Manpages - DBI.3pm

General

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 http://dbi.perl.org/support/

You might also like to read the Advanced DBI Tutorial at http://www.slideshare.net/Tim.Bunce/dbi-advanced-tutorial-2007

To help you make the best use of the dbi-users mailing list, and any other lists or forums you may use, I recommend that you read Getting Answers by Mike Ash: http://mikeash.com/getting_answers.html.

Mailing Lists

If you have questions about DBI, or DBD driver modules, you can get help from the dbi-users@perl.org mailing list. This is the best way to get help. 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.

Please note that Tim Bunce does not maintain the mailing lists or the web pages (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 is very likely to just forward it to the mailing list.

IRC

DBI IRC Channel: #dbi on irc.perl.org (<irc://irc.perl.org/#dbi>)

Online

StackOverflow has a DBI tag http://stackoverflow.com/questions/tagged/dbi with over 800 questions.

The DBI home page at http://dbi.perl.org/ and the DBI FAQ at http://faq.dbi-support.com/ may be worth a visit. They include links to other resources, but are rather out-dated.

Reporting a Bug

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

If you think you’ve found a memory leak then read Memory Leaks.

Your problem is most likely related to the specific DBD driver module you’re using. If that’s the case then click on the ’Bugs’ link on the http://metacpan.org page for your driver. Only submit a bug report against the DBI itself if you’re sure that your issue isn’t related to the driver you’re using.

This is the DBI specification that corresponds to DBI version 1.642 (see DBI::Changes for details).

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 is 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 https://metacpan.org/search?q=DBIx. And all modules related to the DBI can be found at https://metacpan.org/search?q=DBI.

`--------–— | 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.

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.

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”; }

For queries that are not executed many times at once, it is often cleaner to use the higher level select wrappers:

$row_hashref = $dbh->selectrow_hashref(“SELECT foo, bar FROM table WHERE baz=?”, undef, $baz); $arrayref_of_row_hashrefs = $dbh->selectall_arrayref( “SELECT foo, bar FROM table WHERE baz BETWEEN ? AND ?”, { Slice => {} }, $baz_min, $baz_max);

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 is a wrapper of prepare and execute that can be simpler for non repeated non-SELECT statements (or with drivers that don’t support placeholders):

$rows_affected = $dbh->do(“UPDATE your_table SET foo = foo + 1”); $rows_affected = $dbh->do(“DELETE FROM table WHERE foo = ?”, undef, $foo);

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;

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.

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/). 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).

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://www.mcjones.org/System_R/SQL_Reunion_95/sqlr95.html

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

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.

Some drivers allow you to prevent the recognition of a placeholder by placing a single backslash character (\) immediately before it. The driver will remove the backslash character and ignore the placeholder, passing it unchanged to the backend. If the driver supports this then get_info(9000) will return true.

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.

1. 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 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 MSAccess N N N N Y N Y

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 has conflicting codes in SQL/CLI and ODBC, DBI uses the ODBC one.

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.

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 “Cant 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 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 true. The RaiseWarn attribute defaults to false.

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=>0,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 transactionsΩ-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.

Note 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. When connect_cached() returns a handle the attributes will be reset to their initial values. This can cause problems, especially with the AutoCommit attribute.

Also, to ensure that the attributes passed are always the same, avoid passing references inline. For example, the Callbacks attribute is specified as a hash reference. Be sure to declare it external to the call to connect_cached(), such that the hash reference is not re-created on every call. A package-level lexical works well:

package MyDBH; my $cb = { connect_cached.reused => sub { delete $_[4]->{AutoCommit} }, }; sub dbh { DBI->connect_cached( $dsn, $username, $auth, { Callbacks => $cb }); }

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 an extra entry for the DBI is added (and DBI::PurePerl if appropriate) and a reference to the hash is returned.

When called in a void context the installed_versions() method will print out a formatted list of the hash contents, one per line, along with some other information about the DBI version and OS.

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.

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 sometimes be handy to store such values in a database. It also doesn’t have much to do with perl hashes, like %foo.

sql_type_cast

$sts = DBI::sql_type_cast($sv, $sql_type, $flags);

sql_type_cast attempts to cast $sv to the SQL type (see DBI Constants) specified in $sql_type. At present only the SQL types SQL_INTEGER, SQL_DOUBLE and SQL_NUMERIC are supported.

For SQL_INTEGER the effect is similar to using the value in an expression that requires an integer. It gives the perl scalar an ’integer aspect’. (Technically the value gains an IV, or possibly a UV or NV if the value is too large for an IV.)

For SQL_DOUBLE the effect is similar to using the value in an expression that requires a general numeric value. It gives the perl scalar a ’numeric aspect’. (Technically the value gains an NV.)

SQL_NUMERIC is similar to SQL_INTEGER or SQL_DOUBLE but more general and more cautious. It will look at the string first and if it looks like an integer (that will fit in an IV or UV) it will act like SQL_INTEGER, if it looks like a floating point value it will act like SQL_DOUBLE, if it looks like neither then it will do nothing - and thereby avoid the warnings that would be generated by SQL_INTEGER and SQL_DOUBLE when given non-numeric data.

$flags may be:

“DBIstcf_DISCARD_STRING”

If this flag is specified then when the driver successfully casts the bound perl scalar to a non-string type then the string portion of the scalar will be discarded.

“DBIstcf_STRICT”

If $sv cannot be cast to the requested $sql_type then by default it is left untouched and no error is generated. If you specify DBIstcf_STRICT and the cast fails, this will generate an error.

The returned $sts value is:

-2 sql_type is not handled -1 sv is undef so unchanged 0 sv could not be cast cleanly and DBIstcf_STRICT was used 1 sv could not be cast and DBIstcf_STRICT was not used 2 sv was cast successfully

This method is exported by the :utils tag and was introduced in DBI 1.611.

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.

The following methods are specified for DBI database handles:

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 any other attributes in \%attr merged over them. Effectively the same as doing:

%attributes_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 unreliable and so use of clone without an argument is deprecated and may cause a warning in a future release.

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

The clone method was added in DBI 1.33.

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.

Note drivers are free to avoid the overhead of creating an DBI statement handle for do(), especially if there are no parameters. In this case error handlers, if invoked during do(), will be passed the database handle.

last_insert_id

$rv = $dbh->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:

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.

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.

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

Type: 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 dont support transactions at all. Databases in which a transaction is always active. Databases in which a transaction must be explicitly started (C<BEGIN WORK>).

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.

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.

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

Type: 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

Type: 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

Type: string, read-only

Returns the statement string passed to the most recent prepare or do 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

Type: 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

Type: string

Returns the username used to connect to the database.

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 number of rows affected for each tuple, if available or -1 if the driver cannot determine this. NOTE, some drivers cannot determine the number of rows affected per tuple but can provide the number of rows affected for the batch. 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 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 (or -1 if unknown). 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 err, errstr and state 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 although it is most often used via the execute_array() method you can use it directly. The main difference between execute_array and execute_for_fetch is the former does column or row-wise binding and the latter uses row-wise binding.

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.

last_insert_id

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

Returns a value ’identifying’ the row inserted by last execution of the statement $sth, if possible.

For some drivers the value may be ’identifying’ the row inserted by the last executed statement, not by $sth.

See database handle method last_insert_id for all details.

The last_insert_id statement method was added in DBI 1.642.

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 recommended 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, so statements like “select foo, foo from bar” will return only a single key from fetchrow_hashref. In these cases use column aliases or fetchrow_arrayref. Note that it is the database server (and not the DBD implementation) which provides the name for fields containing functions like “count(*) or max(c_foo)” and they may clash with existing column names (most databases don’t care about duplicate column names in a result-set). If you want these to return as unique names that are the same across databases, use aliases, as in “select count(*) as cnt or select max(c_foo) mx_foo, ...” depending on the syntax your database supports.

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 called on an inactive statement handle, fetchall_arrayref returns undef.

If there are no rows left to return from an active statement handle, 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.

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]);

Those two examples both return a reference to an array of array refs.

If $slice is a hash reference, fetchall_arrayref fetches each row as a hash reference. If the $slice hash is empty then the keys in the hashes have whatever name lettercase is returned by default. (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 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, regardless of the original capitalization):

$tbl_ary_ref = $sth->fetchall_arrayref({ foo=>1, BAR=>1 });

Those two examples both return a reference to an array of hash refs.

If $slice is a reference to a hash reference, that hash is used to select and rename columns. The keys are 0-based column index numbers and the values are the corresponding keys for the returned row hashes.

For example, to fetch only the first and second columns of every row as a hash ref (with keys named k and v regardless of their original names):

$tbl_ary_ref = $sth->fetchall_arrayref( \{ 0 => k, 1 => v } );

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. You almost certainly do not need to call this method.

Adding calls to finish after loop that fetches all rows is a common mistake, don’t do it, it can mask genuine problems like uncaught fetch errors.

When all the data has been fetched from a SELECT statement, the driver will automatically call finish for you. So you should not call it explicitly except when you know that you’ve not fetched all the data from a statement handle and the handle won’t be destroyed soon.

The most common example is when you only want to fetch just one row, but in that case the selectrow_* methods are usually better anyway.

Consider a query like:

SELECT foo FROM table WHERE bar=? ORDER BY baz

on 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 just a few rows, 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. If you do set a column type the type should remain sticky through further calls to bind_col for the same column if the type is not overridden (this is important for instance when you are using a slice in fetchall_arrayref).

The TYPE attribute for bind_col() was first specified in DBI 1.41.

From DBI 1.611, drivers can use the TYPE attribute to attempt to cast the bound scalar to a perl type which more closely matches TYPE. At present DBI supports SQL_INTEGER, SQL_DOUBLE and SQL_NUMERIC. See sql_type_cast for details of how types are cast.

Other attributes for Column Binding

The \%attr parameter may also contain the following attributes:

“StrictlyTyped”

If a TYPE attribute is passed to bind_col, then the driver will attempt to change the bound perl scalar to match the type more closely. If the bound value cannot be cast to the requested TYPE then by default it is left untouched and no error is generated. If you specify StrictlyTyped as 1 and the cast fails, this will generate an error. This attribute was first added in DBI 1.611. When 1.611 was released few drivers actually supported this attribute but DBD::Oracle and DBD::ODBC should from versions 1.24.

“DiscardString”

When the TYPE attribute is passed to bind_col and the driver successfully casts the bound perl scalar to a non-string type then if DiscardString is set to 1, the string portion of the scalar will be discarded. By default, DiscardString is not set. This attribute was first added in DBI 1.611. When 1.611 was released few drivers actually supported this attribute but DBD::Oracle and DBD::ODBC should from versions 1.24.

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 Perls \(…) 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.

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

Type: 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

Type: integer, read-only

The number of parameters (placeholders) in the prepared statement. See SUBSTITUTION VARIABLES below for more details.

NAME

Type: 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”;

Also note that the name returned for (aggregate) functions like count(*) or max(c_foo) is determined by the database server and not by DBI or the DBD backend.

NAME_lc

Type: array-ref, read-only

Like /NAME but always returns lowercase names.

NAME_uc

Type: array-ref, read-only

Like /NAME but always returns uppercase names.

NAME_hash

Type: hash-ref, read-only

NAME_lc_hash

Type: hash-ref, read-only

NAME_uc_hash

Type: 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

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

Type: 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

Type: 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

Type: 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

Type: 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

Type: dbh, read-only

Returns the parent $dbh of the statement handle.

Statement

Type: string, read-only

Returns the statement string passed to the prepare method.

ParamValues

Type: 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.

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 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 enable RaiseError and catch the error that’s ’thrown’ as an exception. For example, using Try::Tiny:

use Try::Tiny; $dbh->{AutoCommit} = 0; # enable transactions, if possible $dbh->{RaiseError} = 1; try { foo(…) # do lots of work here bar(…) # including inserts baz(…) # and updates $dbh->commit; # commit the changes if we get this far } catch { warn “Transaction aborted because $_”; # Try::Tiny copies $@ into $_ # 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.

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.

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 “Cant connect to $data_source: $DBI::errstr”; my $sth = $dbh->prepare( q{ SELECT name, phone FROM mytelbook }) or die “Cant prepare statement: $DBI::errstr”; my $rc = $sth->execute or die “Cant 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.

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 its 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.

[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: my $failed; eval { local $SIG{ALRM} = sub { die “TIMEOUT\n” }; # N.B. \n required eval { alarm($seconds); … code to execute with timeout here (which may die) … 1; } or $failed = 1; # outer eval catches alarm that might fire JUST before this alarm(0) alarm(0); # cancel alarm (if code ran fast) die “$@” if $failed; 1; } or $failed = 1; if ( $failed ) { if ( defined $@ and $@ eq “TIMEOUT\n” ) { … } else { … } # some other error } The first (outer) eval is used to avoid the unlikely but possible chance that the code to execute dies and the alarm fires before it is cancelled. Without the outer eval, if this happened your program will die if you have no ALRM handler or a non-local alarm handler will be called. 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 qw(:signal_h); my $mask = POSIX::SigSet->new( SIGALRM ); # signals to mask in the handler my $action = POSIX::SigAction->new( sub { die “connect timeout\n” }, # the handler code ref $mask, # not using (perl 5.8.2 and later) safe switch or sa_flags ); my $oldaction = POSIX::SigAction->new(); sigaction( SIGALRM, $action, $oldaction ); my $dbh; my $failed; eval { eval { alarm(5); # seconds before time out $dbh = DBI->connect(“dbi:Oracle:$dsn” … ); 1; } or $failed = 1; alarm(0); # cancel alarm (if connect worked fast) die “$@\n” if $failed; # connect died 1; } or $failed = 1; sigaction( SIGALRM, $oldaction ); # restore original signal handler if ( $failed ) { if ( defined $@ and $@ eq “connect timeout\n” ) {…} else { # connect died } }

See previous example for the reasoning around the double eval.

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 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.

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.

When tracking down memory leaks using tools like Devel::Leak you’ll find that some DBI internals are reported as ’leaking’ memory. This is very unlikely to be a real leak. The DBI has various caches to improve performance and the apparrent leaks are simply the normal operation of these caches.

The most frequent sources of the apparrent leaks are ChildHandles, prepare_cached and connect_cached.

For example http://stackoverflow.com/questions/13338308/perl-dbi-memory-leak

Given how widely the DBI is used, you can rest assured that if a new release of the DBI did have a real leak it would be discovered, reported, and fixed immediately. The leak you’re looking for is probably elsewhere. Good luck!

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 its 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 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 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 defines these 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) CON - trace connection process ENC - trace encoding (unicode translations etc) (not yet implemented in DBI but implemented in some DBDs) DBD - trace only DBD messages (not implemented by all DBDs yet) TXN - trace transactions (not implemented in all DBDs yet)

The parse_trace_flags and parse_trace_flag methods are used to convert trace flag names into the corresponding integer bit flags.

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”;

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.

NOTE:

• Tied filehandles are not currently supported, as tie operations are not available to the PerlIO methods used by the DBI.
• 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

An old variable that should no longer be used; equivalent to DBI_TRACE.

The DBI_PROFILE environment variable can be used to enable profiling of DBI method calls. See DBI::Profile for more information.

The DBI_PUREPERL environment variable can be used to enable the use of DBI::PurePerl. See DBI::PurePerl for more information.

• 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.
• 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.)
• The DBD driver implementation is incomplete. Consult the author.
• 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).

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.

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

To find documentation on the ODBC function you can use the MSDN search facility at:

http://msdn.microsoft.com/Search

and search for something like "SQLColumns returns".

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

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

You can find more information about SQL standards online by searching for the appropriate standard names and numbers. For example, searching for ANSI/ISO/IEC International Standard (IS) Database Language SQL - Part 1: SQL/Framework you’ll find a copy at:

ftp://ftp.iks-jena.de/mitarb/lutz/standards/sql/ansi-iso-9075-1-1999.pdf

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

Index of DBI related modules available from CPAN:

Lhttps://metacpan.org/search?q=DBD%3A%3A Lhttps://metacpan.org/search?q=DBIx%3A%3A Lhttps://metacpan.org/search?q=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

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:

L<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/

The DBI Home Page:

http://dbi.perl.org/

Other DBI related links:

http://www.perlmonks.org/?node=DBI%20recipes http://www.perlmonks.org/?node=Speeding%20up%20the%20DBI

Other database related links:

http://www.connectionstrings.com/

Security, especially the SQL Injection attack:

http://bobby-tables.com/ http://online.securityfocus.com/infocus/1644

See http://faq.dbi-support.com/

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.

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.

Use https://github.com/perl5-dbi/dbi

The DBI source code is maintained using Git. To access the source you’ll need to install a Git client. Then, to get the source code, do:

git clone https://github.com/perl5-dbi/dbi.git DBI-git

The source code will now be available in the new subdirectory DBI-git.

When you want to synchronize later, issue the command

git pull –all

Make your changes, test them, test them again until everything passes. If there are no tests for the new feature you added or a behaviour change, the change should include a new test. Then commit the changes. Either use

git gui

or

git commit -a -m Message to my changes

If you get any conflicts reported you’ll need to fix them first.

Then generate the patch file to be mailed:

git format-patch -1 –attach

which will create a file 0001-*.patch (where * relates to the commit message). Read the patch file, as a sanity check, and then email it to dbi-dev@perl.org.

If you have a github https://github.com account, you can also fork the repository, commit your changes to the forked repository and then do a pull request.

Unpack a fresh copy of the distribution:

wget http://cpan.metacpan.org/authors/id/T/TI/TIMB/DBI-1.627.tar.gz tar xfz DBI-1.627.tar.gz

Rename the newly created top level directory:

mv DBI-1.627 DBI-1.627.your_foo

Edit the contents of DBI-1.627.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-1.627.tar.gz

Then create a patch file by performing a recursive diff on the two top level directories:

diff -purd DBI-1.627 DBI-1.627.your_foo > DBI-1.627.your_foo.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.

You can also reach the developers on IRC (chat). If they are on-line, the most likely place to talk to them is the #dbi channel on irc.perl.org