $fk_schema, $fk_table
                                      , \%attr );

         # then $sth->fetchall_arrayref or $sth->fetchall_hashref etc

       Returns an active statement handle that can be used to fetch
       information about foreign keys in and/or referencing the specified
       table(s).  The arguments don't accept search patterns (unlike
       table_info()).

       $pk_catalog, $pk_schema, $pk_table identify the primary (unique) key
       table (PKT).

       $fk_catalog, $fk_schema, $fk_table identify the foreign key table
       (FKT).

       If both PKT and FKT are given, the function returns the foreign key, if
       any, in table FKT that refers to the primary (unique) key of table PKT.
       (Note: In SQL/CLI, the result is implementation-defined.)

       If only PKT is given, then the result set contains the primary key of
       that table and all foreign keys that refer to it.

       If only FKT is given, then the result set contains all foreign keys in
       that table and the primary keys to which they refer.  (Note: In
       SQL/CLI, the result includes unique keys too.)

       For example:

         $sth = $dbh->foreign_key_info( undef, $user, 'master');
         $sth = $dbh->foreign_key_info( undef, undef,   undef , undef, $user, 'detail');
         $sth = $dbh->foreign_key_info( undef, $user, 'master', undef, $user, 'detail');

         # then $sth->fetchall_arrayref or $sth->fetchall_hashref etc

       Note: The support for the selection criteria, such as $catalog, is
       driver specific.  If the driver doesn't support catalogs and/or
       schemas, it may ignore these criteria.

       The statement handle returned has the following fields in the order
       shown below.  Because ODBC never includes unique keys, they define
       different columns in the result set than SQL/CLI. SQL/CLI column names
       are shown in parentheses.

       PKTABLE_CAT    ( UK_TABLE_CAT      ): The primary (unique) key table
       catalog identifier.  This field is NULL ("undef") if not applicable to
       the data source, which is often the case.  This field is empty if not
       applicable to the table.

       PKTABLE_SCHEM  ( UK_TABLE_SCHEM    ): The primary (unique) key table
       schema identifier.  This field is NULL ("undef") if not applicable to
       the data source, and empty if not applicable to the table.

       PKTABLE_NAME   ( UK_TABLE_NAME     ): The primary (unique) key table
       identifier.

       PKCOLUMN_NAME  (UK_COLUMN_NAME    ): The primary (unique) key column
       identifier.

       FKTABLE_CAT    ( FK_TABLE_CAT      ): The foreign key table catalog
       identifier.  This field is NULL ("undef") if not applicable to the data
       source, which is often the case.  This field is empty if not applicable
       to the table.

       FKTABLE_SCHEM  ( FK_TABLE_SCHEM    ): The foreign key table schema
       identifier.  This field is NULL ("undef") if not applicable to the data
       source, and empty if not applicable to the table.

       FKTABLE_NAME   ( FK_TABLE_NAME     ): The foreign key table identifier.

       FKCOLUMN_NAME  ( FK_COLUMN_NAME    ): The foreign key column
       identifier.

       KEY_SEQ        ( ORDINAL_POSITION  ): The column sequence number
       (starting with 1).

       UPDATE_RULE    ( UPDATE_RULE       ): The referential action for the
       UPDATE rule.  The following codes are defined:

         CASCADE              0
         RESTRICT             1
         SET NULL             2
         NO ACTION            3
         SET DEFAULT          4

       DELETE_RULE    ( DELETE_RULE       ): The referential action for the
       DELETE rule.  The codes are the same as for UPDATE_RULE.

       FK_NAME        ( FK_NAME           ): The foreign key name.

       PK_NAME        ( UK_NAME           ): The primary (unique) key name.

       DEFERRABILITY  ( DEFERABILITY      ): The deferrability of the foreign
       key constraint.  The following codes are defined:

         INITIALLY DEFERRED   5
         INITIALLY IMMEDIATE  6
         NOT DEFERRABLE       7

                      ( UNIQUE_OR_PRIMARY ): This column is necessary if a
       driver includes all candidate (i.e. primary and alternate) keys in the
       result set (as specified by SQL/CLI).  The value of this column is
       UNIQUE if the foreign key references an alternate key and PRIMARY if
       the foreign key references a primary key, or it may be undefined if the
       driver doesn't have access to the information.

       See also "Catalog Methods" and "Standards Reference Information".

       "statistics_info"

       Warning: This method is experimental and may change.

         $sth = $dbh->statistics_info( $catalog, $schema, $table, $unique_only, $quick );

         # then $sth->fetchall_arrayref or $sth->fetchall_hashref etc

       Returns an active statement handle that can be used to fetch
       statistical information about a table and its indexes.

       The arguments don't accept search patterns (unlike "table_info").

       If the boolean argument $unique_only is true, only UNIQUE indexes will
       be returned in the result set, otherwise all indexes will be returned.

       If the boolean argument $quick is set, the actual statistical
       information columns (CARDINALITY and PAGES) will only be returned if
       they are readily available from the server, and might not be current.
       Some databases may return stale statistics or no statistics at all with
       this flag set.

       The statement handle will return at most one row per column name per
       index, plus at most one row for the entire table itself, ordered by
       NON_UNIQUE, TYPE, INDEX_QUALIFIER, INDEX_NAME, and ORDINAL_POSITION.

       Note: The support for the selection criteria, such as $catalog, is
       driver specific.  If the driver doesn't support catalogs and/or
       schemas, it may ignore these criteria.

       The statement handle returned has at least the following fields in the
       order shown below. Other fields, after these, may also be present.

       TABLE_CAT: The catalog identifier.  This field is NULL ("undef") if not
       applicable to the data source, which is often the case.  This field is
       empty if not applicable to the table.

       TABLE_SCHEM: The schema identifier.  This field is NULL ("undef") if
       not applicable to the data source, and empty if not applicable to the
       table.

       TABLE_NAME: The table identifier.

       NON_UNIQUE: Unique index indicator.  Returns 0 for unique indexes, 1
       for non-unique indexes

       INDEX_QUALIFIER: Index qualifier identifier.  The identifier that is
       used to qualify the index name when doing a "DROP INDEX"; NULL
       ("undef") is returned if an index qualifier is not supported by the
       data source.  If a non-NULL (defined) value is returned in this column,
       it must be used to qualify the index name on a "DROP INDEX" statement;
       otherwise, the TABLE_SCHEM should be used to qualify the index name.

       INDEX_NAME: The index identifier.

       TYPE: The type of information being returned.  Can be any of the
       following values: 'table', 'btree', 'clustered', 'content', 'hashed',
       or 'other'.

       In the case that this field is 'table', all fields other than
       TABLE_CAT, TABLE_SCHEM, TABLE_NAME, TYPE, CARDINALITY, and PAGES will
       be NULL ("undef").

       ORDINAL_POSITION: Column sequence number (starting with 1).

       COLUMN_NAME: The column identifier.

       ASC_OR_DESC: Column sort sequence.  "A" for Ascending, "D" for
       Descending, or NULL ("undef") if not supported for this index.

       CARDINALITY: Cardinality of the table or index.  For indexes, this is
       the number of unique values in the index.  For tables, this is the
       number of rows in the table.  If not supported, the value will be NULL
       ("undef").

       PAGES: Number of storage pages used by this table or index.  If not
       supported, the value will be NULL ("undef").

       FILTER_CONDITION: The index filter condition as a string.  If the index
       is not a filtered index, or it cannot be determined whether the index
       is a filtered index, this value is NULL ("undef").  If the index is a
       filtered index, but the filter condition cannot be determined, this
       value is the empty string ''.  Otherwise it will be the literal filter
       condition as a string, such as "SALARY <= 4500".

       See also "Catalog Methods" and "Standards Reference Information".

       "tables"

         @names = $dbh->tables( $catalog, $schema, $table, $type );
         @names = $dbh->tables;        # deprecated

       Simple interface to table_info(). Returns a list of matching table
       names, possibly including a catalog/schema prefix.

       See "table_info" for a description of the parameters.

       If "$dbh->get_info(29)" returns true (29 is SQL_IDENTIFIER_QUOTE_CHAR)
       then the table names are constructed and quoted by "quote_identifier"
       to ensure they are usable even if they contain whitespace or reserved
       words etc. This means that the table names returned will include quote
       characters.

       "type_info_all"

         $type_info_all = $dbh->type_info_all;

       Returns a reference to an array which holds information about each data
       type variant supported by the database and driver. The array and its
       contents should be treated as read-only.

       The first item is a reference to an 'index' hash of "Name ="> "Index"
       pairs.  The items following that are references to arrays, one per
       supported data type variant. The leading index hash defines the names
       and order of the fields within the arrays that follow it.  For example:

         $type_info_all = [
           {   TYPE_NAME         => 0,
               DATA_TYPE         => 1,
               COLUMN_SIZE       => 2,     # was PRECISION originally
               LITERAL_PREFIX    => 3,
               LITERAL_SUFFIX    => 4,
               CREATE_PARAMS     => 5,
               NULLABLE          => 6,
               CASE_SENSITIVE    => 7,
               SEARCHABLE        => 8,
               UNSIGNED_ATTRIBUTE=> 9,
               FIXED_PREC_SCALE  => 10,    # was MONEY originally
               AUTO_UNIQUE_VALUE => 11,    # was AUTO_INCREMENT originally
               LOCAL_TYPE_NAME   => 12,
               MINIMUM_SCALE     => 13,
               MAXIMUM_SCALE     => 14,
               SQL_DATA_TYPE     => 15,
               SQL_DATETIME_SUB  => 16,
               NUM_PREC_RADIX    => 17,
               INTERVAL_PRECISION=> 18,
           },
           [ 'VARCHAR', SQL_VARCHAR,
               undef, "'","'", undef,0, 1,1,0,0,0,undef,1,255, undef
           ],
           [ 'INTEGER', SQL_INTEGER,
               undef,  "", "", undef,0, 0,1,0,0,0,undef,0,  0, 10
           ],
         ];

       More than one row may have the same value in the "DATA_TYPE" field if
       there are different ways to spell the type name and/or there are
       variants of the type with different attributes (e.g., with and without
       "AUTO_UNIQUE_VALUE" set, with and without "UNSIGNED_ATTRIBUTE", etc).

       The rows are ordered by "DATA_TYPE" first and then by how closely each
       type maps to the corresponding ODBC SQL data type, closest first.

       The meaning of the fields is described in the documentation for the
       "type_info" method.

       An 'index' hash is provided so you don't need to rely on index values
       defined above.  However, using DBD::ODBC with some old ODBC drivers may
       return older names, shown as comments in the example above.  Another
       issue with the index hash is that the lettercase of the keys is not
       defined. It is usually uppercase, as show here, but drivers may return
       names with any lettercase.

       Drivers are also free to return extra driver-specific columns of
       information - though it's recommended that they start at column index
       50 to leave room for expansion of the DBI/ODBC specification.

       The type_info_all() method is not normally used directly.  The
       "type_info" method provides a more usable and useful interface to the
       data.

       "type_info"

         @type_info = $dbh->type_info($data_type);

       Returns a list of hash references holding information about one or more
       variants of $data_type. The list is ordered by "DATA_TYPE" first and
       then by how closely each type maps to the corresponding ODBC SQL data
       type, closest first.  If called in a scalar context then only the first
       (best) element is returned.

       If $data_type is undefined or "SQL_ALL_TYPES", then the list will
       contain hashes for all data type variants supported by the database and
       driver.

       If $data_type is an array reference then "type_info" returns the
       information for the first type in the array that has any matches.

       The keys of the hash follow the same letter case conventions as the
       rest of the DBI (see "Naming Conventions and Name Space"). The
       following uppercase items should always exist, though may be undef:

       TYPE_NAME (string)
           Data type name for use in CREATE TABLE statements etc.

       DATA_TYPE (integer)
           SQL data type number.

       COLUMN_SIZE (integer)
           For numeric types, this is either the total number of digits (if
           the NUM_PREC_RADIX value is 10) or the total number of bits allowed
           in the column (if NUM_PREC_RADIX is 2).

           For string types, this is the maximum size of the string in
           characters.

           For date and interval types, this is the maximum number of
           characters needed to display the value.

       LITERAL_PREFIX (string)
           Characters used to prefix a literal. A typical prefix is ""'"" for
           characters, or possibly ""0x"" for binary values passed as
           hexadecimal.  NULL ("undef") is returned for data types for which
           this is not applicable.

       LITERAL_SUFFIX (string)
           Characters used to suffix a literal. Typically ""'"" for
           characters.  NULL ("undef") is returned for data types where this
           is not applicable.

       CREATE_PARAMS (string)
           Parameter names for data type definition. For example,
           "CREATE_PARAMS" for a "DECIMAL" would be ""precision,scale"" if the
           DECIMAL type should be declared as "DECIMAL("precision,scale")"
           where precision and scale are integer values.  For a "VARCHAR" it
           would be ""max length"".  NULL ("undef") is returned for data types
           for which this is not applicable.

       NULLABLE (integer)
           Indicates whether the data type accepts a NULL value: 0 or an empty
           string = no, 1 = yes, 2 = unknown.

       CASE_SENSITIVE (boolean)
           Indicates whether the data type is case sensitive in collations and
           comparisons.

       SEARCHABLE (integer)
           Indicates how the data type can be used in a WHERE clause, as
           follows:

             0 - Cannot be used in a WHERE clause
             1 - Only with a LIKE predicate
             2 - All comparison operators except LIKE
             3 - Can be used in a WHERE clause with any comparison operator

       UNSIGNED_ATTRIBUTE (boolean)
           Indicates whether the data type is unsigned.  NULL ("undef") is
           returned for data types for which this is not applicable.

       FIXED_PREC_SCALE (boolean)
           Indicates whether the data type always has the same precision and
           scale (such as a money type).  NULL ("undef") is returned for data
           types for which this is not applicable.

       AUTO_UNIQUE_VALUE (boolean)
           Indicates whether a column of this data type is automatically set
           to a unique value whenever a new row is inserted.  NULL ("undef")
           is returned for data types for which this is not applicable.

       LOCAL_TYPE_NAME (string)
           Localized version of the "TYPE_NAME" for use in dialog with users.
           NULL ("undef") is returned if a localized name is not available (in
           which case "TYPE_NAME" should be used).

       MINIMUM_SCALE (integer)
           The minimum scale of the data type. If a data type has a fixed
           scale, then "MAXIMUM_SCALE" holds the same value.  NULL ("undef")
           is returned for data types for which this is not applicable.

       MAXIMUM_SCALE (integer)
           The maximum scale of the data type. If a data type has a fixed
           scale, then "MINIMUM_SCALE" holds the same value.  NULL ("undef")
           is returned for data types for which this is not applicable.

       SQL_DATA_TYPE (integer)
           This column is the same as the "DATA_TYPE" column, except for
           interval and datetime data types.  For interval and datetime data
           types, the "SQL_DATA_TYPE" field will return "SQL_INTERVAL" or
           "SQL_DATETIME", and the "SQL_DATETIME_SUB" field below will return
           the subcode for the specific interval or datetime data type. If
           this field is NULL, then the driver does not support or report on
           interval or datetime subtypes.

       SQL_DATETIME_SUB (integer)
           For interval or datetime data types, where the "SQL_DATA_TYPE"
           field above is "SQL_INTERVAL" or "SQL_DATETIME", this field will
           hold the subcode for the specific interval or datetime data type.
           Otherwise it will be NULL ("undef").

           Although not mentioned explicitly in the standards, it seems there
           is a simple relationship between these values:

             DATA_TYPE == (10 * SQL_DATA_TYPE) + SQL_DATETIME_SUB

       NUM_PREC_RADIX (integer)
           The radix value of the data type. For approximate numeric types,
           "NUM_PREC_RADIX" contains the value 2 and "COLUMN_SIZE" holds the
           number of bits. For exact numeric types, "NUM_PREC_RADIX" contains
           the value 10 and "COLUMN_SIZE" holds the number of decimal digits.
           NULL ("undef") is returned either for data types for which this is
           not applicable or if the driver cannot report this information.

       INTERVAL_PRECISION (integer)
           The interval leading precision for interval types. NULL is returned
           either for data types for which this is not applicable or if the
           driver cannot report this information.

       For example, to find the type name for the fields in a select statement
       you can do:

         @names = map { scalar $dbh->type_info($_)->{TYPE_NAME} } @{ $sth->{TYPE} }

       Since DBI and ODBC drivers vary in how they map their types into the
       ISO standard types you may need to search for more than one type.
       Here's an example looking for a usable type to store a date:

         $my_date_type = $dbh->type_info( [ SQL_DATE, SQL_TIMESTAMP ] );

       Similarly, to more reliably find a type to store small integers, you
       could use a list starting with "SQL_SMALLINT", "SQL_INTEGER",
       "SQL_DECIMAL", etc.

       See also "Standards Reference Information".

       "quote"

         $sql = $dbh->quote($value);
         $sql = $dbh->quote($value, $data_type);

       Quote a string literal for use as a literal value in an SQL statement,
       by escaping any special characters (such as quotation marks) contained
       within the string and adding the required type of outer quotation
       marks.

         $sql = sprintf "SELECT foo FROM bar WHERE baz = %s",
                       $dbh->quote("Don't");

       For most database types, at least those that conform to SQL standards,
       quote would return 'Don''t' (including the outer quotation marks). For
       others it may return something like 'Don\'t'

       An undefined $value value will be returned as the string "NULL"
       (without single quotation marks) to match how NULLs are represented in
       SQL.

       If $data_type is supplied, it is used to try to determine the required
       quoting behaviour by using the information returned by "type_info".  As
       a special case, the standard numeric types are optimized to return
       $value without calling "type_info".

       Quote will probably not be able to deal with all possible input (such
       as binary data or data containing newlines), and is not related in any
       way with escaping or quoting shell meta-characters.

       It is valid for the quote() method to return an SQL expression that
       evaluates to the desired string. For example:

         $quoted = $dbh->quote("one\ntwo\0three")

       may return something like:

         CONCAT('one', CHAR(12), 'two', CHAR(0), 'three')

       The quote() method should not be used with "Placeholders and Bind
       Values".

       "quote_identifier"

         $sql = $dbh->quote_identifier( $name );
         $sql = $dbh->quote_identifier( $catalog, $schema, $table, \%attr );

       Quote an identifier (table name etc.) for use in an SQL statement, by
       escaping any special characters (such as double quotation marks) it
       contains and adding the required type of outer quotation marks.

       Undefined names are ignored and the remainder are quoted and then
       joined together, typically with a dot (".") character. For example:

         $id = $dbh->quote_identifier( undef, 'Her schema', 'My table' );

       would, for most database types, return "Her schema"."My table"
       (including all the double quotation marks).

       If three names are supplied then the first is assumed to be a catalog
       name and special rules may be applied based on what "get_info" returns
       for SQL_CATALOG_NAME_SEPARATOR (41) and SQL_CATALOG_LOCATION (114).
       For example, for Oracle:

         $id = $dbh->quote_identifier( 'link', 'schema', 'table' );

       would return "schema"."table"@"link".

       "take_imp_data"

         $imp_data = $dbh->take_imp_data;

       Leaves the $dbh in an almost dead, zombie-like, state and returns a
       binary string of raw implementation data from the driver which
       describes the current database connection. Effectively it detaches the
       underlying database API connection data from the DBI handle.  After
       calling take_imp_data(), all other methods except "DESTROY" will
       generate a warning and return undef.

       Why would you want to do this? You don't, forget I even mentioned it.
       Unless, that is, you're implementing something advanced like a multi-
       threaded connection pool. See DBI::Pool.

       The returned $imp_data can be passed as a "dbi_imp_data" attribute to a
       later connect() call, even in a separate thread in the same process,
       where the driver can use it to 'adopt' the existing connection that the
       implementation data was taken from.

       Some things to keep in mind...

       * the $imp_data holds the only reference to the underlying database API
       connection data. That connection is still 'live' and won't be cleaned
       up properly unless the $imp_data is used to create a new $dbh which is
       then allowed to disconnect() normally.

       * using the same $imp_data to create more than one other new $dbh at a
       time may well lead to unpleasant problems. Don't do that.

       Any child statement handles are effectively destroyed when
       take_imp_data() is called.

       The "take_imp_data" method was added in DBI 1.36 but wasn't useful till
       1.49.

   Database Handle Attributes
       This section describes attributes specific to database handles.

       Changes to these database handle attributes do not affect any other
       existing or future database handles.

       Attempting to set or get the value of an unknown attribute generates a
       warning, except for private driver-specific attributes (which all have
       names starting with a lowercase letter).

       Example:

         $h->{AutoCommit} = ...;       # set/write
         ... = $h->{AutoCommit};       # get/read

       "AutoCommit"  (boolean)

       If true, then database changes cannot be rolled-back (undone).  If
       false, then database changes automatically occur within a
       "transaction", which must either be committed or rolled back using the
       "commit" or "rollback" methods.

       Drivers should always default to "AutoCommit" mode (an unfortunate
       choice largely forced on the DBI by ODBC and JDBC conventions.)

       Attempting to set "AutoCommit" to an unsupported value is a fatal
       error.  This is an important feature of the DBI. Applications that need
       full transaction behaviour can set "$dbh->{AutoCommit} = 0" (or set
       "AutoCommit" to 0 via "connect") without having to check that the value
       was assigned successfully.

       For the purposes of this description, we can divide databases into
       three categories:

         Databases which don't support transactions at all.
         Databases in which a transaction is always active.
         Databases in which a transaction must be explicitly started (C<'BEGIN WORK'>).

       * Databases which don't support transactions at all

       For these databases, attempting to turn "AutoCommit" off is a fatal
       error.  "commit" and "rollback" both issue warnings about being
       ineffective while "AutoCommit" is in effect.

       * Databases in which a transaction is always active

       These are typically mainstream commercial relational databases with
       "ANSI standard" transaction behaviour.  If "AutoCommit" is off, then
       changes to the database won't have any lasting effect unless "commit"
       is called (but see also "disconnect"). If "rollback" is called then any
       changes since the last commit are undone.

       If "AutoCommit" is on, then the effect is the same as if the DBI called
       "commit" automatically after every successful database operation. So
       calling "commit" or "rollback" explicitly while "AutoCommit" is on
       would be ineffective because the changes would have already been
       committed.

       Changing "AutoCommit" from off to on will trigger a "commit".

       For databases which don't support a specific auto-commit mode, the
       driver has to commit each statement automatically using an explicit
       "COMMIT" after it completes successfully (and roll it back using an
       explicit "ROLLBACK" if it fails).  The error information reported to
       the application will correspond to the statement which was executed,
       unless it succeeded and the commit or rollback failed.

       * Databases in which a transaction must be explicitly started

       For these databases, the intention is to have them act like databases
       in which a transaction is always active (as described above).

       To do this, the driver will automatically begin an explicit transaction
       when "AutoCommit" is turned off, or after a "commit" or "rollback" (or
       when the application issues the next database operation after one of
       those events).

       In this way, the application does not have to treat these databases as
       a special case.

       See "commit", "disconnect" and "Transactions" for other important notes
       about transactions.

       "Driver"  (handle)

       Holds the handle of the parent driver. The only recommended use for
       this is to find the name of the driver using:

         $dbh->{Driver}->{Name}

       "Name"  (string)

       Holds the "name" of the database. Usually (and recommended to be) the
       same as the ""dbi:DriverName:..."" string used to connect to the
       database, but with the leading ""dbi:DriverName:"" removed.

       "Statement"  (string, read-only)

       Returns the statement string passed to the most recent "prepare" method
       called in this database handle, even if that method failed. This is
       especially useful where "RaiseError" is enabled and the exception
       handler checks $@ and sees that a 'prepare' method call failed.

       "RowCacheSize"  (integer)

       A hint to the driver indicating the size of the local row cache that
       the application would like the driver to use for future "SELECT"
       statements.  If a row cache is not implemented, then setting
       "RowCacheSize" is ignored and getting the value returns "undef".

       Some "RowCacheSize" values have special meaning, as follows:

         0 - Automatically determine a reasonable cache size for each C<SELECT>
         1 - Disable the local row cache
        >1 - Cache this many rows
        <0 - Cache as many rows that will fit into this much memory for each C<SELECT>.

       Note that large cache sizes may require a very large amount of memory
       (cached rows * maximum size of row). Also, a large cache will cause a
       longer delay not only for the first fetch, but also whenever the cache
       needs refilling.

       See also the "RowsInCache" statement handle attribute.

       "Username"  (string)

       Returns the username used to connect to the database.

DBI STATEMENT HANDLE OBJECTS
       This section lists the methods and attributes associated with DBI
       statement handles.

   Statement Handle Methods
       The DBI defines the following methods for use on DBI statement handles:

       "bind_param"

         $sth->bind_param($p_num, $bind_value)
         $sth->bind_param($p_num, $bind_value, \%attr)
         $sth->bind_param($p_num, $bind_value, $bind_type)

       The "bind_param" method takes a copy of $bind_value and associates it
       (binds it) with a placeholder, identified by $p_num, embedded in the
       prepared statement. Placeholders are indicated with question mark
       character ("?"). For example:

         $dbh->{RaiseError} = 1;        # save having to check each method call
         $sth = $dbh->prepare("SELECT name, age FROM people WHERE name LIKE ?");
         $sth->bind_param(1, "John%");  # placeholders are numbered from 1
         $sth->execute;
         DBI::dump_results($sth);

       See "Placeholders and Bind Values" for more information.

       Data Types for Placeholders

       The "\%attr" parameter can be used to hint at the data type the
       placeholder should have. This is rarely needed. Typically, the driver
       is only interested in knowing if the placeholder should be bound as a
       number or a string.

         $sth->bind_param(1, $value, { TYPE => SQL_INTEGER });

       As a short-cut for the common case, the data type can be passed
       directly, in place of the "\%attr" hash reference. This example is
       equivalent to the one above:

         $sth->bind_param(1, $value, SQL_INTEGER);

       The "TYPE" value indicates the standard (non-driver-specific) type for
       this parameter. To specify the driver-specific type, the driver may
       support a driver-specific attribute, such as "{ ora_type => 97 }".

       The SQL_INTEGER and other related constants can be imported using

         use DBI qw(:sql_types);

       See "DBI Constants" for more information.

       The data type is 'sticky' in that bind values passed to execute() are
       bound with the data type specified by earlier bind_param() calls, if
       any.  Portable applications should not rely on being able to change the
       data type after the first "bind_param" call.

       Perl only has string and number scalar data types. All database types
       that aren't numbers are bound as strings and must be in a format the
       database will understand except where the bind_param() TYPE attribute
       specifies a type that implies a particular format. For example, given:

         $sth->bind_param(1, $value, SQL_DATETIME);

       the driver should expect $value to be in the ODBC standard SQL_DATETIME
       format, which is 'YYYY-MM-DD HH:MM:SS'. Similarly for SQL_DATE,
       SQL_TIME etc.

       As an alternative to specifying the data type in the "bind_param" call,
       you can let the driver pass the value as the default type ("VARCHAR").
       You can then use an SQL function to convert the type within the
       statement.  For example:

         INSERT INTO price(code, price) VALUES (?, CONVERT(MONEY,?))

       The "CONVERT" function used here is just an example. The actual
       function and syntax will vary between different databases and is non-
       portable.

       See also "Placeholders and Bind Values" for more information.

       "bind_param_inout"

         $rc = $sth->bind_param_inout($p_num, \$bind_value, $max_len)  or die $sth->errstr;
         $rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, \%attr)     or ...
         $rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, $bind_type) or ...

       This method acts like "bind_param", but also enables values to be
       updated by the statement. The statement is typically a call to a stored
       procedure. The $bind_value must be passed as a reference to the actual
       value to be used.

       Note that unlike "bind_param", the $bind_value variable is not copied
       when "bind_param_inout" is called. Instead, the value in the variable
       is read at the time "execute" is called.

       The additional $max_len parameter specifies the minimum amount of
       memory to allocate to $bind_value for the new value. If the value
       returned from the database is too big to fit, then the execution should
       fail. If unsure what value to use, pick a generous length, i.e., a
       length larger than the longest value that would ever be returned.  The
       only cost of using a larger value than needed is wasted memory.

       Undefined values or "undef" are used to indicate null values.  See also
       "Placeholders and Bind Values" for more information.

       "bind_param_array"

         $rc = $sth->bind_param_array($p_num, $array_ref_or_value)
         $rc = $sth->bind_param_array($p_num, $array_ref_or_value, \%attr)
         $rc = $sth->bind_param_array($p_num, $array_ref_or_value, $bind_type)

       The "bind_param_array" method is used to bind an array of values to a
       placeholder embedded in the prepared statement which is to be executed
       with "execute_array". For example:

         $dbh->{RaiseError} = 1;        # save having to check each method call
         $sth = $dbh->prepare("INSERT INTO staff (first_name, last_name, dept) VALUES(?, ?, ?)");
         $sth->bind_param_array(1, [ 'John', 'Mary', 'Tim' ]);
         $sth->bind_param_array(2, [ 'Booth', 'Todd', 'Robinson' ]);
         $sth->bind_param_array(3, "SALES"); # scalar will be reused for each row
         $sth->execute_array( { ArrayTupleStatus => \my @tuple_status } );

       The %attr ($bind_type) argument is the same as defined for
       "bind_param".  Refer to "bind_param" for general details on using
       placeholders.

       (Note that bind_param_array() can not be used to expand a placeholder
       into a list of values for a statement like "SELECT foo WHERE bar IN
       (?)".  A placeholder can only ever represent one value per execution.)

       Scalar values, including "undef", may also be bound by
       "bind_param_array". In which case the same value will be used for each
       "execute" call. Driver-specific implementations may behave differently,
       e.g., when binding to a stored procedure call, some databases may
       permit mixing scalars and arrays as arguments.

       The default implementation provided by DBI (for drivers that have not
       implemented array binding) is to iteratively call "execute" for each
       parameter tuple provided in the bound arrays.  Drivers may provide more
       optimized implementations using whatever bulk operation support the
       database API provides. The default driver behaviour should match the
       default DBI behaviour, but always consult your driver documentation as
       there may be driver specific issues to consider.

       Note that the default implementation currently only supports non-data
       returning statements (INSERT, UPDATE, but not SELECT). Also,
       "bind_param_array" and "bind_param" cannot be mixed in the same
       statement execution, and "bind_param_array" must be used with
       "execute_array"; using "bind_param_array" will have no effect for
       "execute".

       The "bind_param_array" method was added in DBI 1.22.

       "execute"

         $rv = $sth->execute                or die $sth->errstr;
         $rv = $sth->execute(@bind_values)  or die $sth->errstr;

       Perform whatever processing is necessary to execute the prepared
       statement.  An "undef" is returned if an error occurs.  A successful
       "execute" always returns true regardless of the number of rows
       affected, even if it's zero (see below). It is always important to
       check the return status of "execute" (and most other DBI methods) for
       errors if you're not using "RaiseError".

       For a non-"SELECT" statement, "execute" returns the number of rows
       affected, if known. If no rows were affected, then "execute" returns
       "0E0", which Perl will treat as 0 but will regard as true. Note that it
       is not an error for no rows to be affected by a statement. If the
       number of rows affected is not known, then "execute" returns -1.

       For "SELECT" statements, execute simply "starts" the query within the
       database engine. Use one of the fetch methods to retrieve the data
       after calling "execute".  The "execute" method does not return the
       number of rows that will be returned by the query (because most
       databases can't tell in advance), it simply returns a true value.

       You can tell if the statement was a "SELECT" statement by checking if
       "$sth->{NUM_OF_FIELDS}" is greater than zero after calling "execute".

       If any arguments are given, then "execute" will effectively call
       "bind_param" for each value before executing the statement.  Values
       bound in this way are usually treated as "SQL_VARCHAR" types unless the
       driver can determine the correct type (which is rare), or unless
       "bind_param" (or "bind_param_inout") has already been used to specify
       the type.

       Note that passing "execute" an empty array is the same as passing no
       arguments at all, which will execute the statement with previously
       bound values.  That's probably not what you want.

       If execute() is called on a statement handle that's still active
       ($sth->{Active} is true) then it should effectively call finish() to
       tidy up the previous execution results before starting this new
       execution.

       "execute_array"

         $tuples = $sth->execute_array(\%attr) or die $sth->errstr;
         $tuples = $sth->execute_array(\%attr, @bind_values) or die $sth->errstr;

         ($tuples, $rows) = $sth->execute_array(\%attr) or die $sth->errstr;
         ($tuples, $rows) = $sth->execute_array(\%attr, @bind_values) or die $sth->errstr;

       Execute the prepared statement once for each parameter tuple (group of
       values) provided either in the @bind_values, or by prior calls to
       "bind_param_array", or via a reference passed in \%attr.

       When called in scalar context the execute_array() method returns the
       number of tuples executed, or "undef" if an error occurred.  Like
       execute(), a successful execute_array() always returns true regardless
       of the number of tuples executed, even if it's zero. If there were any
       errors the ArrayTupleStatus array can be used to discover which tuples
       failed and with what errors.

       When called in list context the execute_array() method returns two
       scalars; $tuples is the same as calling execute_array() in scalar
       context and $rows is the sum of the number of rows affected for each
       tuple, if available or -1 if the driver cannot determine this.  If you
       are doing an update operation the returned rows affected may not be
       what you expect if, for instance, one or more of the tuples affected
       the same row multiple times.  Some drivers may not yet support list
       context, in which case $rows will be undef, or may not be able to
       provide the number of rows affected when performing this batch
       operation, in which case $rows will be -1.

       Bind values for the tuples to be executed may be supplied row-wise by
       an "ArrayTupleFetch" attribute, or else column-wise in the @bind_values
       argument, or else column-wise by prior calls to "bind_param_array".

       Where column-wise binding is used (via the @bind_values argument or
       calls to bind_param_array()) the maximum number of elements in any one
       of the bound value arrays determines the number of tuples executed.
       Placeholders with fewer values in their parameter arrays are treated as
       if padded with undef (NULL) values.

       If a scalar value is bound, instead of an array reference, it is
       treated as a variable length array with all elements having the same
       value. It's does not influence the number of tuples executed, so if all
       bound arrays have zero elements then zero tuples will be executed. If
       all bound values are scalars then one tuple will be executed, making
       execute_array() act just like execute().

       The "ArrayTupleFetch" attribute can be used to specify a reference to a
       subroutine that will be called to provide the bind values for each
       tuple execution. The subroutine should return an reference to an array
       which contains the appropriate number of bind values, or return an
       undef if there is no more data to execute.

       As a convenience, the "ArrayTupleFetch" attribute can also be used to
       specify a statement handle. In which case the fetchrow_arrayref()
       method will be called on the given statement handle in order to provide
       the bind values for each tuple execution.

       The values specified via bind_param_array() or the @bind_values
       parameter may be either scalars, or arrayrefs.  If any @bind_values are
       given, then "execute_array" will effectively call "bind_param_array"
       for each value before executing the statement.  Values bound in this
       way are usually treated as "SQL_VARCHAR" types unless the driver can
       determine the correct type (which is rare), or unless "bind_param",
       "bind_param_inout", "bind_param_array", or "bind_param_inout_array" has
       already been used to specify the type.  See "bind_param_array" for
       details.

       The "ArrayTupleStatus" attribute can be used to specify a reference to
       an array which will receive the execute status of each executed
       parameter tuple. Note the "ArrayTupleStatus" attribute was mandatory
       until DBI 1.38.

       For tuples which are successfully executed, the element at the same
       ordinal position in the status array is the resulting rowcount.  If the
       execution of a tuple causes an error, then the corresponding status
       array element will be set to a reference to an array containing the
       error code and error string set by the failed execution.

       If any tuple execution returns an error, "execute_array" will return
       "undef". In that case, the application should inspect the status array
       to determine which parameter tuples failed.  Some databases may not
       continue executing tuples beyond the first failure. In this case the
       status array will either hold fewer elements, or the elements beyond
       the failure will be undef.

       If all parameter tuples are successfully executed, "execute_array"
       returns the number tuples executed.  If no tuples were executed, then
       execute_array() returns "0E0", just like execute() does, which Perl
       will treat as 0 but will regard as true.

       For example:

         $sth = $dbh->prepare("INSERT INTO staff (first_name, last_name) VALUES (?, ?)");
         my $tuples = $sth->execute_array(
             { ArrayTupleStatus => \my @tuple_status },
             \@first_names,
             \@last_names,
         );
         if ($tuples) {
             print "Successfully inserted $tuples records\n";
         }
         else {
             for my $tuple (0..@last_names-1) {
                 my $status = $tuple_status[$tuple];
                 $status = [0, "Skipped"] unless defined $status;
                 next unless ref $status;
                 printf "Failed to insert (%s, %s): %s\n",
                     $first_names[$tuple], $last_names[$tuple], $status->[1];
             }
         }

       Support for data returning statements such as SELECT is driver-specific
       and subject to change. At present, the default implementation provided
       by DBI only supports non-data returning statements.

       Transaction semantics when using array binding are driver and database
       specific.  If "AutoCommit" is on, the default DBI implementation will
       cause each parameter tuple to be individually committed (or rolled back
       in the event of an error). If "AutoCommit" is off, the application is
       responsible for explicitly committing the entire set of bound parameter
       tuples.  Note that different drivers and databases may have different
       behaviours when some parameter tuples cause failures. In some cases,
       the driver or database may automatically rollback the effect of all
       prior parameter tuples that succeeded in the transaction; other drivers
       or databases may retain the effect of prior successfully executed
       parameter tuples. Be sure to check your driver and database for its
       specific behaviour.

       Note that, in general, performance will usually be better with
       "AutoCommit" turned off, and using explicit "commit" after each
       "execute_array" call.

       The "execute_array" method was added in DBI 1.22, and ArrayTupleFetch
       was added in 1.36.

       "execute_for_fetch"

         $tuples = $sth->execute_for_fetch($fetch_tuple_sub);
         $tuples = $sth->execute_for_fetch($fetch_tuple_sub, \@tuple_status);

         ($tuples, $rows) = $sth->execute_for_fetch($fetch_tuple_sub);
         ($tuples, $rows) = $sth->execute_for_fetch($fetch_tuple_sub, \@tuple_status);

       The execute_for_fetch() method is used to perform bulk operations and
       is most often used via the execute_array() method, not directly.

       The fetch subroutine, referenced by $fetch_tuple_sub, is expected to
       return a reference to an array (known as a 'tuple') or undef.

       The execute_for_fetch() method calls $fetch_tuple_sub, without any
       parameters, until it returns a false value. Each tuple returned is used
       to provide bind values for an $sth->execute(@$tuple) call.

       In scalar context execute_for_fetch() returns "undef" if there were any
       errors and the number of tuples executed otherwise. Like execute() and
       execute_array() a zero is returned as "0E0" so execute_for_fetch() is
       only false on error.  If there were any errors the @tuple_status array
       can be used to discover which tuples failed and with what errors.

       When called in list context execute_for_fetch() returns two scalars;
       $tuples is the same as calling execute_for_fetch() in scalar context
       and $rows is the sum of the number of rows affected for each tuple, if
       available or -1 if the driver cannot determine this.  If you are doing
       an update operation the returned rows affected may not be what you
       expect if, for instance, one or more of the tuples affected the same
       row multiple times.  Some drivers may not yet support list context, in
       which case $rows will be undef, or may not be able to provide the
       number of rows affected when performing this batch operation, in which
       case $rows will be -1.

       If \@tuple_status is passed then the execute_for_fetch method uses it
       to return status information. The tuple_status array holds one element
       per tuple. If the corresponding execute() did not fail then the element
       holds the return value from execute(), which is typically a row count.
       If the execute() did fail then the element holds a reference to an
       array containing ($sth->err, $sth->errstr, $sth->state).

       If the driver detects an error that it knows means no further tuples
       can be executed then it may return, with an error status, even though
       $fetch_tuple_sub may still have more tuples to be executed.

       Although each tuple returned by $fetch_tuple_sub is effectively used to
       call $sth->execute(@$tuple_array_ref) the exact timing may vary.
       Drivers are free to accumulate sets of tuples to pass to the database
       server in bulk group operations for more efficient execution.  However,
       the $fetch_tuple_sub is specifically allowed to return the same array
       reference each time (which is what fetchrow_arrayref() usually does).

       For example:

         my $sel = $dbh1->prepare("select foo, bar from table1");
         $sel->execute;

         my $ins = $dbh2->prepare("insert into table2 (foo, bar) values (?,?)");
         my $fetch_tuple_sub = sub { $sel->fetchrow_arrayref };

         my @tuple_status;
         $rc = $ins->execute_for_fetch($fetch_tuple_sub, \@tuple_status);
         my @errors = grep { ref $_ } @tuple_status;

       Similarly, if you already have an array containing the data rows to be
       processed you'd use a subroutine to shift off and return each array ref
       in turn:

         $ins->execute_for_fetch( sub { shift @array_of_arrays }, \@tuple_status);

       The "execute_for_fetch" method was added in DBI 1.38.

       "fetchrow_arrayref"

         $ary_ref = $sth->fetchrow_arrayref;
         $ary_ref = $sth->fetch;    # alias

       Fetches the next row of data and returns a reference to an array
       holding the field values.  Null fields are returned as "undef" values
       in the array.  This is the fastest way to fetch data, particularly if
       used with "$sth->bind_columns".

       If there are no more rows or if an error occurs, then
       "fetchrow_arrayref" returns an "undef". You should check "$sth->err"
       afterwards (or use the "RaiseError" attribute) to discover if the
       "undef" returned was due to an error.

       Note that the same array reference is returned for each fetch, so don't
       store the reference and then use it after a later fetch.  Also, the
       elements of the array are also reused for each row, so take care if you
       want to take a reference to an element. See also "bind_columns".

       "fetchrow_array"

        @ary = $sth->fetchrow_array;

       An alternative to "fetchrow_arrayref". Fetches the next row of data and
       returns it as a list containing the field values.  Null fields are
       returned as "undef" values in the list.

       If there are no more rows or if an error occurs, then "fetchrow_array"
       returns an empty list. You should check "$sth->err" afterwards (or use
       the "RaiseError" attribute) to discover if the empty list returned was
       due to an error.

       If called in a scalar context for a statement handle that has more than
       one column, it is undefined whether the driver will return the value of
       the first column or the last. So don't do that.  Also, in a scalar
       context, an "undef" is returned if there are no more rows or if an
       error occurred. That "undef" can't be distinguished from an "undef"
       returned because the first field value was NULL.  For these reasons you
       should exercise some caution if you use "fetchrow_array" in a scalar
       context.

       "fetchrow_hashref"

        $hash_ref = $sth->fetchrow_hashref;
        $hash_ref = $sth->fetchrow_hashref($name);

       An alternative to "fetchrow_arrayref". Fetches the next row of data and
       returns it as a reference to a hash containing field name and field
       value pairs.  Null fields are returned as "undef" values in the hash.

       If there are no more rows or if an error occurs, then
       "fetchrow_hashref" returns an "undef". You should check "$sth->err"
       afterwards (or use the "RaiseError" attribute) to discover if the
       "undef" returned was due to an error.

       The optional $name parameter specifies the name of the statement handle
       attribute. For historical reasons it defaults to ""NAME"", however
       using either ""NAME_lc"" or ""NAME_uc"" is recomended for portability.

       The keys of the hash are the same names returned by "$sth->{$name}". If
       more than one field has the same name, there will only be one entry in
       the returned hash for those fields.

       Because of the extra work "fetchrow_hashref" and Perl have to perform,
       it is not as efficient as "fetchrow_arrayref" or "fetchrow_array".

       By default a reference to a new hash is returned for each row.  It is
       likely that a future version of the DBI will support an attribute which
       will enable the same hash to be reused for each row. This will give a
       significant performance boost, but it won't be enabled by default
       because of the risk of breaking old code.

       "fetchall_arrayref"

         $tbl_ary_ref = $sth->fetchall_arrayref;
         $tbl_ary_ref = $sth->fetchall_arrayref( $slice );
         $tbl_ary_ref = $sth->fetchall_arrayref( $slice, $max_rows  );

       The "fetchall_arrayref" method can be used to fetch all the data to be
       returned from a prepared and executed statement handle. It returns a
       reference to an array that contains one reference per row.

       If there are no rows to return, "fetchall_arrayref" returns a reference
       to an empty array. If an error occurs, "fetchall_arrayref" returns the
       data fetched thus far, which may be none.  You should check "$sth->err"
       afterwards (or use the "RaiseError" attribute) to discover if the data
       is complete or was truncated due to an error.

       If $slice is an array reference, "fetchall_arrayref" uses
       "fetchrow_arrayref" to fetch each row as an array ref. If the $slice
       array is not empty then it is used as a slice to select individual
       columns by perl array index number (starting at 0, unlike column and
       parameter numbers which start at 1).

       With no parameters, or if $slice is undefined, "fetchall_arrayref" acts
       as if passed an empty array ref.

       If $slice is a hash reference, "fetchall_arrayref" uses
       "fetchrow_hashref" to fetch each row as a hash reference. If the $slice
       hash is empty then fetchrow_hashref() is simply called in a tight loop
       and the keys in the hashes have whatever name lettercase is returned by
       default from fetchrow_hashref.  (See "FetchHashKeyName" attribute.) If
       the $slice hash is not empty, then it is used as a slice to select
       individual columns by name.  The values of the hash should be set to 1.
       The key names of the returned hashes match the letter case of the names
       in the parameter hash, regardless of the "FetchHashKeyName" attribute.

       For example, to fetch just the first column of every row:

         $tbl_ary_ref = $sth->fetchall_arrayref([0]);

       To fetch the second to last and last column of every row:

         $tbl_ary_ref = $sth->fetchall_arrayref([-2,-1]);

       To fetch all fields of every row as a hash ref:

         $tbl_ary_ref = $sth->fetchall_arrayref({});

       To fetch only the fields called "foo" and "bar" of every row as a hash
       ref (with keys named "foo" and "BAR"):

         $tbl_ary_ref = $sth->fetchall_arrayref({ foo=>1, BAR=>1 });

       The first two examples return a reference to an array of array refs.
       The third and forth return a reference to an array of hash refs.

       If $max_rows is defined and greater than or equal to zero then it is
       used to limit the number of rows fetched before returning.
       fetchall_arrayref() can then be called again to fetch more rows.  This
       is especially useful when you need the better performance of
       fetchall_arrayref() but don't have enough memory to fetch and return
       all the rows in one go.

       Here's an example (assumes RaiseError is enabled):

         my $rows = []; # cache for batches of rows
         while( my $row = ( shift(@$rows) || # get row from cache, or reload cache:
                            shift(@{$rows=$sth->fetchall_arrayref(undef,10_000)||[]}) )
         ) {
           ...
         }

       That might be the fastest way to fetch and process lots of rows using
       the DBI, but it depends on the relative cost of method calls vs memory
       allocation.

       A standard "while" loop with column binding is often faster because the
       cost of allocating memory for the batch of rows is greater than the
       saving by reducing method calls. It's possible that the DBI may provide
       a way to reuse the memory of a previous batch in future, which would
       then shift the balance back towards fetchall_arrayref().

       "fetchall_hashref"

         $hash_ref = $sth->fetchall_hashref($key_field);

       The "fetchall_hashref" method can be used to fetch all the data to be
       returned from a prepared and executed statement handle. It returns a
       reference to a hash containing a key for each distinct value of the
       $key_field column that was fetched. For each key the corresponding
       value is a reference to a hash containing all the selected columns and
       their values, as returned by fetchrow_hashref().

       If there are no rows to return, "fetchall_hashref" returns a reference
       to an empty hash. If an error occurs, "fetchall_hashref" returns the
       data fetched thus far, which may be none.  You should check "$sth->err"
       afterwards (or use the "RaiseError" attribute) to discover if the data
       is complete or was truncated due to an error.

       The $key_field parameter provides the name of the field that holds the
       value to be used for the key for the returned hash.  For example:

         $dbh->{FetchHashKeyName} = 'NAME_lc';
         $sth = $dbh->prepare("SELECT FOO, BAR, ID, NAME, BAZ FROM TABLE");
         $sth->execute;
         $hash_ref = $sth->fetchall_hashref('id');
         print "Name for id 42 is $hash_ref->{42}->{name}\n";

       The $key_field parameter can also be specified as an integer column
       number (counting from 1).  If $key_field doesn't match any column in
       the statement, as a name first then as a number, then an error is
       returned.

       For queries returning more than one 'key' column, you can specify
       multiple column names by passing $key_field as a reference to an array
       containing one or more key column names (or index numbers).  For
       example:

         $sth = $dbh->prepare("SELECT foo, bar, baz FROM table");
         $sth->execute;
         $hash_ref = $sth->fetchall_hashref( [ qw(foo bar) ] );
         print "For foo 42 and bar 38, baz is $hash_ref->{42}->{38}->{baz}\n";

       The fetchall_hashref() method is normally used only where the key
       fields values for each row are unique.  If multiple rows are returned
       with the same values for the key fields then later rows overwrite
       earlier ones.

       "finish"

         $rc  = $sth->finish;

       Indicate that no more data will be fetched from this statement handle
       before it is either executed again or destroyed.  The "finish" method
       is rarely needed, and frequently overused, but can sometimes be helpful
       in a few very specific situations to allow the server to free up
       resources (such as sort buffers).

       When all the data has been fetched from a "SELECT" statement, the
       driver should automatically call "finish" for you. So you should not
       normally need to call it explicitly except when you know that you've
       not fetched all the data from a statement handle.  The most common
       example is when you only want to fetch one row, but in that case the
       "selectrow_*" methods are usually better anyway.  Adding calls to
       "finish" after each fetch loop is a common mistake, don't do it, it can
       mask genuine problems like uncaught fetch errors.

       Consider a query like:

         SELECT foo FROM table WHERE bar=? ORDER BY foo

       where you want to select just the first (smallest) "foo" value from a
       very large table. When executed, the database server will have to use
       temporary buffer space to store the sorted rows. If, after executing
       the handle and selecting one row, the handle won't be re-executed for
       some time and won't be destroyed, the "finish" method can be used to
       tell the server that the buffer space can be freed.

       Calling "finish" resets the "Active" attribute for the statement.  It
       may also make some statement handle attributes (such as "NAME" and
       "TYPE") unavailable if they have not already been accessed (and thus
       cached).

       The "finish" method does not affect the transaction status of the
       database connection.  It has nothing to do with transactions. It's
       mostly an internal "housekeeping" method that is rarely needed.  See
       also "disconnect" and the "Active" attribute.

       The "finish" method should have been called "discard_pending_rows".

       "rows"

         $rv = $sth->rows;

       Returns the number of rows affected by the last row affecting command,
       or -1 if the number of rows is not known or not available.

       Generally, you can only rely on a row count after a non-"SELECT"
       "execute" (for some specific operations like "UPDATE" and "DELETE"), or
       after fetching all the rows of a "SELECT" statement.

       For "SELECT" statements, it is generally not possible to know how many
       rows will be returned except by fetching them all.  Some drivers will
       return the number of rows the application has fetched so far, but
       others may return -1 until all rows have been fetched.  So use of the
       "rows" method or $DBI::rows with "SELECT" statements is not
       recommended.

       One alternative method to get a row count for a "SELECT" is to execute
       a "SELECT COUNT(*) FROM ..." SQL statement with the same "..." as your
       query and then fetch the row count from that.

       "bind_col"

         $rc = $sth->bind_col($column_number, \$var_to_bind);
         $rc = $sth->bind_col($column_number, \$var_to_bind, \%attr );
         $rc = $sth->bind_col($column_number, \$var_to_bind, $bind_type );

       Binds a Perl variable and/or some attributes to an output column
       (field) of a "SELECT" statement.  Column numbers count up from 1.  You
       do not need to bind output columns in order to fetch data.  For maximum
       portability between drivers, bind_col() should be called after
       execute() and not before.  See also "bind_columns" for an example.

       The binding is performed at a low level using Perl aliasing.  Whenever
       a row is fetched from the database $var_to_bind appears to be
       automatically updated simply because it now refers to the same memory
       location as the corresponding column value.  This makes using bound
       variables very efficient.  Binding a tied variable doesn't work,
       currently.

       The "bind_param" method performs a similar, but opposite, function for
       input variables.

       Data Types for Column Binding

       The "\%attr" parameter can be used to hint at the data type formatting
       the column should have. For example, you can use:

         $sth->bind_col(1, undef, { TYPE => SQL_DATETIME });

       to specify that you'd like the column (which presumably is some kind of
       datetime type) to be returned in the standard format for SQL_DATETIME,
       which is 'YYYY-MM-DD HH:MM:SS', rather than the native formatting the
       database would normally use.

       There's no $var_to_bind in that example to emphasize the point that
       bind_col() works on the underlying column and not just a particular
       bound variable.

       As a short-cut for the common case, the data type can be passed
       directly, in place of the "\%attr" hash reference. This example is
       equivalent to the one above:

         $sth->bind_col(1, undef, SQL_DATETIME);

       The "TYPE" value indicates the standard (non-driver-specific) type for
       this parameter. To specify the driver-specific type, the driver may
       support a driver-specific attribute, such as "{ ora_type => 97 }".

       The SQL_DATETIME and other related constants can be imported using

         use DBI qw(:sql_types);

       See "DBI Constants" for more information.

       Few drivers support specifying a data type via a "bind_col" call (most
       will simply ignore the data type). Fewer still allow the data type to
       be altered once set.

       The TYPE attribute for bind_col() was first specified in DBI 1.41.

       "bind_columns"

         $rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);

       Calls "bind_col" for each column of the "SELECT" statement.

       The list of references should have the same number of elements as the
       number of columns in the "SELECT" statement. If it doesn't then
       "bind_columns" will bind the elements given, up to the number of
       columns, and then return an error.

       For maximum portability between drivers, bind_columns() should be
       called after execute() and not before.

       For example:

         $dbh->{RaiseError} = 1; # do this, or check every call for errors
         $sth = $dbh->prepare(q{ SELECT region, sales FROM sales_by_region });
         $sth->execute;
         my ($region, $sales);

         # Bind Perl variables to columns:
         $rv = $sth->bind_columns(\$region, \$sales);

         # you can also use Perl's \(...) syntax (see perlref docs):
         #     $sth->bind_columns(\($region, $sales));

         # Column binding is the most efficient way to fetch data
         while ($sth->fetch) {
             print "$region: $sales\n";
         }

       For compatibility with old scripts, the first parameter will be ignored
       if it is "undef" or a hash reference.

       Here's a more fancy example that binds columns to the values inside a
       hash (thanks to H.Merijn Brand):

         $sth->execute;
         my %row;
         $sth->bind_columns( \( @row{ @{$sth->{NAME_lc} } } ));
         while ($sth->fetch) {
             print "$row{region}: $row{sales}\n";
         }

       "dump_results"

         $rows = $sth->dump_results($maxlen, $lsep, $fsep, $fh);

       Fetches all the rows from $sth, calls "DBI::neat_list" for each row,
       and prints the results to $fh (defaults to "STDOUT") separated by $lsep
       (default "\n"). $fsep defaults to ", " and $maxlen defaults to 35.

       This method is designed as a handy utility for prototyping and testing
       queries. Since it uses "neat_list" to format and edit the string for
       reading by humans, it is not recommended for data transfer
       applications.

   Statement Handle Attributes
       This section describes attributes specific to statement handles. Most
       of these attributes are read-only.

       Changes to these statement handle attributes do not affect any other
       existing or future statement handles.

       Attempting to set or get the value of an unknown attribute generates a
       warning, except for private driver specific attributes (which all have
       names starting with a lowercase letter).

       Example:

         ... = $h->{NUM_OF_FIELDS};    # get/read

       Some drivers cannot provide valid values for some or all of these
       attributes until after "$sth->execute" has been successfully called.
       Typically the attribute will be "undef" in these situations.

       Some attributes, like NAME, are not appropriate to some types of
       statement, like SELECT. Typically the attribute will be "undef" in
       these situations.

       For drivers which support stored procedures and multiple result sets
       (see "more_results") these attributes relate to the current result set.

       See also "finish" to learn more about the effect it may have on some
       attributes.

       "NUM_OF_FIELDS"  (integer, read-only)

       Number of fields (columns) in the data the prepared statement may
       return.  Statements that don't return rows of data, like "DELETE" and
       "CREATE" set "NUM_OF_FIELDS" to 0 (though it may be undef in some
       drivers).

       "NUM_OF_PARAMS"  (integer, read-only)

       The number of parameters (placeholders) in the prepared statement.  See
       SUBSTITUTION VARIABLES below for more details.

       "NAME"  (array-ref, read-only)

       Returns a reference to an array of field names for each column. The
       names may contain spaces but should not be truncated or have any
       trailing space. Note that the names have the letter case (upper, lower
       or mixed) as returned by the driver being used. Portable applications
       should use "NAME_lc" or "NAME_uc".

         print "First column name: $sth->{NAME}->[0]\n";

       "NAME_lc"  (array-ref, read-only)

       Like "NAME" but always returns lowercase names.

       "NAME_uc"  (array-ref, read-only)

       Like "NAME" but always returns uppercase names.

       "NAME_hash"  (hash-ref, read-only)

       "NAME_lc_hash"  (hash-ref, read-only)

       "NAME_uc_hash"  (hash-ref, read-only)

       The "NAME_hash", "NAME_lc_hash", and "NAME_uc_hash" attributes return
       column name information as a reference to a hash.

       The keys of the hash are the names of the columns.  The letter case of
       the keys corresponds to the letter case returned by the "NAME",
       "NAME_lc", and "NAME_uc" attributes respectively (as described above).

       The value of each hash entry is the perl index number of the
       corresponding column (counting from 0). For example:

         $sth = $dbh->prepare("select Id, Name from table");
         $sth->execute;
         @row = $sth->fetchrow_array;
         print "Name $row[ $sth->{NAME_lc_hash}{name} ]\n";

       "TYPE"  (array-ref, read-only)

       Returns a reference to an array of integer values for each column. The
       value indicates the data type of the corresponding column.

       The values correspond to the international standards (ANSI X3.135 and
       ISO/IEC 9075) which, in general terms, means ODBC. Driver-specific
       types that don't exactly match standard types should generally return
       the same values as an ODBC driver supplied by the makers of the
       database. That might include private type numbers in ranges the vendor
       has officially registered with the ISO working group:

         ftp://sqlstandards.org/SC32/SQL_Registry/

       Where there's no vendor-supplied ODBC driver to be compatible with, the
       DBI driver can use type numbers in the range that is now officially
       reserved for use by the DBI: -9999 to -9000.

       All possible values for "TYPE" should have at least one entry in the
       output of the "type_info_all" method (see "type_info_all").

       "PRECISION"  (array-ref, read-only)

       Returns a reference to an array of integer values for each column.

       For numeric columns, the value is the maximum number of digits (without
       considering a sign character or decimal point). Note that the "display
       size" for floating point types (REAL, FLOAT, DOUBLE) can be up to 7
       characters greater than the precision (for the sign + decimal point +
       the letter E + a sign + 2 or 3 digits).

       For any character type column the value is the OCTET_LENGTH, in other
       words the number of bytes, not characters.

       (More recent standards refer to this as COLUMN_SIZE but we stick with
       PRECISION for backwards compatibility.)

       "SCALE"  (array-ref, read-only)

       Returns a reference to an array of integer values for each column.
       NULL ("undef") values indicate columns where scale is not applicable.

       "NULLABLE"  (array-ref, read-only)

       Returns a reference to an array indicating the possibility of each
       column returning a null.  Possible values are 0 (or an empty string) =
       no, 1 = yes, 2 = unknown.

         print "First column may return NULL\n" if $sth->{NULLABLE}->[0];

       "CursorName"  (string, read-only)

       Returns the name of the cursor associated with the statement handle, if
       available. If not available or if the database driver does not support
       the "where current of ..." SQL syntax, then it returns "undef".

       "Database"  (dbh, read-only)

       Returns the parent $dbh of the statement handle.

       "Statement"  (string, read-only)

       Returns the statement string passed to the "prepare" method.

       "ParamValues"  (hash ref, read-only)

       Returns a reference to a hash containing the values currently bound to
       placeholders.  The keys of the hash are the 'names' of the
       placeholders, typically integers starting at 1.  Returns undef if not
       supported by the driver.

       See "ShowErrorStatement" for an example of how this is used.

       * Keys:

       If the driver supports "ParamValues" but no values have been bound yet
       then the driver should return a hash with placeholders names in the
       keys but all the values undef, but some drivers may return a ref to an
       empty hash because they can't pre-determine the names.

       It is possible that the keys in the hash returned by "ParamValues" are
       not exactly the same as those implied by the prepared statement.  For
       example, DBD::Oracle translates '"?"' placeholders into '":pN"' where N
       is a sequence number starting at 1.

       * Values:

       It is possible that the values in the hash returned by "ParamValues"
       are not exactly the same as those passed to bind_param() or execute().
       The driver may have slightly modified values in some way based on the
       TYPE the value was bound with. For example a floating point value bound
       as an SQL_INTEGER type may be returned as an integer.  The values
       returned by "ParamValues" can be passed to another bind_param() method
       with the same TYPE and will be seen by the database as the same value.
       See also "ParamTypes" below.

       The "ParamValues" attribute was added in DBI 1.28.

       "ParamTypes"  (hash ref, read-only)

       Returns a reference to a hash containing the type information currently
       bound to placeholders.  Returns undef if not supported by the driver.

       * Keys:

       See "ParamValues" above.

       * Values:

       The hash values are hashrefs of type information in the same form as
       that passed to the various bind_param() methods (See "bind_param" for
       the format and values).

       It is possible that the values in the hash returned by "ParamTypes" are
       not exactly the same as those passed to bind_param() or execute().
       Param attributes specified using the abreviated form, like this:

           $sth->bind_param(1, SQL_INTEGER);

       are returned in the expanded form, as if called like this:

           $sth->bind_param(1, { TYPE => SQL_INTEGER });

       The driver may have modified the type information in some way based on
       the bound values, other hints provided by the prepare()'d SQL
       statement, or alternate type mappings required by the driver or target
       database system. The driver may also add private keys (with names
       beginning with the drivers reserved prefix, e.g., odbc_xxx).

       * Example:

       The keys and values in the returned hash can be passed to the various
       bind_param() methods to effectively reproduce a previous param binding.
       For example:

         # assuming $sth1 is a previously prepared statement handle
         my $sth2 = $dbh->prepare( $sth1->{Statement} );
         my $ParamValues = $sth1->{ParamValues} || {};
         my $ParamTypes  = $sth1->{ParamTypes}  || {};
         $sth2->bind_param($_, $PV->{$_} $PT->{$_})
           for keys %{ %$PV, %$PT };
         $sth2->execute();

       The "ParamTypes" attribute was added in DBI 1.49. Implementation is the
       responsibility of individual drivers; the DBI layer default
       implementation simply returns undef.

       "ParamArrays"  (hash ref, read-only)

       Returns a reference to a hash containing the values currently bound to
       placeholders with "execute_array" or "bind_param_array".  The keys of
       the hash are the 'names' of the placeholders, typically integers
       starting at 1.  Returns undef if not supported by the driver or no
       arrays of parameters are bound.

       Each key value is an array reference containing a list of the bound
       parameters for that column.

       For example:

         $sth = $dbh->prepare("INSERT INTO staff (id, name) values (?,?)");
         $sth->execute_array({},[1,2], ['fred','dave']);
         if ($sth->{ParamArrays}) {
             foreach $param (keys %{$sth->{ParamArrays}}) {
                 printf "Parameters for %s : %s\n", $param,
                 join(",", @{$sth->{ParamArrays}->{$param}});
             }
         }

       It is possible that the values in the hash returned by "ParamArrays"
       are not exactly the same as those passed to "bind_param_array" or
       "execute_array".  The driver may have slightly modified values in some
       way based on the TYPE the value was bound with. For example a floating
       point value bound as an SQL_INTEGER type may be returned as an integer.

       It is also possible that the keys in the hash returned by "ParamArrays"
       are not exactly the same as those implied by the prepared statement.
       For example, DBD::Oracle translates '"?"'  placeholders into '":pN"'
       where N is a sequence number starting at 1.

       "RowsInCache"  (integer, read-only)

       If the driver supports a local row cache for "SELECT" statements, then
       this attribute holds the number of un-fetched rows in the cache. If the
       driver doesn't, then it returns "undef". Note that some drivers pre-
       fetch rows on execute, whereas others wait till the first fetch.

       See also the "RowCacheSize" database handle attribute.

FURTHER INFORMATION
   Catalog Methods
       An application can retrieve metadata information from the DBMS by
       issuing appropriate queries on the views of the Information Schema.
       Unfortunately, "INFORMATION_SCHEMA" views are seldom supported by the
       DBMS.  Special methods (catalog methods) are available to return result
       sets for a small but important portion of that metadata:

         column_info
         foreign_key_info
         primary_key_info
         table_info
         statistics_info

       All catalog methods accept arguments in order to restrict the result
       sets.  Passing "undef" to an optional argument does not constrain the
       search for that argument.  However, an empty string ('') is treated as
       a regular search criteria and will only match an empty value.

       Note: SQL/CLI and ODBC differ in the handling of empty strings. An
       empty string will not restrict the result set in SQL/CLI.

       Most arguments in the catalog methods accept only ordinary values, e.g.
       the arguments of "primary_key_info()".  Such arguments are treated as a
       literal string, i.e. the case is significant and quote characters are
       taken literally.

       Some arguments in the catalog methods accept search patterns (strings
       containing '_' and/or '%'), e.g. the $table argument of
       "column_info()".  Passing '%' is equivalent to leaving the argument
       "undef".

       Caveat: The underscore ('_') is valid and often used in SQL
       identifiers.  Passing such a value to a search pattern argument may
       return more rows than expected!  To include pattern characters as
       literals, they must be preceded by an escape character which can be
       achieved with

         $esc = $dbh->get_info( 14 );  # SQL_SEARCH_PATTERN_ESCAPE
         $search_pattern =~ s/([_%])/$esc$1/g;

       The ODBC and SQL/CLI specifications define a way to change the default
       behaviour described above: All arguments (except list value arguments)
       are treated as identifier if the "SQL_ATTR_METADATA_ID" attribute is
       set to "SQL_TRUE".  Quoted identifiers are very similar to ordinary
       values, i.e. their body (the string within the quotes) is interpreted
       literally.  Unquoted identifiers are compared in UPPERCASE.

       The DBI (currently) does not support the "SQL_ATTR_METADATA_ID"
       attribute, i.e. it behaves like an ODBC driver where
       "SQL_ATTR_METADATA_ID" is set to "SQL_FALSE".

   Transactions
       Transactions are a fundamental part of any robust database system. They
       protect against errors and database corruption by ensuring that sets of
       related changes to the database take place in atomic (indivisible, all-
       or-nothing) units.

       This section applies to databases that support transactions and where
       "AutoCommit" is off.  See "AutoCommit" for details of using
       "AutoCommit" with various types of databases.

       The recommended way to implement robust transactions in Perl
       applications is to use "RaiseError" and "eval { ... }" (which is very
       fast, unlike "eval "...""). For example:

         $dbh->{AutoCommit} = 0;  # enable transactions, if possible
         $dbh->{RaiseError} = 1;
         eval {
             foo(...)        # do lots of work here
             bar(...)        # including inserts
             baz(...)        # and updates
             $dbh->commit;   # commit the changes if we get this far
         };
         if ($@) {
             warn "Transaction aborted because $@";
             # now rollback to undo the incomplete changes
             # but do it in an eval{} as it may also fail
             eval { $dbh->rollback };
             # add other application on-error-clean-up code here
         }

       If the "RaiseError" attribute is not set, then DBI calls would need to
       be manually checked for errors, typically like this:

         $h->method(@args) or die $h->errstr;

       With "RaiseError" set, the DBI will automatically "die" if any DBI
       method call on that handle (or a child handle) fails, so you don't have
       to test the return value of each method call. See "RaiseError" for more
       details.

       A major advantage of the "eval" approach is that the transaction will
       be properly rolled back if any code (not just DBI calls) in the inner
       application dies for any reason. The major advantage of using the
       "$h->{RaiseError}" attribute is that all DBI calls will be checked
       automatically. Both techniques are strongly recommended.

       After calling "commit" or "rollback" many drivers will not let you
       fetch from a previously active "SELECT" statement handle that's a child
       of the same database handle. A typical way round this is to connect the
       the database twice and use one connection for "SELECT" statements.

       See "AutoCommit" and "disconnect" for other important information about
       transactions.

   Handling BLOB / LONG / Memo Fields
       Many databases support "blob" (binary large objects), "long", or
       similar datatypes for holding very long strings or large amounts of
       binary data in a single field. Some databases support variable length
       long values over 2,000,000,000 bytes in length.

       Since values of that size can't usually be held in memory, and because
       databases can't usually know in advance the length of the longest long
       that will be returned from a "SELECT" statement (unlike other data
       types), some special handling is required.

       In this situation, the value of the "$h->{LongReadLen}" attribute is
       used to determine how much buffer space to allocate when fetching such
       fields.  The "$h->{LongTruncOk}" attribute is used to determine how to
       behave if a fetched value can't fit into the buffer.

       See the description of "LongReadLen" for more information.

       When trying to insert long or binary values, placeholders should be
       used since there are often limits on the maximum size of an "INSERT"
       statement and the "quote" method generally can't cope with binary data.
       See "Placeholders and Bind Values".

   Simple Examples
       Here's a complete example program to select and fetch some data:

         my $data_source = "dbi::DriverName:db_name";
         my $dbh = DBI->connect($data_source, $user, $password)
             or die "Can't connect to $data_source: $DBI::errstr";

         my $sth = $dbh->prepare( q{
                 SELECT name, phone
                 FROM mytelbook
         }) or die "Can't prepare statement: $DBI::errstr";

         my $rc = $sth->execute
             or die "Can't execute statement: $DBI::errstr";

         print "Query will return $sth->{NUM_OF_FIELDS} fields.\n\n";
         print "Field names: @{ $sth->{NAME} }\n";

         while (($name, $phone) = $sth->fetchrow_array) {
             print "$name: $phone\n";
         }
         # check for problems which may have terminated the fetch early
         die $sth->errstr if $sth->err;

         $dbh->disconnect;

       Here's a complete example program to insert some data from a file.
       (This example uses "RaiseError" to avoid needing to check each call).

         my $dbh = DBI->connect("dbi:DriverName:db_name", $user, $password, {
             RaiseError => 1, AutoCommit => 0
         });

         my $sth = $dbh->prepare( q{
             INSERT INTO table (name, phone) VALUES (?, ?)
         });

         open FH, "<phone.csv" or die "Unable to open phone.csv: $!";
         while (<FH>) {
             chomp;
             my ($name, $phone) = split /,/;
             $sth->execute($name, $phone);
         }
         close FH;

         $dbh->commit;
         $dbh->disconnect;

       Here's how to convert fetched NULLs (undefined values) into empty
       strings:

         while($row = $sth->fetchrow_arrayref) {
           # this is a fast and simple way to deal with nulls:
           foreach (@$row) { $_ = '' unless defined }
           print "@$row\n";
         }

       The "q{...}" style quoting used in these examples avoids clashing with
       quotes that may be used in the SQL statement. Use the double-quote like
       "qq{...}" operator if you want to interpolate variables into the
       string.  See "Quote and Quote-like Operators" in perlop for more
       details.

   Threads and Thread Safety
       Perl 5.7 and later support a new threading model called iThreads.  (The
       old "5.005 style" threads are not supported by the DBI.)

       In the iThreads model each thread has it's own copy of the perl
       interpreter.  When a new thread is created the original perl
       interpreter is 'cloned' to create a new copy for the new thread.

       If the DBI and drivers are loaded and handles created before the thread
       is created then it will get a cloned copy of the DBI, the drivers and
       the handles.

       However, the internal pointer data within the handles will refer to the
       DBI and drivers in the original interpreter. Using those handles in the
       new interpreter thread is not safe, so the DBI detects this and croaks
       on any method call using handles that don't belong to the current
       thread (except for DESTROY).

       Because of this (possibly temporary) restriction, newly created threads
       must make their own connections to the database. Handles can't be
       shared across threads.

       But BEWARE, some underlying database APIs (the code the DBD driver uses
       to talk to the database, often supplied by the database vendor) are not
       thread safe. If it's not thread safe, then allowing more than one
       thread to enter the code at the same time may cause subtle/serious
       problems. In some cases allowing more than one thread to enter the
       code, even if not at the same time, can cause problems. You have been
       warned.

       Using DBI with perl threads is not yet recommended for production
       environments. For more information see
       <http://www.perlmonks.org/index.pl?node_id=288022>

       Note: There is a bug in perl 5.8.2 when configured with threads and
       debugging enabled (bug #24463) which causes a DBI test to fail.

   Signal Handling and Canceling Operations
       [The following only applies to systems with unix-like signal handling.
       I'd welcome additions for other systems, especially Windows.]

       The first thing to say is that signal handling in Perl versions less
       than 5.8 is not safe. There is always a small risk of Perl crashing
       and/or core dumping when, or after, handling a signal because the
       signal could arrive and be handled while internal data structures are
       being changed. If the signal handling code used those same internal
       data structures it could cause all manner of subtle and not-so-subtle
       problems.  The risk was reduced with 5.4.4 but was still present in all
       perls up through 5.8.0.

       Beginning in perl 5.8.0 perl implements 'safe' signal handling if your
       system has the POSIX sigaction() routine. Now when a signal is
       delivered perl just makes a note of it but does not run the %SIG
       handler. The handling is 'deferred' until a 'safe' moment.

       Although this change made signal handling safe, it also lead to a
       problem with signals being deferred for longer than you'd like.  If a
       signal arrived while executing a system call, such as waiting for data
       on a network connection, the signal is noted and then the system call
       that was executing returns with an EINTR error code to indicate that it
       was interrupted. All fine so far.

       The problem comes when the code that made the system call sees the
       EINTR code and decides it's going to call it again. Perl doesn't do
       that, but database code sometimes does. If that happens then the signal
       handler doesn't get called until later. Maybe much later.

       Fortunately there are ways around this which we'll discuss below.
       Unfortunately they make signals unsafe again.

       The two most common uses of signals in relation to the DBI are for
       canceling operations when the user types Ctrl-C (interrupt), and for
       implementing a timeout using "alarm()" and $SIG{ALRM}.

       Cancel
           The DBI provides a "cancel" method for statement handles. The
           "cancel" method should abort the current operation and is designed
           to be called from a signal handler.  For example:

             $SIG{INT} = sub { $sth->cancel };

           However, few drivers implement this (the DBI provides a default
           method that just returns "undef") and, even if implemented, there
           is still a possibility that the statement handle, and even the
           parent database handle, will not be usable afterwards.

           If "cancel" returns true, then it has successfully invoked the
           database engine's own cancel function.  If it returns false, then
           "cancel" failed. If it returns "undef", then the database driver
           does not have cancel implemented - very few do.

       Timeout
           The traditional way to implement a timeout is to set $SIG{ALRM} to
           refer to some code that will be executed when an ALRM signal
           arrives and then to call alarm($seconds) to schedule an ALRM signal
           to be delivered $seconds in the future. For example:

             eval {
               local $SIG{ALRM} = sub { die "TIMEOUT\n" };
               alarm($seconds);
               ... code to execute with timeout here ...
               alarm(0);  # cancel alarm (if code ran fast)
             };
             alarm(0);    # cancel alarm (if eval failed)
             if ( $@ eq "TIMEOUT\n" ) { ... }

           Unfortunately, as described above, this won't always work as
           expected, depending on your perl version and the underlying
           database code.

           With Oracle for instance (DBD::Oracle), if the system which hosts
           the database is down the DBI->connect() call will hang for several
           minutes before returning an error.

       The solution on these systems is to use the "POSIX::sigaction()"
       routine to gain low level access to how the signal handler is
       installed.

       The code would look something like this (for the DBD-Oracle connect()):

          use POSIX ':signal_h';

          my $mask = POSIX::SigSet->new( SIGALRM ); # signals to mask in the handler
          my $action = POSIX::SigAction->new(
              sub { die "connect timeout" },        # the handler code ref
              $mask,
              # not using (perl 5.8.2 and later) 'safe' switch or sa_flags
          );
          my $oldaction = POSIX::SigAction->new();
          sigaction( 'ALRM', $action, $oldaction );
          my $dbh;
          eval {
             alarm(5); # seconds before time out
             $dbh = DBI->connect("dbi:Oracle:$dsn" ... );
             alarm(0); # cancel alarm (if connect worked fast)
          };
          alarm(0);    # cancel alarm (if eval failed)
          sigaction( 'ALRM', $oldaction );  # restore original signal handler
          if ( $@ ) ....

       Similar techniques can be used for canceling statement execution.

       Unfortunately, this solution is somewhat messy, and it does not work
       with perl versions less than perl 5.8 where "POSIX::sigaction()"
       appears to be broken.

       For a cleaner implementation that works across perl versions, see
       Lincoln Baxter's Sys::SigAction module at
       http://search.cpan.org/~lbaxter/Sys-SigAction/
       <http://search.cpan.org/~lbaxter/Sys-SigAction/>.  The documentation
       for Sys::SigAction includes an longer discussion of this problem, and a
       DBD::Oracle test script.

       Be sure to read all the signal handling sections of the perlipc manual.

       And finally, two more points to keep firmly in mind. Firstly, remember
       that what we've done here is essentially revert to old style unsafe
       handling of these signals. So do as little as possible in the handler.
       Ideally just die(). Secondly, the handles in use at the time the signal
       is handled may not be safe to use afterwards.

   Subclassing the DBI
       DBI can be subclassed and extended just like any other object oriented
       module.  Before we talk about how to do that, it's important to be
       clear about the various DBI classes and how they work together.

       By default "$dbh = DBI->connect(...)" returns a $dbh blessed into the
       "DBI::db" class.  And the "$dbh->prepare" method returns an $sth
       blessed into the "DBI::st" class (actually it simply changes the last
       four characters of the calling handle class to be "::st").

       The leading '"DBI"' is known as the 'root class' and the extra '"::db"'
       or '"::st"' are the 'handle type suffixes'. If you want to subclass the
       DBI you'll need to put your overriding methods into the appropriate
       classes.  For example, if you want to use a root class of "MySubDBI"
       and override the do(), prepare() and execute() methods, then your do()
       and prepare() methods should be in the "MySubDBI::db" class and the
       execute() method should be in the "MySubDBI::st" class.

       To setup the inheritance hierarchy the @ISA variable in "MySubDBI::db"
       should include "DBI::db" and the @ISA variable in "MySubDBI::st" should
       include "DBI::st".  The "MySubDBI" root class itself isn't currently
       used for anything visible and so, apart from setting @ISA to include
       "DBI", it can be left empty.

       So, having put your overriding methods into the right classes, and
       setup the inheritance hierarchy, how do you get the DBI to use them?
       You have two choices, either a static method call using the name of
       your subclass:

         $dbh = MySubDBI->connect(...);

       or specifying a "RootClass" attribute:

         $dbh = DBI->connect(..., { RootClass => 'MySubDBI' });

       If both forms are used then the attribute takes precedence.

       The only differences between the two are that using an explicit
       RootClass attribute will a) make the DBI automatically attempt to load
       a module by that name if the class doesn't exist, and b) won't call
       your MySubDBI::connect() method, if you have one.

       When subclassing is being used then, after a successful new connect,
       the DBI->connect method automatically calls:

         $dbh->connected($dsn, $user, $pass, \%attr);

       The default method does nothing. The call is made just to simplify any
       post-connection setup that your subclass may want to perform.  The
       parameters are the same as passed to DBI->connect.  If your subclass
       supplies a connected method, it should be part of the MySubDBI::db
       package.

       One more thing to note: you must let the DBI do the handle creation.
       If you want to override the connect() method in your *::dr class then
       it must still call SUPER::connect to get a $dbh to work with.
       Similarly, an overridden prepare() method in *::db must still call
       SUPER::prepare to get a $sth.  If you try to create your own handles
       using bless() then you'll find the DBI will reject them with an "is not
       a DBI handle (has no magic)" error.

       Here's a brief example of a DBI subclass.  A more thorough example can
       be found in t/subclass.t in the DBI distribution.

         package MySubDBI;

         use strict;

         use DBI;
         use vars qw(@ISA);
         @ISA = qw(DBI);

         package MySubDBI::db;
         use vars qw(@ISA);
         @ISA = qw(DBI::db);

         sub prepare {
           my ($dbh, @args) = @_;
           my $sth = $dbh->SUPER::prepare(@args)
               or return;
           $sth->{private_mysubdbi_info} = { foo => 'bar' };
           return $sth;
         }

         package MySubDBI::st;
         use vars qw(@ISA);
         @ISA = qw(DBI::st);

         sub fetch {
           my ($sth, @args) = @_;
           my $row = $sth->SUPER::fetch(@args)
               or return;
           do_something_magical_with_row_data($row)
               or return $sth->set_err(1234, "The magic failed", undef, "fetch");
           return $row;
         }

       When calling a SUPER::method that returns a handle, be careful to check
       the return value before trying to do other things with it in your
       overridden method. This is especially important if you want to set a
       hash attribute on the handle, as Perl's autovivification will bite you
       by (in)conveniently creating an unblessed hashref, which your method
       will then return with usually baffling results later on like the error
       "dbih_getcom handle HASH(0xa4451a8) is not a DBI handle (has no magic".
       It's best to check right after the call and return undef immediately on
       error, just like DBI would and just like the example above.

       If your method needs to record an error it should call the set_err()
       method with the error code and error string, as shown in the example
       above. The error code and error string will be recorded in the handle
       and available via "$h->err" and $DBI::errstr etc.  The set_err() method
       always returns an undef or empty list as appropriate. Since your method
       should nearly always return an undef or empty list as soon as an error
       is detected it's handy to simply return what set_err() returns, as
       shown in the example above.

       If the handle has "RaiseError", "PrintError", or "HandleError" etc. set
       then the set_err() method will honour them. This means that if
       "RaiseError" is set then set_err() won't return in the normal way but
       will 'throw an exception' that can be caught with an "eval" block.

       You can stash private data into DBI handles via "$h->{private_..._*}".
       See the entry under "ATTRIBUTES COMMON TO ALL HANDLES" for info and
       important caveats.

TRACING
       The DBI has a powerful tracing mechanism built in. It enables you to
       see what's going on 'behind the scenes', both within the DBI and the
       drivers you're using.

   Trace Settings
       Which details are written to the trace output is controlled by a
       combination of a trace level, an integer from 0 to 15, and a set of
       trace flags that are either on or off. Together these are known as the
       trace settings and are stored together in a single integer.  For normal
       use you only need to set the trace level, and generally only to a value
       between 1 and 4.

       Each handle has it's own trace settings, and so does the DBI.  When you
       call a method the DBI merges the handles settings into its own for the
       duration of the call: the trace flags of the handle are OR'd into the
       trace flags of the DBI, and if the handle has a higher trace level then
       the DBI trace level is raised to match it.  The previous DBI trace
       settings are restored when the called method returns.

   Trace Levels
       Trace levels are as follows:

         0 - Trace disabled.
         1 - Trace top-level DBI method calls returning with results or errors.
         2 - As above, adding tracing of top-level method entry with parameters.
         3 - As above, adding some high-level information from the driver
             and some internal information from the DBI.
         4 - As above, adding more detailed information from the driver.
             This is the first level to trace all the rows being fetched.
         5 to 15 - As above but with more and more internal information.

       Trace level 1 is best for a simple overview of what's happening.  Trace
       levels 2 thru 4 a good choice for general purpose tracing.  Levels 5
       and above are best reserved for investigating a specific problem, when
       you need to see "inside" the driver and DBI.

       The trace output is detailed and typically very useful. Much of the
       trace output is formatted using the "neat" function, so strings in the
       trace output may be edited and truncated by that function.

   Trace Flags
       Trace flags are used to enable tracing of specific activities within
       the DBI and drivers. The DBI defines some trace flags and drivers can
       define others. DBI trace flag names begin with a capital letter and
       driver specific names begin with a lowercase letter, as usual.

       Currently the DBI only defines two trace flags:

         ALL - turn on all DBI and driver flags (not recommended)
         SQL - trace SQL statements executed
               (not yet implemented in DBI but implemented in some DBDs)

       The "parse_trace_flags" and "parse_trace_flag" methods are used to
       convert trace flag names into the corresponding integer bit flags.

   Enabling Trace
       The "$h->trace" method sets the trace settings for a handle and
       "DBI->trace" does the same for the DBI.

       In addition to the "trace" method, you can enable the same trace
       information, and direct the output to a file, by setting the
       "DBI_TRACE" environment variable before starting Perl.  See "DBI_TRACE"
       for more information.

       Finally, you can set, or get, the trace settings for a handle using the
       "TraceLevel" attribute.

       All of those methods use parse_trace_flags() and so allow you set both
       the trace level and multiple trace flags by using a string containing
       the trace level and/or flag names separated by vertical bar (""|"") or
       comma ("","") characters. For example:

         local $h->{TraceLevel} = "3|SQL|foo";

   Trace Output
       Initially trace output is written to "STDERR".  Both the "$h->trace"
       and "DBI->trace" methods take an optional $trace_file parameter, which
       may be either the name of a file to be opened by DBI in append mode, or
       a reference to an existing writable (possibly layered) filehandle. If
       $trace_file is a filename, and can be opened in append mode, or
       $trace_file is a writable filehandle, then all trace output (currently
       including that from other handles) is redirected to that file. A
       warning is generated if $trace_file can't be opened or is not writable.

       Further calls to trace() without $trace_file do not alter where the
       trace output is sent. If $trace_file is undefined, then trace output is
       sent to "STDERR" and, if the prior trace was opened with $trace_file as
       a filename, the previous trace file is closed; if $trace_file was a
       filehandle, the filehandle is not closed.

       NOTE: If $trace_file is specified as a filehandle, the filehandle
       should not be closed until all DBI operations are completed, or the
       application has reset the trace file via another call to "trace()" that
       changes the trace file.

   Tracing to Layered Filehandles
       NOTE:

       o   Tied filehandles are not currently supported, as tie operations are
           not available to the PerlIO methods used by the DBI.

       o   PerlIO layer support requires Perl version 5.8 or higher.

       As of version 5.8, Perl provides the ability to layer various
       "disciplines" on an open filehandle via the PerlIO module.

       A simple example of using PerlIO layers is to use a scalar as the
       output:

           my $scalar = '';
           open( my $fh, "+>:scalar", \$scalar );
           $dbh->trace( 2, $fh );

       Now all trace output is simply appended to $scalar.

       A more complex application of tracing to a layered filehandle is the
       use of a custom layer (Refer to Perlio::via for details on creating
       custom PerlIO layers.). Consider an application with the following
       logger module:

           package MyFancyLogger;

           sub new
           {
               my $self = {};
               my $fh;
               open $fh, '>', 'fancylog.log';
               $self->{_fh} = $fh;
               $self->{_buf} = '';
               return bless $self, shift;
           }

           sub log
           {
               my $self = shift;
               return unless exists $self->{_fh};
               my $fh = $self->{_fh};
               $self->{_buf} .= shift;
           #
           # DBI feeds us pieces at a time, so accumulate a complete line
           # before outputing
           #
               print $fh "At ", scalar localtime(), ':', $self->{_buf}, "\n" and
               $self->{_buf} = ''
                   if $self->{_buf}=~tr/\n//;
           }

           sub close {
               my $self = shift;
               return unless exists $self->{_fh};
               my $fh = $self->{_fh};
               print $fh "At ", scalar localtime(), ':', $self->{_buf}, "\n" and
               $self->{_buf} = ''
                   if $self->{_buf};
               close $fh;
               delete $self->{_fh};
           }

           1;

       To redirect DBI traces to this logger requires creating a package for
       the layer:

           package PerlIO::via::MyFancyLogLayer;

           sub PUSHED
           {
               my ($class,$mode,$fh) = @_;
               my $logger;
               return bless \$logger,$class;
           }

           sub OPEN {
               my ($self, $path, $mode, $fh) = @_;
               #
               # $path is actually our logger object
               #
               $$self = $path;
               return 1;
           }

           sub WRITE
           {
               my ($self, $buf, $fh) = @_;
               $$self->log($buf);
               return length($buf);
           }

           sub CLOSE {
               my $self = shift;
               $$self->close();
               return 0;
           }

           1;

       The application can then cause DBI traces to be routed to the logger
       using

           use PerlIO::via::MyFancyLogLayer;

           open my $fh, '>:via(MyFancyLogLayer)', MyFancyLogger->new();

           $dbh->trace('SQL', $fh);

       Now all trace output will be processed by MyFancyLogger's log() method.

   Trace Content
       Many of the values embedded in trace output are formatted using the
       neat() utility function. This means they may be quoted, sanitized, and
       possibly truncated if longer than $DBI::neat_maxlen. See "neat" for
       more details.

   Tracing Tips
       You can add tracing to your own application code using the "trace_msg"
       method.

       It can sometimes be handy to compare trace files from two different
       runs of the same script. However using a tool like "diff" on the
       original log output doesn't work well because the trace file is full of
       object addresses that may differ on each run.

       The DBI includes a handy utility called dbilogstrip that can be used to
       'normalize' the log content. It can be used as a filter like this:

           DBI_TRACE=2 perl yourscript.pl ...args1... 2>&1 | dbilogstrip > dbitrace1.log
           DBI_TRACE=2 perl yourscript.pl ...args2... 2>&1 | dbilogstrip > dbitrace2.log
           diff -u dbitrace1.log dbitrace2.log

       See dbilogstrip for more information.

DBI ENVIRONMENT VARIABLES
       The DBI module recognizes a number of environment variables, but most
       of them should not be used most of the time.  It is better to be
       explicit about what you are doing to avoid the need for environment
       variables, especially in a web serving system where web servers are
       stingy about which environment variables are available.

   DBI_DSN
       The DBI_DSN environment variable is used by DBI->connect if you do not
       specify a data source when you issue the connect.  It should have a
       format such as "dbi:Driver:databasename".

   DBI_DRIVER
       The DBI_DRIVER environment variable is used to fill in the database
       driver name in DBI->connect if the data source string starts "dbi::"
       (thereby omitting the driver).  If DBI_DSN omits the driver name,
       DBI_DRIVER can fill the gap.

   DBI_AUTOPROXY
       The DBI_AUTOPROXY environment variable takes a string value that starts
       "dbi:Proxy:" and is typically followed by "hostname=...;port=...".  It
       is used to alter the behaviour of DBI->connect.  For full details, see
       DBI::Proxy documentation.

   DBI_USER
       The DBI_USER environment variable takes a string value that is used as
       the user name if the DBI->connect call is given undef (as distinct from
       an empty string) as the username argument.  Be wary of the security
       implications of using this.

   DBI_PASS
       The DBI_PASS environment variable takes a string value that is used as
       the password if the DBI->connect call is given undef (as distinct from
       an empty string) as the password argument.  Be extra wary of the
       security implications of using this.

   DBI_DBNAME (obsolete)
       The DBI_DBNAME environment variable takes a string value that is used
       only when the obsolescent style of DBI->connect (with driver name as
       fourth parameter) is used, and when no value is provided for the first
       (database name) argument.

   DBI_TRACE
       The DBI_TRACE environment variable specifies the global default trace
       settings for the DBI at startup. Can also be used to direct trace
       output to a file. When the DBI is loaded it does:

         DBI->trace(split /=/, $ENV{DBI_TRACE}, 2) if $ENV{DBI_TRACE};

       So if "DBI_TRACE" contains an ""="" character then what follows it is
       used as the name of the file to append the trace to.

       output appended to that file. If the name begins with a number followed
       by an equal sign ("="), then the number and the equal sign are stripped
       off from the name, and the number is used to set the trace level. For
       example:

         DBI_TRACE=1=dbitrace.log perl your_test_script.pl

       On Unix-like systems using a Bourne-like shell, you can do this easily
       on the command line:

         DBI_TRACE=2 perl your_test_script.pl

       See "TRACING" for more information.

   PERL_DBI_DEBUG (obsolete)
       An old variable that should no longer be used; equivalent to DBI_TRACE.

   DBI_PROFILE
       The DBI_PROFILE environment variable can be used to enable profiling of
       DBI method calls. See DBI::Profile for more information.

   DBI_PUREPERL
       The DBI_PUREPERL environment variable can be used to enable the use of
       DBI::PurePerl.  See DBI::PurePerl for more information.

WARNING AND ERROR MESSAGES
   Fatal Errors
       Can't call method "prepare" without a package or object reference
           The $dbh handle you're using to call "prepare" is probably
           undefined because the preceding "connect" failed. You should always
           check the return status of DBI methods, or use the "RaiseError"
           attribute.

       Can't call method "execute" without a package or object reference
           The $sth handle you're using to call "execute" is probably
           undefined because the preceding "prepare" failed. You should always
           check the return status of DBI methods, or use the "RaiseError"
           attribute.

       DBI/DBD internal version mismatch
           The DBD driver module was built with a different version of DBI
           than the one currently being used.  You should rebuild the DBD
           module under the current version of DBI.

           (Some rare platforms require "static linking". On those platforms,
           there may be an old DBI or DBD driver version actually embedded in
           the Perl executable being used.)

       DBD driver has not implemented the AutoCommit attribute
           The DBD driver implementation is incomplete. Consult the author.

       Can't [sg]et %s->{%s}: unrecognised attribute
           You attempted to set or get an unknown attribute of a handle.  Make
           sure you have spelled the attribute name correctly; case is
           significant (e.g., "Autocommit" is not the same as "AutoCommit").

Pure-Perl DBI
       A pure-perl emulation of the DBI is included in the distribution for
       people using pure-perl drivers who, for whatever reason, can't install
       the compiled DBI. See DBI::PurePerl.

SEE ALSO
   Driver and Database Documentation
       Refer to the documentation for the DBD driver that you are using.

       Refer to the SQL Language Reference Manual for the database engine that
       you are using.

   ODBC and SQL/CLI Standards Reference Information
       More detailed information about the semantics of certain DBI methods
       that are based on ODBC and SQL/CLI standards is available on-line via
       microsoft.com, for ODBC, and www.jtc1sc32.org for the SQL/CLI standard:

        DBI method        ODBC function     SQL/CLI Working Draft
        ----------        -------------     ---------------------
        column_info       SQLColumns        Page 124
        foreign_key_info  SQLForeignKeys    Page 163
        get_info          SQLGetInfo        Page 214
        primary_key_info  SQLPrimaryKeys    Page 254
        table_info        SQLTables         Page 294
        type_info         SQLGetTypeInfo    Page 239
        statistics_info   SQLStatistics

       For example, for ODBC information on SQLColumns you'd visit:

         http://msdn.microsoft.com/library/en-us/odbc/htm/odbcsqlcolumns.asp

       If that URL ceases to work then use the MSDN search facility at:

         http://search.microsoft.com/us/dev/

       and search for "SQLColumns returns" using the exact phrase option.  The
       link you want will probably just be called "SQLColumns" and will be
       part of the Data Access SDK.

       And for SQL/CLI standard information on SQLColumns you'd read page 124
       of the (very large) SQL/CLI Working Draft available from:

         http://jtc1sc32.org/doc/N0701-0750/32N0744T.pdf

   Standards Reference Information
       A hyperlinked, browsable version of the BNF syntax for SQL92 (plus
       Oracle 7 SQL and PL/SQL) is available here:

         http://cui.unige.ch/db-research/Enseignement/analyseinfo/SQL92/BNFindex.html

       A BNF syntax for SQL3 is available here:

         http://www.sqlstandards.org/SC32/WG3/Progression_Documents/Informal_working_drafts/iso-9075-2-1999.bnf

       The following links provide further useful information about SQL.  Some
       of these are rather dated now but may still be useful.

         http://www.jcc.com/SQLPages/jccs_sql.htm
         http://www.contrib.andrew.cmu.edu/~shadow/sql.html
         http://www.altavista.com/query?q=sql+tutorial

   Books and Articles
       Programming the Perl DBI, by Alligator Descartes and Tim Bunce.
       <http://books.perl.org/book/154>

       Programming Perl 3rd Ed. by Larry Wall, Tom Christiansen & Jon Orwant.
       <http://books.perl.org/book/134>

       Learning Perl by Randal Schwartz.  <http://books.perl.org/book/101>

       Details of many other books related to perl can be found at
       <http://books.perl.org>

   Perl Modules
       Index of DBI related modules available from CPAN:

        http://search.cpan.org/search?mode=module&query=DBIx%3A%3A
        http://search.cpan.org/search?mode=doc&query=DBI

       For a good comparison of RDBMS-OO mappers and some OO-RDBMS mappers
       (including Class::DBI, Alzabo, and DBIx::RecordSet in the former
       category and Tangram and SPOPS in the latter) see the Perl Object-
       Oriented Persistence project pages at:

        http://poop.sourceforge.net

       A similar page for Java toolkits can be found at:

        http://c2.com/cgi-bin/wiki?ObjectRelationalToolComparison

   Mailing List
       The dbi-users mailing list is the primary means of communication among
       users of the DBI and its related modules. For details send email to:

        dbi-users-help@perl.org

       There are typically between 700 and 900 messages per month.  You have
       to subscribe in order to be able to post. However you can opt for a
       'post-only' subscription.

       Mailing list archives (of variable quality) are held at:

        http://groups.google.com/groups?group=perl.dbi.users
        http://www.xray.mpe.mpg.de/mailing-lists/dbi/
        http://www.mail-archive.com/dbi-users%40perl.org/

   Assorted Related WWW Links
       The DBI "Home Page":

        http://dbi.perl.org/

       Other DBI related links:

        http://tegan.deltanet.com/~phlip/DBUIdoc.html
        http://dc.pm.org/perl_db.html
        http://wdvl.com/Authoring/DB/Intro/toc.html
        http://www.hotwired.com/webmonkey/backend/tutorials/tutorial1.html
        http://bumppo.net/lists/macperl/1999/06/msg00197.html
        http://gmax.oltrelinux.com/dbirecipes.html

       Other database related links:

        http://www.jcc.com/sql_stnd.html
        http://cuiwww.unige.ch/OSG/info/FreeDB/FreeDB.home.html
        http://www.connectionstrings.com/

       Security, especially the "SQL Injection" attack:

        http://www.ngssoftware.com/research/papers.html
        http://www.ngssoftware.com/papers/advanced_sql_injection.pdf
        http://www.ngssoftware.com/papers/more_advanced_sql_injection.pdf
        http://www.esecurityplanet.com/trends/article.php/2243461
        http://www.spidynamics.com/papers/SQLInjectionWhitePaper.pdf
        http://www.imperva.com/application_defense_center/white_papers/blind_sql_server_injection.html
        http://online.securityfocus.com/infocus/1644

       Commercial and Data Warehouse Links

        http://www.dwinfocenter.org
        http://www.datawarehouse.com
        http://www.datamining.org
        http://www.olapcouncil.org
        http://www.idwa.org
        http://www.knowledgecenters.org/dwcenter.asp

       Recommended Perl Programming Links

        http://language.perl.com/style/

   FAQ
       See http://faq.dbi-support.com/ <http://faq.dbi-support.com/>

AUTHORS
       DBI by Tim Bunce, <http://www.tim.bunce.name>

       This pod text by Tim Bunce, J. Douglas Dunlop, Jonathan Leffler and
       others.  Perl by Larry Wall and the "perl5-porters".

COPYRIGHT
       The DBI module is Copyright (c) 1994-2009 Tim Bunce. Ireland.  All
       rights reserved.

       You may distribute under the terms of either the GNU General Public
       License or the Artistic License, as specified in the Perl 5.10.0 README
       file.

SUPPORT / WARRANTY
       The DBI is free Open Source software. IT COMES WITHOUT WARRANTY OF ANY
       KIND.

   Support
       My consulting company, Data Plan Services, offers annual and multi-
       annual support contracts for the DBI. These provide sustained support
       for DBI development, and sustained value for you in return.  Contact me
       for details.

   Sponsor Enhancements
       The DBI Roadmap is available at
       <http://search.cpan.org/~timb/DBI/Roadmap.pod>

       If your company would benefit from a specific new DBI feature, please
       consider sponsoring its development.  Work is performed rapidly, and
       usually on a fixed-price payment-on-delivery basis.  Contact me for
       details.

       Using such targeted financing allows you to contribute to DBI
       development, and rapidly get something specific and valuable in return.

ACKNOWLEDGEMENTS
       I would like to acknowledge the valuable contributions of the many
       people I have worked with on the DBI project, especially in the early
       years (1992-1994). In no particular order: Kevin Stock, Buzz Moschetti,
       Kurt Andersen, Ted Lemon, William Hails, Garth Kennedy, Michael
       Peppler, Neil S. Briscoe, Jeff Urlwin, David J. Hughes, Jeff Stander,
       Forrest D Whitcher, Larry Wall, Jeff Fried, Roy Johnson, Paul Hudson,
       Georg Rehfeld, Steve Sizemore, Ron Pool, Jon Meek, Tom Christiansen,
       Steve Baumgarten, Randal Schwartz, and a whole lot more.

       Then, of course, there are the poor souls who have struggled through
       untold and undocumented obstacles to actually implement DBI drivers.
       Among their ranks are Jochen Wiedmann, Alligator Descartes, Jonathan
       Leffler, Jeff Urlwin, Michael Peppler, Henrik Tougaard, Edwin Pratomo,
       Davide Migliavacca, Jan Pazdziora, Peter Haworth, Edmund Mergl, Steve
       Williams, Thomas Lowery, and Phlip Plumlee. Without them, the DBI would
       not be the practical reality it is today.  I'm also especially grateful
       to Alligator Descartes for starting work on the first edition of the
       "Programming the Perl DBI" book and letting me jump on board.

       The DBI and DBD::Oracle were originally developed while I was Technical
       Director (CTO) of Ingeneering in the UK (<http://www.ig.co.uk>)
       (formerly known as the Paul Ingram Group).  So I'd especially like to
       thank Paul for his generosity and vision in supporting this work for
       many years.

       A couple of specific DBI features have been sponsored by enlightened
       companies:

       The development of the swap_inner_handle() method was sponsored by
       BizRate.com (<http://BizRate.com>)

       The development of DBD::Gofer and related modules was sponsored by
       Shopzilla.com (<http://Shopzilla.com>), where I currently work.

CONTRIBUTING
       As you can see above, many people have contributed to the DBI and
       drivers in many ways over many years.

       If you'd like to help then see <http://dbi.perl.org/contributing> and
       <http://search.cpan.org/~timb/DBI/Roadmap.pod>

       If you'd like the DBI to do something new or different then a good way
       to make that happen is to do it yourself and send me a patch to the
       source code that shows the changes. (But read "Speak before you patch"
       below.)

   Browsing the source code repository
       Use http://svn.perl.org/modules/dbi/trunk (basic) or
       http://svn.perl.org/viewcvs/modules/ (more useful)

   How to create a patch using Subversion
       The DBI source code is maintained using Subversion (a replacement for
       CVS, see <http://subversion.tigris.org/>). To access the source you'll
       need to install a Subversion client. Then, to get the source code, do:

         svn checkout http://svn.perl.org/modules/dbi/trunk

       If it prompts for a username and password use your perl.org account if
       you have one, else just 'guest' and 'guest'. The source code will be in
       a new subdirectory called "trunk".

       To keep informed about changes to the source you can send an empty
       email to svn-commit-modules-dbi-subscribe@perl.org after which you'll
       get an email with the change log message and diff of each change
       checked-in to the source.

       After making your changes you can generate a patch file, but before you
       do, make sure your source is still up to date using:

         svn update

       If you get any conflicts reported you'll need to fix them first.  Then
       generate the patch file from within the "trunk" directory using:

         svn diff > foo.patch

       Read the patch file, as a sanity check, and then email it to
       dbi-dev@perl.org.

   How to create a patch without Subversion
       Unpack a fresh copy of the distribution:

         tar xfz DBI-1.40.tar.gz

       Rename the newly created top level directory:

         mv DBI-1.40 DBI-1.40.your_foo

       Edit the contents of DBI-1.40.your_foo/* till it does what you want.

       Test your changes and then remove all temporary files:

         make test && make distclean

       Go back to the directory you originally unpacked the distribution:

         cd ..

       Unpack another copy of the original distribution you started with:

         tar xfz DBI-1.40.tar.gz

       Then create a patch file by performing a recursive "diff" on the two
       top level directories:

         diff -r -u DBI-1.40 DBI-1.40.your_foo > DBI-1.40.your_foo.patch

   Speak before you patch
       For anything non-trivial or possibly controversial it's a good idea to
       discuss (on dbi-dev@perl.org) the changes you propose before actually
       spending time working on them. Otherwise you run the risk of them being
       rejected because they don't fit into some larger plans you may not be
       aware of.

TRANSLATIONS
       A German translation of this manual (possibly slightly out of date) is
       available, thanks to O'Reilly, at:

         http://www.oreilly.de/catalog/perldbiger/

       Some other translations:

        http://cronopio.net/perl/                              - Spanish
        http://member.nifty.ne.jp/hippo2000/dbimemo.htm        - Japanese

TRAINING
       References to DBI related training resources. No recommendation
       implied.

         http://www.treepax.co.uk/
         http://www.keller.com/dbweb/

       (If you offer professional DBI related training services, please send
       me your details so I can add them here.)

OTHER RELATED WORK AND PERL MODULES
       Apache::DBI by E.Mergl@bawue.de
           To be used with the Apache daemon together with an embedded Perl
           interpreter like "mod_perl". Establishes a database connection
           which remains open for the lifetime of the HTTP daemon. This way
           the CGI connect and disconnect for every database access becomes
           superfluous.

       SQL Parser
           See also the SQL::Statement module, SQL parser and engine.

perl v5.12.1                      2009-06-05                            DBI(3)
 

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