DBI - Database independent interface for Perl
use DBI;
@driver_names = DBI->available_drivers; @data_sources = DBI->data_sources($driver_name);
$dbh = DBI->connect($data_source, $username, $auth, \%attr);
$rv = $dbh->do($statement); $rv = $dbh->do($statement, \%attr); $rv = $dbh->do($statement, \%attr, @bind_values);
$ary_ref = $dbh->selectall_arrayref($statement); @row_ary = $dbh->selectrow_array($statement); $ary_ref = $dbh->selectcol_arrayref($statement);
$sth = $dbh->prepare($statement); $sth = $dbh->prepare_cached($statement);
$rv = $sth->bind_param($p_num, $bind_value); $rv = $sth->bind_param($p_num, $bind_value, $bind_type); $rv = $sth->bind_param($p_num, $bind_value, \%attr);
$rv = $sth->execute; $rv = $sth->execute(@bind_values);
$rc = $sth->bind_col($col_num, \$col_variable); $rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
@row_ary = $sth->fetchrow_array; $ary_ref = $sth->fetchrow_arrayref; $hash_ref = $sth->fetchrow_hashref;
$ary_ref = $sth->fetchall_arrayref;
$rv = $sth->rows;
$rc = $dbh->commit; $rc = $dbh->rollback;
$sql = $dbh->quote($string);
$rc = $h->err; $str = $h->errstr; $rv = $h->state;
$rc = $dbh->disconnect;
The synopsis above only lists the major methods.
If you have questions about DBI, you can get help from the dbi-users@isc.org mailing list. You can subscribe to the list by visiting:
http://www.isc.org/dbi-lists.html
Also worth a visit is the DBI home page at:
http://www.symbolstone.org/technology/perl/DBI
Before asking any questions, reread this document, consult the
archives and read the DBI FAQ. The archives are listed
at the end of this document.
The FAQ is installed as a DBI::FAQ module so
you can read it by executing perldoc DBI::FAQ.
Please note that Tim Bunce does not maintain the mailing lists or the web page (generous volunteers do that). So please don't send mail directly to him; he just doesn't have the time to answer questions personally. The dbi-users mailing list has lots of experienced people who should be able to help you if you need it.
This is the DBI specification that corresponds to the DBI version 1.14
($Date: 2000/06/14 20:04:03 $).
The DBI specification is evolving at a steady pace, so it's important to check that you have the latest copy. The RECENT CHANGES section below has a summary of user-visible changes. The Changes file supplied with the DBI holds more detailed change information.
Note also that whenever the DBI changes, the drivers take some time to catch up. Recent versions of the DBI have added new features (marked NEW in the text) that may not yet be supported by the drivers you use. Talk to the authors of those drivers if you need the features.
Extensions to the DBI and other DBI related modules use the DBIx::*
namespace. See Naming Conventions and Name Space and:
http://www.perl.com/CPAN/modules/by-module/DBIx/
Here is a brief summary of significant user-visible changes in recent versions. (If a recent version isn't mentioned, it simply means that there were no significant user-visible changes in that version.)
Added $dbh->selectcol_arrayref($statement) method.
Connect now allows you to specify attribute settings within the DSN.
For example:
dbi:Driver(RaiseError=>1,Taint=>1,AutoCommit=>0):dbname"
Added $h->{Taint}, $sth->{NAME_uc}, and $sth->{NAME_lc} attributes.
The DBI is a database access module for the Perl programming language. It defines a set of methods, variables, and conventions that provide a consistent database interface, independent of the actual database being used.
It is important to remember that the DBI is just an interface. The DBI is a layer of ``glue'' between an application and one or more database driver modules. It is the driver modules which do most of the real work. The DBI provides a standard interface and framework for the drivers to operate within.
|<- Scope of DBI ->|
.-. .--------------. .-------------.
.-------. | |---| XYZ Driver |---| XYZ Engine |
| Perl | | | `--------------' `-------------'
| script| |A| |D| .--------------. .-------------.
| using |--|P|--|B|---|Oracle Driver |---|Oracle Engine|
| DBI | |I| |I| `--------------' `-------------'
| API | | |...
|methods| | |... Other drivers
`-------' | |...
`-'
The API, or Application Programming Interface, defines the call interface and variables for Perl scripts to use. The API is implemented by the Perl DBI extension.
The DBI ``dispatches'' the method calls to the appropriate driver for actual execution. The DBI is also responsible for the dynamic loading of drivers, error checking and handling, providing default implementations for methods, and many other non-database specific duties.
Each driver contains implementations of the DBI methods using the private interface functions of the corresponding database engine. Only authors of sophisticated/multi-database applications or generic library functions need be concerned with drivers.
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 behavior is strongly
recommended and may become mandatory in a later version. This
determines whether changes are automatically committed to the
database when executed, or need to be explicitly committed later.
The DBI allows an application to ``prepare'' statements for later
execution. A prepared statement is identified by a statement handle
held in a Perl variable.
We'll call the Perl variable $sth in our examples.
The typical method call sequence for a SELECT statement is:
prepare,
execute, fetch, fetch, ...
execute, fetch, fetch, ...
execute, fetch, fetch, ...
for example:
$sth = $dbh->prepare("SELECT foo, bar FROM table WHERE baz=?");
$sth->execute( $baz );
while ( @row = $sth->fetchrow_array ) {
print "@row\n";
}
The typical method call sequence for a non-SELECT statement is:
prepare,
execute,
execute,
execute.
for example:
$sth = $dbh->prepare("INSERT INTO table(foo,bar,baz) VALUES (?,?,?)");
while(<CSV>) {
chop;
my ($foo,$bar,$baz) = split /,/;
$sth->execute( $foo, $bar, $baz );
}
The do() method can be used for non repeated non-SELECT statement
(or with drivers that don't support placeholders):
$rows_affected = $dbh->do("UPDATE your_table SET foo = foo + 1");
To commit your changes to the database (when AutoCommit is off):
$dbh->commit; # or call $dbh->rollback; to undo changes
Finally, when you have finished working with the data source, you should disconnect from it:
$dbh->disconnect;
The DBI does not have a concept of a ``current session''. Every session
has a handle object (i.e., a $dbh) returned from the connect method.
That handle object is used to invoke database related methods.
Most data is returned to the Perl script as strings. (Null values are
returned as undef.) This allows arbitrary precision numeric data to be
handled without loss of accuracy. Beware that Perl may not preserve
the same accuracy when the string is used as a number.
Dates and times are returned as character strings in the native format of the corresponding database engine. Time zone effects are database/driver dependent.
Perl supports binary data in Perl strings, and the DBI will pass binary data to and from the driver without change. It is up to the driver implementors to decide how they wish to handle such binary data.
Most databases that understand multiple character sets have a default global charset. Text stored in the database is, or should be, stored in that charset; if not, then that's the fault of either the database or the application that inserted the data. When text is fetched it should be automatically converted to the charset of the client, presumably based on the locale. If a driver needs to set a flag to get that behavior, then it should do so; it should not require the application to do that.
Multiple SQL statements may not be combined in a single statement
handle ($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).
Driver Specific Prefix Registry:
ado_ DBD::ADO best_ DBD::BestWins csv_ DBD::CSV db2_ DBD::DB2 f_ DBD::File file_ DBD::TextFile ib_ DBD::InterBase ing_ DBD::Ingres ix_ DBD::Informix msql_ DBD::mSQL mysql_ DBD::mysql odbc_ DBD::ODBC ora_ DBD::Oracle pg_ DBD::Pg proxy_ DBD::Proxy solid_ DBD::Solid syb_ DBD::Sybase tuber_ DBD::Tuber xbase_ DBD::XBase
Most DBI drivers require applications to use a dialect of SQL (Structured Query Language) to interact with the database engine. The following links provide useful information and further links about SQL:
http://www.altavista.com/query?q=sql+tutorial http://www.jcc.com/sql_stnd.html http://www.contrib.andrew.cmu.edu/~shadow/sql.html
The DBI itself does not mandate or require any particular language to be used; it is language independent. In ODBC terms, the DBI is in ``pass-thru'' mode, although individual drivers might not be. The only requirement is that queries and other statements must be expressed as a single string of characters passed as the first argument to the prepare or do methods.
For an interesting diversion on the real history of RDBMS and SQL, from the people who made it happen, see:
http://ftp.digital.com/pub/DEC/SRC/technical-notes/SRC-1997-018-html/sqlr95.html
Follow the ``And the rest'' and ``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.
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, can be used to indicate null values.
However, care must be taken in the particular case of trying to use
null values to qualify a SELECT statement. Consider:
SELECT description FROM products WHERE product_code = ?
Binding an undef (NULL) to the placeholder will not select rows
which have a NULL product_code! 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 product_code IS NULL'' and to make that
general you have to say:
... WHERE (product_code = ? OR (? IS NULL AND product_code IS NULL))
and bind the same value to both placeholders.
Performance
Without using placeholders, the insert statement shown previously would have to
contain the literal values to be inserted and would have to be
re-prepared and re-executed for each row. With placeholders, the insert
statement only needs to be prepared once. The bind values for each row
can be given to the execute method each time it's called. By avoiding
the need to re-prepare the statement for each row, the application
typically runs many times faster. Here's an example:
my $sth = $dbh->prepare(q{
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
}) || die $dbh->errstr;
while (<>) {
chop;
my ($product_code, $qty, $price) = split /,/;
$sth->execute($product_code, $qty, $price) || die $dbh->errstr;
}
$dbh->commit || die $dbh->errstr;
See execute and bind_param for more details.
The q{...} style quoting used in this example avoids clashing with
quotes that may be used in the SQL statement. Use the double-quote like
qq{...} operator if you want to interpolate variables into the string.
See perlop/``Quote and Quote-like Operators'' for more details.
See also the bind_column method, which is used to associate Perl
variables with the output columns of a SELECT statement.
In this section, we cover the DBI class methods, utility functions, and the dynamic attributes associated with generic DBI handles.
The following methods are provided by the DBI class:
connect
$dbh = DBI->connect($data_source, $username, $password)
|| die $DBI::errstr;
$dbh = DBI->connect($data_source, $username, $password, \%attr)
|| 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 set $!, etc.) 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 should 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. (Where a driver author needs
to define a syntax for the $data_source, it is recommended that
they follow the ODBC style, shown in the last example above.)
If the environment variable DBI_AUTOPROXY is defined (and the driver in
$data_source is not ``Proxy'') then the connect request will
automatically be changed to:
dbi:Proxy:$ENV{DBI_AUTOPROXY};dsn=$data_source
and passed to the DBD::Proxy module. DBI_AUTOPROXY is typically set as
``hostname=...;port=...''. 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.
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.
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. Future versions of
the DBI may issue a warning if AutoCommit is not explicitly defined.
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
});
You can also define connection attribute values within the $data_source
parameter. For example:
dbi:DriverName(PrintError=>0,Taint=>1):...
Individual attributes values specified in this way take precedence over
any conflicting values specified via the \%attr parameter to connect.
Where possible, each session ($dbh) is independent from the transactions
in other sessions. This is useful when you need to hold cursors open
across transactions--for example, if you use one session for your long lifespan
cursors (typically read-only) and another for your short update
transactions.
For compatibility with old DBI scripts, the driver can be specified by
passing its name as the fourth argument to connect (instead of \%attr):
$dbh = DBI->connect($data_source, $user, $pass, $driver);
In this ``old-style'' form of connect, the $data_source should not start
with ``dbi:driver_name:''. (If it does, the embedded driver_name
will be ignored). Also note that in this older form of connect,
the $dbh->{AutoCommit} attribute is undefined, the
$dbh->{PrintError} attribute is off, and the old DBI_DBNAME
environment variable is
checked if DBI_DSN is not defined. Beware that this ``old-style''
connect will be withdrawn in a future version of DBI.
connect_cached NEW
$dbh = DBI->connect_cached($data_source, $username, $password)
|| die $DBI::errstr;
$dbh = DBI->connect_cached($data_source, $username, $password, \%attr)
|| 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 $dbh will be returned if it is still valid.
The cached database handle is replaced with a new connection if it
has been disconnected or if the ping method fails.
Note that the behavior of this method differs in several respects from the behavior of presistent connections implemented by Apache::DBI.
Caching can be useful in some applications, but it can also cause problems and should be used with care. The exact behavior of this method is liable to change, so if you intend to use it in any production applications you should discuss your needs on the dbi-users mailing list.
The cache can be accessed (and cleared) via the CachedKids attribute.
available_drivers@ary = DBI->available_drivers; @ary = DBI->available_drivers($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 $quiet will inhibit the warning.
data_sources@ary = DBI->data_sources($driver); @ary = DBI->data_sources($driver, \%attr);
Returns a list of all data sources (databases) available via the named
driver. The driver will be loaded if it hasn't been already. If $driver is empty
or undef, then the value of the DBI_DRIVER environment variable is
used.
Data sources are returned in a form suitable for passing to the
connect method (that is, they will include the ``dbi:$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.
traceDBI->trace($trace_level) DBI->trace($trace_level, $trace_filename)
DBI trace information can be enabled for all handles using the trace
DBI class method. To enable trace information for a specific handle, use
the similar $h->trace method described elsewhere.
Trace levels are as follows:
0 - Trace disabled.
1 - Trace DBI method calls returning with results or errors.
2 - Trace method entry with parameters and returning with results.
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.
Also includes DBI mutex information when using threaded Perl.
5 and above - As above but with more and more obscure information.
Trace level 1 is best for a simple overview of what's happening. Trace level 2 is a good choice for general purpose tracing. Levels 3 and above (up to 9) 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.
Initially trace output is written to STDERR. If $trace_filename is
specified, then the file is opened in append mode and all trace
output (including that from other handles) is redirected to that file.
Further calls to trace without a $trace_filename do not alter where
the trace output is sent. If $trace_filename is undefined, then
trace output is sent to STDERR and the previous trace file is closed.
See also the $h->trace and $h->trace_msg methods and the
DEBUGGING section
for information about the DBI_TRACE environment variable.
In addition to the methods listed in the previous section, the DBI package also provides these utility functions:
neat$str = DBI::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). 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 = DBI::neat_list(\@listref, $maxlen, $field_sep);
Calls DBI::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 = DBI::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.
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.
$DBI::err$h->err.
$DBI::errstr$h->errstr.
$DBI::state$h->state.
$DBI::rows$h->rows. Please refer to the documentation
for the rows method.
The following methods can be used by all types of DBI handles.
err$rv = $h->err;
Returns the native database engine error code from the last driver method called. The code is typically an integer but you should not assume that.
The DBI resets $h->err to undef before most DBI method calls, so the value only has a short lifespan. Also, most drivers share the same error variables across all their handles, so calling a method on one handle will typically reset the error on all the other handles that are children of that driver.
If you need to test for individual errors and have your program be portable to different database engines, then you'll need to determine what the corresponding error codes are for all those engines and test for all of them.
errstr$str = $h->errstr;
Returns the native database engine error message from the last driver method called. This has the same lifespan issues as the err method described above.
state$str = $h->state;
Returns an error code in the standard SQLSTATE five character format.
Note that the specific success code 00000 is translated to 0
(false). If the driver does not support SQLSTATE (and most don't),
then state will return S1000 (General Error) for all errors.
trace$h->trace($trace_level); $h->trace($trace_level, $trace_filename);
DBI trace information can be enabled for a specific handle (and any
future children of that handle) by setting the trace level using the
trace method.
Trace level 1 is best for a simple overview of what's happening.
Trace level 2 is a good choice for general purpose tracing. Levels 3
and above (up to 9) are best reserved for investigating a
specific problem, when you need to see ``inside'' the driver and DBI.
Set $trace_level to 0 to disable the trace.
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.
Initially, trace output is written to STDERR. If $trace_filename is
specified, then the file is opened in append mode and all trace
output (including that from other handles) is redirected to that file.
Further calls to trace without a $trace_filename do not alter where
the trace output is sent. If $trace_filename is undefined, then
trace output is sent to STDERR and the previous trace file is closed.
See also the DBI->trace method and DEBUGGING for information
about the DBI_TRACE environment variable.
trace_msg$h->trace_msg($message_text); $h->trace_msg($message_text, $min_level);
Writes $message_text to the trace file if trace is enabled for $h or
for the DBI as a whole. Can also be called as DBI->trace_msg($msg).
See trace.
If $min_level is defined, then the message is output only if the trace
level is equal to or greater than that level. $min_level defaults to 1.
func$h->func(@func_arguments, $func_name);
The func method can be used to call private non-standard and
non-portable methods implemented by the driver. Note that the function
name is given as the last argument.
This method is not directly related to calling stored procedures. Calling stored procedures is currently not defined by the DBI. Some drivers, such as DBD::Oracle, support it in non-portable ways. See driver documentation for more details.
These attributes are common to all types of DBI handles.
Some attributes are inherited by child handles. That is, the value of an inherited attribute in a newly created statement handle is the same as the value in the parent database handle. Changes to attributes in the new statement handle do not affect the parent database handle and changes to the database handle do not affect existing statement handles, only future ones.
Attempting to set or get the value of an unknown attribute is fatal, except for private driver specific attributes (which all have names starting with a lowercase letter).
Example:
$h->{AttributeName} = ...; # set/write
... = $h->{AttributeName}; # get/read
Warn (boolean, inherited)warn function, they can be
intercepted using the Perl $SIG{__WARN__} hook.
Active (boolean, read-only)$dbh->disconnect sets Active off). For
a statement handle it typically means that the handle is a SELECT
that may have more data to fetch. (Fetching all the data or calling $sth->finish
sets Active off.)
Kids (integer, read-only)Kids is the number of currently existing database
handles that were created from that driver handle. For a database
handle, Kids is the number of currently existing statement handles that
were created from that database handle.
ActiveKids (integer, read-only)Kids, but only counting those that are Active (as above).
CachedKids (hash ref)CompatMode (boolean, inherited)InactiveDestroy (boolean)For a database handle, this attribute does not disable an explicit call to the disconnect method, only the implicit call from DESTROY.
This attribute is specifically designed for use in Unix applications
that ``fork'' child processes. Either the parent or the child process,
but not both, should set InactiveDestroy on all their shared handles.
Note that some databases, including Oracle, don't support passing a
database connection across a fork.
PrintError (boolean, inherited)warn) in addition to returning error codes in the normal way. When set
``on'', any method which results in an error occuring will cause the DBI to
effectively do a warn("$class $method failed: $DBI::errstr") where $class
is the driver class and $method is the name of the method which failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
By default, DBI->connect sets PrintError ``on''.
If desired, the warnings can be caught and processed using a $SIG{__WARN__}
handler or modules like CGI::Carp and CGI::ErrorWrap.
RaiseError (boolean, inherited)die("$class $method failed: $DBI::errstr"),
where $class is the driver class and $method is the name of the method
that failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
If PrintError is also on, then the PrintError is done before the
RaiseError unless no __DIE__ handler has been defined, in which case
PrintError is skipped since the die will print the message.
If you want to temporarily turn RaiseError off (inside a library function
that is likely to fail, for example), the recommended way is like this:
{
local $h->{RaiseError}; # localize and turn off for this block
...
}
The original value will automatically and reliably be restored by Perl,
regardless of how the block is exited.
The same logic applies to other attributes, including PrintError.
Sadly, this doesn't work for Perl versions up to and including 5.004_04.
For backwards compatibility, you could just use eval { ... } instead.
ChopBlanks (boolean, inherited)The default is false (although it is possible that the default may change). Applications that need specific behavior should set the attribute as needed. Emulation interfaces should set the attribute to match the behavior of the interface they are emulating.
Drivers are not required to support this attribute, but any driver which
does not support it must arrange to return undef as the attribute value.
LongReadLen (unsigned integer, inherited)LongReadLen attribute only relates to fetching and reading long values; it
is not involved in inserting or updating them.
A value of 0 means not to automatically fetch any long data. (fetch
should return undef for long fields when LongReadLen is 0.)
The default is typically 0 (zero) bytes but may vary between drivers. Applications fetching long fields should set this value to slightly larger than the longest long field value to be fetched.
Some databases return some long types encoded as pairs of hex digits.
For these types, LongReadLen relates to the underlying data length and not the
doubled-up length of the encoded string.
Changing the value of LongReadLen for a statement handle after it
has been prepare'd will typically have no effect, so it's common to
set LongReadLen on the $dbh before calling prepare.
Note that the value used here has a direct effect on the memory used by the application, so don't be too generous.
See LongTruncOk for more information on truncation behavior.
LongTruncOk (boolean, inherited)LongReadLen attribute).
By default, LongTruncOk is false and so fetching a long value that
needs to be truncated will cause the fetch to fail.
(Applications should always be sure to
check for errors after a fetch loop in case an error, such as a divide
by zero or long field truncation, caused the fetch to terminate
prematurely.)
If a fetch fails due to a long field truncation when LongTruncOk is
false, many drivers will allow you to continue fetching further rows.
See also LongReadLen.
Taint (boolean, inherited)-T option), then all data
fetched from the database is tainted, and the arguments to most DBI
method calls are checked for being tainted. This may change.
The attribute defaults to off, even if Perl is in taint mode. See perlsec for more about taint mode. If Perl is not running in taint mode, this attribute has no effect.
When fetching data that you trust you can turn off the Taint attribute, for that statement handle, for the duration of the fetch loop.
Currently only fetched data is tainted. It is possible that the results of other DBI method calls, and the value of fetched attributes, may also be tainted in future versions. That change may well break your applications unless you take great care now. If you use DBI Taint mode, please report your experience and any suggestions for changes.
private_*private_''. It is strongly
recommended that you use just one private attribute (e.g., use a
hash ref) and give it a long and unambiguous name that includes the
module or application name that the attribute relates to (e.g.,
``private_YourFullModuleName_thingy'').
This section covers the methods and attributes associated with database handles.
The following methods are specified for DBI database handles:
do$rc = $dbh->do($statement) || die $dbh->errstr; $rc = $dbh->do($statement, \%attr) || die $dbh->errstr; $rv = $dbh->do($statement, \%attr, @bind_values) || ...
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 or is 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') || 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 perlop/``Quote and Quote-like Operators'' for more details.
selectrow_array@row_ary = $dbh->selectrow_array($statement); @row_ary = $dbh->selectrow_array($statement, \%attr); @row_ary = $dbh->selectrow_array($statement, \%attr, @bind_values);
This utility method combines prepare, execute and
fetchrow_array into a single call. If called in a list context, it
returns the first row of data from the statement. If called in a scalar
context, it returns the first field of the first row. The $statement
parameter can be a previously prepared statement handle, in which case
the prepare is skipped.
If any method fails, and RaiseError is not set, selectrow_array
will return an empty list.
In a scalar context, selectrow_array returns the value of the first
field. An undef is returned if there are no matching rows or an error
occurred. Since that undef can't be distinguished from an undef returned
because the first field value was NULL, calling selectrow_array in
a scalar context should be used with caution.
selectall_arrayref$ary_ref = $dbh->selectall_arrayref($statement); $ary_ref = $dbh->selectall_arrayref($statement, \%attr); $ary_ref = $dbh->selectall_arrayref($statement, \%attr, @bind_values);
This utility method combines prepare, execute and fetchall_arrayref into a single call. It returns a reference to an array containing a reference to an array for each row of data fetched.
The $statement parameter can be a previously prepared statement handle,
in which case the prepare is skipped. This is recommended if the
statement is going to be executed many times.
If any method except fetch fails, and RaiseError is not set,
selectall_arrayref will return undef. If fetch fails and
RaiseError is not set, then it will return with whatever data it
has fetched thus far.
selectcol_arrayref$ary_ref = $dbh->selectcol_arrayref($statement); $ary_ref = $dbh->selectcol_arrayref($statement, \%attr); $ary_ref = $dbh->selectcol_arrayref($statement, \%attr, @bind_values);
This utility method combines prepare, execute, and fetching one column from all the rows, into a single call. It returns a reference to an array containing the values of the first column from each row.
The $statement parameter can be a previously prepared statement handle,
in which case the prepare is skipped. This is recommended if the
statement is going to be executed many times.
If any method except fetch fails, and RaiseError is not set,
selectall_arrayref will return undef. If fetch fails and
RaiseError is not set, then it will return with whatever data it
has fetched thus far.
prepare$sth = $dbh->prepare($statement) || die $dbh->errstr; $sth = $dbh->prepare($statement, \%attr) || die $dbh->errstr;
Prepares a single statement for later execution by the database engine and returns a reference to a statement handle object.
The returned statement handle can be used to get attributes of the statement and invoke the execute method. See Statement Handle Methods.
Drivers for engines without the concept of preparing a
statement will typically just store the statement in the returned
handle and process it when $sth->execute is called. Such drivers are
unlikely to give much useful information about the
statement, such as $sth->{NUM_OF_FIELDS}, until after $sth->execute
has been called. Portable applications should take this into account.
In general, DBI drivers do not parse the contents of the statement (other than simply counting any Placeholders). The statement is passed directly to the database engine, sometimes known as pass-thru mode. This has advantages and disadvantages. On the plus side, you can access all the functionality of the engine being used. On the downside, you're limited if you're using a simple engine, and you need to take extra care if writing applications intended to be portable between engines.
Portable applications should not assume that a new statement can be prepared and/or executed while still fetching results from a previous statement.
Some command-line SQL tools use statement terminators, like a semicolon, to indicate the end of a statement. Such terminators should not normally be used with the DBI.
prepare_cached$sth = $dbh->prepare_cached($statement) $sth = $dbh->prepare_cached($statement, \%attr) $sth = $dbh->prepare_cached($statement, \%attr, $allow_active)
Like prepare except that the statement handle returned will be
stored in a hash associated with the $dbh. If another call is made to
prepare_cached with the same $statement and %attr values, then the
corresponding cached $sth will be returned without contacting the
database server.
This caching can be useful in some applications, but it can also cause
problems and should be used with care. If the cached $sth being
returned is active (i.e., is a SELECT that may still have data to be
fetched) then a warning will be generated and finish will be called
for you. The warning can be suppressed by setting $allow_active to
true. The cache can be accessed (and cleared) via the CachedKids
attribute.
Here's an example of one possible use of prepare_cached:
while ( ($field, $value) = each %search_fields ) {
push @sql, "$field = ?";
push @values, $value;
}
$qualifier = "";
$qualifier = "where ".join(" and ", @sql) if @sql;
$sth = $dbh->prepare_cached("SELECT * FROM table $qualifier");
$sth->execute(@values);
commit$rc = $dbh->commit || die $dbh->errstr;
Commit (make permanent) the most recent series of database changes if the database supports transactions and AutoCommit is off.
If AutoCommit is on, then calling
commit will issue a ``commit ineffective with AutoCommit'' warning.
See also Transactions in the FURTHER INFORMATION section below.
rollback$rc = $dbh->rollback || die $dbh->errstr;
Rollback (undo) the most recent series of uncommitted database changes if the database supports transactions and AutoCommit is off.
If AutoCommit is on, then calling
rollback will issue a ``rollback ineffective with AutoCommit'' warning.
See also Transactions in the FURTHER INFORMATION section below.
disconnect$rc = $dbh->disconnect || warn $dbh->errstr;
Disconnects the database from the database handle. disconnect is typically only used
before exiting the program. The handle is of little use after disconnecting.
The transaction behavior of the disconnect method is, sadly,
undefined. Some database systems (such as Oracle and Ingres) will
automatically commit any outstanding changes, but others (such as
Informix) will rollback any outstanding changes. Applications not
using AutoCommit should explicitly call commit or rollback before
calling disconnect.
The database is automatically disconnected by the DESTROY method if
still connected when there are no longer any references to the handle.
The DESTROY method for each driver should implicitly call rollback to
undo any uncommitted changes. This is vital behavior to ensure that
incomplete transactions don't get committed simply because Perl calls
DESTROY on every object before exiting. Also, do not rely on the order
of object destruction during ``global destruction'', as it is undefined.
Generally, if you want your changes to be commited or rolled back when you disconnect, then you should explicitly call commit or rollback before disconnecting.
If you disconnect from a database while you still have active statement
handles, you will get a warning. The statement handles should either be
cleared (destroyed) before disconnecting, or the finish method
should be called on
each one.
ping$rc = $dbh->ping;
Attempts to determine, in a reasonably efficient way, if the database server is still running and the connection to it is still working. Individual drivers should implement this function in the most suitable manner for their database engine.
The current default implementation always returns true without
actually doing anything. Actually, it returns ``0 but true'' which is
true but zero. That way you can tell if the return value is genuine or
just the default. Drivers should override this method with one that
does the right thing for their type of database.
Few applications would have use for this method. See the specialized Apache::DBI module for one example usage.
table_info NEW$sth = $dbh->table_info;
Returns an active statement handle that can be used to fetch information about tables and views that exist in the database.
The handle has at least the following fields in the order show below. Other fields, after these, may also be present.
TABLE_CAT: Table catalog identifier. This field is NULL (undef) if not
applicable to the data source, which is usually the case. This field
is empty if not applicable to the table.
TABLE_SCHEM: The name of the schema containing the TABLE_NAME value.
This field is NULL (undef) if not applicable to data source, and
empty if not applicable to the table.
TABLE_NAME: Name of the table (or view, synonym, etc).
TABLE_TYPE: One of the following: ``TABLE'', ``VIEW'', ``SYSTEM TABLE'', ``GLOBAL TEMPORARY'', ``LOCAL TEMPORARY'', ``ALIAS'', ``SYNONYM'' or a type identifier that is specific to the data source.
REMARKS: A description of the table. May be NULL (undef).
Note that table_info might not return records for all tables.
Applications can use any valid table regardless of whether it's
returned by table_info. See also tables.
For more detailed information about the fields and their meanings, you can refer to:
http://msdn.microsoft.com/library/psdk/dasdk/odch6wqb.htm
If that URL ceases to work then use the MSDN search facility at:
http://search.microsoft.com/us/dev/
and search for SQLTables returns using the exact phrase option.
The link you want will probably just be called SQLTables and will
be part of the Data Access SDK.
tables NEW@names = $dbh->tables;
Returns a list of table and view names, possibly including a schema prefix.
This list should include all
tables that can be used in a SELECT statement without further
qualification.
Note that table_info might not return records for all tables.
Applications can use any valid table regardless of whether it's
returned by tables. See also table_info.
type_info_all NEW$type_info_all = $dbh->type_info_all;
Returns a reference to an array which holds information about each data type variant supported by the database and driver. The array and its contents should be treated as read-only.
The first item is a reference to a hash of Name => Index pairs. The
following items are references to arrays, one per supported data type
variant. The leading hash defines the names and order of the fields
within the following list of arrays. For example:
$type_info_all = [
{ TYPE_NAME => 0,
DATA_TYPE => 1,
COLUMN_SIZE => 2, # was PRECISION originally
LITERAL_PREFIX => 3,
LITERAL_SUFFIX => 4,
CREATE_PARAMS => 5,
NULLABLE => 6,
CASE_SENSITIVE => 7,
SEARCHABLE => 8,
UNSIGNED_ATTRIBUTE=> 9,
FIXED_PREC_SCALE => 10, # was MONEY originally
AUTO_UNIQUE_VALUE => 11, # was AUTO_INCREMENT originally
LOCAL_TYPE_NAME => 12,
MINIMUM_SCALE => 13,
MAXIMUM_SCALE => 14,
NUM_PREC_RADIX => 15,
},
[ 'VARCHAR', SQL_VARCHAR,
undef, "'","'", undef,0, 1,1,0,0,0,undef,1,255, undef
],
[ 'INTEGER', SQL_INTEGER,
undef, "", "", undef,0, 0,1,0,0,0,undef,0, 0, 10
],
];
Note that more than one row may have the same value in the DATA_TYPE
field if there are different ways to spell the type name and/or there
are variants of the type with different attributes (e.g., with and
without AUTO_UNIQUE_VALUE set, with and without UNSIGNED_ATTRIBUTE, etc).
The rows are ordered by DATA_TYPE first and then by how closely each
type maps to the corresponding ODBC SQL data type, closest first.
The meaning of the fields is described in the documentation for
the type_info method. The index values shown above (e.g.,
NULLABLE => 6) are for illustration only. Drivers may define the
fields with a different order.
This method is not normally used directly. The type_info method provides a more useful interface to the data.
type_info NEW@type_info = $dbh->type_info($data_type);
Returns a list of hash references holding information about one or more
variants of $data_type. The list is ordered by DATA_TYPE first and
then by how closely each type maps to the corresponding ODBC SQL data
type, closest first. If called in a scalar context then only the first
(best) element is returned.
If $data_type is undefined or SQL_ALL_TYPES, then the list will
contain hashes for all data type variants supported by the database and driver.
If $data_type is an array reference then type_info returns the
information for the first type in the array that has any matches.
The keys of the hash follow the same letter case conventions as the rest of the DBI (see Naming Conventions and Name Space). The following items should exist:
For string types, this is the maximum size of the string in bytes.
For date and interval types, this is the maximum number of characters needed to display the value.
''' for characters,
or possibly ``0x'' for binary values passed as hexadecimal. NULL (undef) is
returned for data types for which this is not applicable.
''' for characters.
NULL (undef) is returned for data types where this is not applicable.
CREATE_PARAMS for a
DECIMAL would be ``precision,scale'' if the DECIMAL type should be
declared as DECIMAL(precision,scale) where precision and scale
are integer values. For a VARCHAR it would be ``max length''.
NULL (undef) is returned for data types for which this is not applicable.
0 or an empty string = no, 1 = yes, 2 = unknown.
0 - Cannot be used in a WHERE clause 1 - Only with a LIKE predicate 2 - All comparison operators except LIKE 3 - Can be used in a WHERE clause with any comparison operator
undef) is returned
for data types for which this is not applicable.
undef) is returned for data types
for which
this is not applicable.
undef) is returned
for data types for which this is not applicable.
TYPE_NAME for use in dialog with users.
NULL (undef) is returned if a localized name is not available (in which
case TYPE_NAME should be used).
MAXIMUM_SCALE holds the same value. NULL (undef) is returned for
data types for which this is not applicable.
MINIMUM_SCALE holds the same value. NULL (undef) is returned for
data types for which this is not applicable.
DATA_TYPE column, except for interval
and datetime data types. For interval and datetime data types, the
SQL_DATA_TYPE field will return SQL_INTERVAL or SQL_DATETIME, and the
SQL_DATETIME_SUB field below will return the subcode for the specific
interval or datetime data type. If this field is NULL, then the driver
does not support or report on interval or date subtypes.
SQL_DATA_TYPE field
above is SQL_INTERVAL or SQL_DATETIME, this field will hold the subcode
for the specific interval or datetime data type. Otherwise it will be
NULL (undef).
NUM_PREC_RADIX
contains the value 2 and COLUMN_SIZE holds the number of bits. For
exact numeric types, NUM_PREC_RADIX contains the value 10 and COLUMN_SIZE holds
the number of decimal digits. NULL (undef) is returned either for data types
for which this is not applicable or if the driver cannot report this information.
Since DBI and ODBC drivers vary in how they map their types into the ISO standard types you may need to search for more than one type. Here's an example looking for a usable type to store a date:
$my_date_type = $dbh->type_info( [ SQL_DATE, SQL_TIMESTAMP ] );
Similarly, to more reliably find a type to store small integers, you could
use a list starting with SQL_SMALLINT, SQL_INTEGER, SQL_DECIMAL, etc.
For more detailed information about these fields and their meanings, you can refer to:
http://msdn.microsoft.com/library/psdk/dasdk/odch6yy7.htm
If that URL ceases to work then use the MSDN search facility at
http://search.microsoft.com/us/dev/
and search the MSDN Library for C<SQLGetTypeInfo returns> using the exact phrase option. The link you want will probably just be called C<SQLGetTypeInfo> (there may be more than one).
The individual data types are currently described here:
http://msdn.microsoft.com/library/psdk/dasdk/odap8fcj.htm
If that URL ceases to work, or to get more general information, use the
MSDN search facility as described above and search for SQL Data Types.
quote$sql = $dbh->quote($value); $sql = $dbh->quote($value, $data_type);
Quote a string literal for use as a literal value in an SQL statement, by escaping any special characters (such as quotation marks) contained within the string and adding the required type of outer quotation marks.
$sql = sprintf "SELECT foo FROM bar WHERE baz = %s",
$dbh->quote("Don't");
For most database types, quote would return 'Don''t' (including the
outer quotation marks).
An undefined $value value will be returned as the string NULL (without
quotation marks) to match how NULLs are represented in SQL.
If $data_type is supplied, it is used to try to determine the required
quoting behavior by using the information returned by type_info.
As a special case, the standard numeric types are optimized to return
$value without calling type_info.
Quote will probably not be able to deal with all possible input (such as binary data or data containing newlines), and is not related in any way with escaping or quoting shell meta-characters. There is no need to quote values being used with Placeholders and Bind Values.
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 is fatal, except for private driver-specific attributes (which all have names starting with a lowercase letter).
Example:
$h->{AutoCommit} = ...; # set/write
... = $h->{AutoCommit}; # get/read
AutoCommit (boolean)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 behavior can set $dbh->{AutoCommit} = 0 (or
set AutoCommit to 0 via connect)
without having to check that the value was assigned successfully.
For the purposes of this description, we can divide databases into three categories:
Databases which don't support transactions at all. Databases in which a transaction is always active. Databases in which a transaction must be explicitly started (C<'BEGIN WORK'>).
* Databases which don't support transactions at all
For these databases, attempting to turn AutoCommit off is a fatal error.
commit and rollback both issue warnings about being ineffective while
AutoCommit is in effect.
* Databases in which a transaction is always active
These are typically mainstream commercial relational databases with
``ANSI standard'' transaction behavior.
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. In other words, calling commit or rollback explicitly while
AutoCommit is on would be ineffective because the changes would
have already been commited.
Changing AutoCommit from off to on should issue a commit in most drivers.
Changing AutoCommit from on to off should have no immediate effect.
For databases which don't support a specific auto-commit mode, the
driver has to commit each statement automatically using an explicit
COMMIT after it completes successfully (and roll it back using an
explicit ROLLBACK if it fails). The error information reported to the
application will correspond to the statement which was executed, unless
it succeeded and the commit or rollback failed.
* Databases in which a transaction must be explicitly started
For these databases, the intention is to have them act like databases in which a transaction is always active (as described above).
To do this, the DBI driver will automatically begin a transaction when
AutoCommit is turned off (from the default ``on'' state) and will
automatically begin another transaction after a commit or rollback.
In this way, the application does not have to treat these databases as a
special case.
See disconnect for other important notes about transactions.
Driver (handle)
$dbh->{Driver}->{Name}
Name (string)dbi:DriverName:...'' string used to connect to the database,
but with the leading ``dbi:DriverName:'' removed.
RowCacheSize (integer) NEWSELECT statements.
If a row cache is not implemented, then setting RowCacheSize is ignored
and getting the value returns undef.
Some RowCacheSize values have special meaning, as follows:
0 - Automatically determine a reasonable cache size for each C<SELECT> 1 - Disable the local row cache >1 - Cache this many rows <0 - Cache as many rows that will fit into this much memory for each C<SELECT>.
Note that large cache sizes may require a very large amount of memory (cached rows * maximum size of row). Also, a large cache will cause a longer delay not only for the first fetch, but also whenever the cache needs refilling.
See also the RowsInCache statement handle attribute.
This section lists the methods and attributes associated with DBI statement handles.
The DBI defines the following methods for use on DBI statement handles:
bind_param$rc = $sth->bind_param($p_num, $bind_value) || die $sth->errstr; $rv = $sth->bind_param($p_num, $bind_value, \%attr) || ... $rv = $sth->bind_param($p_num, $bind_value, $bind_type) || ...
The bind_param method can be used to bind a value
with a placeholder 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);
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. Undefined bind values or undef can be used to
indicate null values.
Some drivers do not support placeholders.
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
Data Types for Placeholders
The \%attr parameter can be used to hint at the data type the
placeholder should have. 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 this 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, $value, SQL_INTEGER);
The TYPE value indicates the standard (non-driver-specific) type for
this parameter. To specify the driver-specific type, the driver may
support a driver-specific attribute, such as { ora_type => 97 }. The
data type for a placeholder cannot be changed after the first
bind_param call. However, it can be left unspecified, in which case it
defaults to the previous value.
Perl only has string and number scalar data types. All database types that aren't numbers are bound as strings and must be in a format the database will understand.
As an alternative to specifying the data type in the bind_param call,
you can let the driver pass the value as the default type (VARCHAR).
You can then use an SQL function to convert the type within the statement.
For example:
INSERT INTO price(code, price) VALUES (?, CONVERT(MONEY,?))
The CONVERT function used here is just an example. The actual function
and syntax will vary between different databases and is non-portable.
See also Placeholders and Bind Values for more information.
bind_param_inout$rc = $sth->bind_param_inout($p_num, \$bind_value, $max_len) || die $sth->errstr; $rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, \%attr) || ... $rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, $bind_type) || ...
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
read 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 $bind_value for the new value. If the value
returned from the database is too
big to fit, then the execution should fail. If unsure what value to use,
pick a generous length, i.e., a length larger than the longest value that would ever be
returned. The only cost of using a larger value than needed is wasted memory.
It is expected that few drivers will support this method. The only driver currently known to do so is DBD::Oracle (DBD::ODBC may support it in a future release). Therefore it should not be used for database independent applications.
Undefined values or undef are used to indicate null values.
See also Placeholders and Bind Values for more information.
execute$rv = $sth->execute || die $sth->errstr; $rv = $sth->execute(@bind_values) || die $sth->errstr;
Perform whatever processing is necessary to execute the prepared
statement. An undef is returned if an error occurs. A successful
execute always returns true regardless of the number of rows affected,
even if it's zero (see below). It is always important to check the
return status of execute (and most other DBI methods) for errors.
For a non-SELECT statement, execute returns the number of rows
affected, if known. If no rows were affected, then execute returns
``0E0'', which Perl will treat as 0 but will regard as true. Note that it
is not an error for no rows to be affected by a statement. If the
number of rows affected is not known, then execute returns -1.
For SELECT statements, execute simply ``starts'' the query within the
database engine. Use one of the fetch methods to retreive the data after
calling execute. The execute method does not return the number of
rows that will be returned by the query (because most databases can't
tell in advance), it simply returns a true value.
If any arguments are given, then execute will effectively call
bind_param for each value before executing the statement.
Values bound in this way are usually treated as SQL_VARCHAR types
unless the driver can determine the correct type (which is rare), or
unless
bind_param (or bind_param_inout) has already been used to specify the
type.
fetchrow_arrayref$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 will currently be returned for each fetch, so don't store the reference and then use it after a later fetch.
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 $sth->err afterwards (or use
the RaiseError attribute) to discover if the empty list returned was
due to an error.
In a scalar context, fetchrow_array returns the value of the first
field. An undef is returned if there are no more rows or if an error
occurred. Since that undef can't be distinguished from an undef returned
because the first field value was NULL, you should exercise some
caution if you use fetchrow_array in a scalar context.
fetchrow_hashref$hash_ref = $sth->fetchrow_hashref; $hash_ref = $sth->fetchrow_hashref($name);
An alternative to fetchrow_arrayref. Fetches the next row of data
and returns it as a reference to a hash containing field name and field
value pairs. Null fields are returned as undef values in the hash.
If there are no more rows or if an error occurs, then fetchrow_hashref
returns an undef. You should check $sth->err afterwards (or use the
RaiseError attribute) to discover if the undef returned was due to an
error.
The optional $name parameter specifies the name of the statement handle
attribute. For historical reasons it defaults to ``NAME'', however using either
``NAME_lc'' or ``NAME_uc'' is recomended for portability.
The keys of the hash are the same names returned by $sth->{$name}. If
more than one field has the same name, there will only be one entry in
the returned hash for those fields.
Because of the extra work fetchrow_hashref and Perl have to perform, it
is not as efficient as fetchrow_arrayref or fetchrow_array.
Currently, a new hash reference is returned for each row. This will change in the future to return the same hash ref each time, so don't rely on the current behaviour.
fetchall_arrayref$tbl_ary_ref = $sth->fetchall_arrayref; $tbl_ary_ref = $sth->fetchall_arrayref( $slice_array_ref ); $tbl_ary_ref = $sth->fetchall_arrayref( $slice_hash_ref );
The fetchall_arrayref method can be used to fetch all the data to be
returned from a prepared and executed statement handle. It returns a
reference to an array that contains one reference per row.
If there are no rows to return, fetchall_arrayref returns a reference
to an empty array. If an error occurs, fetchall_arrayref returns the
data fetched thus far, which may be none. You should check $sth->err
afterwards (or use the RaiseError attribute) to discover if the data is
complete or was truncated due to an error.
When passed an array reference, fetchall_arrayref uses fetchrow_arrayref
to fetch each row as an array ref. If the parameter array is not empty
then it is used as a slice to select individual columns by index number.
With no parameters, fetchall_arrayref acts as if passed an empty array ref.
When passed a hash reference, fetchall_arrayref uses fetchrow_hashref
to fetch each row as a hash reference. If the parameter hash is not empty,
then it is used as a slice to select individual columns by name. The
names should be lower case regardless of the letter case in $sth->{NAME}.
The values of the hash should be set to 1.
For example, to fetch just the first column of every row:
$tbl_ary_ref = $sth->fetchall_arrayref([0]);
To fetch the second to last and last column of every row:
$tbl_ary_ref = $sth->fetchall_arrayref([-2,-1]);
To fetch all fields of every row as a hash ref:
$tbl_ary_ref = $sth->fetchall_arrayref({});
To fetch only the fields called ``foo'' and ``bar'' of every row as a hash ref:
$tbl_ary_ref = $sth->fetchall_arrayref({ foo=>1, bar=>1 });
The first two examples return a reference to an array of array refs. The last returns a reference to an array of hash refs.
finish$rc = $sth->finish;
Indicates that no more data will be fetched from this statement handle
before it is either executed again or destroyed. The finish method
is rarely needed,
but can sometimes be helpful in very specific situations to
allow the server to free up resources (such as
sort buffers).
When all the data has been fetched from a SELECT statement, the driver
should automatically call finish for you. So you should not normally
need to call it explicitly.
Consider a query like:
SELECT foo FROM table WHERE bar=? ORDER BY foo
where you want to select just the first (smallest) ``foo'' value from a
very large table. When executed, the database server will have to use
temporary buffer space to store the sorted rows. If, after executing
the handle and selecting one row, the handle won't be re-executed for
some time and won't be destroyed, the finish method can be used to tell
the server that the buffer space can be freed.
Calling finish resets the Active attribute for the statement. It
may also make some statement handle attributes (such as NAME and TYPE)
unavailable if they have not already been accessed (and thus cached).
The finish method does not affect the transaction status of the
database connection. It has nothing to do with transactions. It's mostly an
internal ``housekeeping'' method that is rarely needed. There's no need
to call finish if you're about to destroy or re-execute the statement
handle. See also disconnect and the Active attribute.
rows$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);
Binds an output column (field) of a SELECT statement to a Perl variable.
See bind_columns below for an example. Note that column numbers count
up from 1.
Whenever a row is fetched from the database, the corresponding Perl variable is automatically updated. There is no need to fetch and assign the values manually. The binding is performed at a very low level using Perl aliasing so there is no extra copying taking place. This makes using bound variables very efficient.
For maximum portability between drivers, bind_col should be called after
execute. This restriction may be removed in a later version of the DBI.
You do not need to bind output columns in order to fetch data, but it can be useful for some applications which need either maximum performance or greater clarity of code. The bind_param method performs a similar but opposite function for input variables.
bind_columns$rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
Calls bind_col for each column of the SELECT statement.
The bind_columns method will die if the number of references does not
match the number of fields.
For maximum portability between drivers, bind_columns should be called
after execute.
For example:
$dbh->{RaiseError} = 1; # do this, or check every call for errors
$sth = $dbh->prepare(q{ SELECT region, sales FROM sales_by_region });
$sth->execute;
my ($region, $sales);
# Bind Perl variables to columns: $rv = $sth->bind_columns(\$region, \$sales);
# you can also use Perl's \(...) syntax (see perlref docs): # $sth->bind_columns(\($region, $sales));
# Column binding is the most efficient way to fetch data
while ($sth->fetch) {
print "$region: $sales\n";
}
For compatibility with old scripts, the first parameter will be
ignored if it is undef or
a hash reference.
dump_results$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 recomended 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 is fatal, except for private driver specific attributes (which all have names starting with a lowercase letter).
Example:
... = $h->{NUM_OF_FIELDS}; # get/read
Note that some drivers cannot provide valid values for some or all of
these attributes until after $sth->execute has been called.
See also finish to learn more about the effect it may have on some attributes.
NUM_OF_FIELDS (integer, read-only)SELECT
statements will have NUM_OF_FIELDS == 0.
NUM_OF_PARAMS (integer, read-only)NAME (array-ref, read-only)
print "First column name: $sth->{NAME}->[0]\n";
NAME_lc (array-ref, read-only)NAME_uc (array-ref, read-only)TYPE (array-ref, read-only) NEWThe 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. See:
ftp://jerry.ece.umassd.edu/isowg3/dbl/SQL_Registry
Where there's no vendor-supplied ODBC driver to be compatible with, the DBI driver can use type numbers in the range that is now officially reserved for use by the DBI: -9999 to -9000.
All possible values for TYPE should have at least one entry in the
output of the type_info_all method (see type_info_all).
PRECISION (array-ref, read-only) NEWSCALE (array-ref, read-only) NEWundef) values indicate columns where scale is not applicable.
NULLABLE (array-ref, read-only)0 = no, 1 = yes, 2 = unknown.
print "First column may return NULL\n" if $sth->{NULLABLE}->[0];
CursorName (string, read-only)"where current of ..." SQL syntax, then it returns undef.
Statement (string, read-only) NEWRowsInCache (integer, read-only)SELECT statements, then
this attribute holds the number of un-fetched rows in the cache. If the
driver doesn't, then it returns undef. Note that some drivers pre-fetch
rows on execute, whereas others wait till the first fetch.
See also the RowCacheSize database handle attribute.
Transactions are a fundamental part of any robust database system. They protect against errors and database corruption by ensuring that sets of related changes to the database take place in atomic (indivisible, all-or-nothing) units.
This section applies to databases that support transactions and where
AutoCommit is off. See AutoCommit for details of using AutoCommit
with various types of databases.
The recommended way to implement robust transactions in Perl
applications is to use RaiseError and eval { ... }
(which is very fast, unlike eval "..."). For example:
$dbh->{AutoCommit} = 0; # enable transactions, if possible
$dbh->{RaiseError} = 1;
eval {
foo(...) # do lots of work here
bar(...) # including inserts
baz(...) # and updates
$dbh->commit; # commit the changes if we get this far
};
if ($@) {
warn "Transaction aborted because $@";
$dbh->rollback; # undo the incomplete changes
# 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) || 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.
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.
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 $dbh = DBI->connect("dbi:DriverName:db_name", $user, $password)
|| die "Can't connect to $data_source: $DBI::errstr";
my $sth = $dbh->prepare( q{
SELECT name, phone
FROM mytelbook
}) || die "Can't prepare statement: $DBI::errstr";
my $rc = $sth->execute
|| die "Can't execute statement: $DBI::errstr";
print "Query will return $sth->{NUM_OF_FIELDS} fields.\n\n";
print "Field names: @{ $sth->{NAME} }\n";
while (($name, $phone) = $sth->fetchrow_array) {
print "$name: $phone\n";
}
# check for problems which may have terminated the fetch early
die $sth->errstr if $sth->err;
$dbh->disconnect;
Here's a complete example program to insert some data from a file.
(This example uses RaiseError to avoid needing to check each call).
my $dbh = DBI->connect("dbi:DriverName:db_name", $user, $password, {
RaiseError => 1, AutoCommit => 0
});
my $sth = $dbh->prepare( q{
INSERT INTO table (name, phone) VALUES (?, ?)
});
open FH, "<phone.csv" or die "Unable to open phone.csv: $!";
while (<FH>) {
chop;
my ($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 perlop/``Quote and Quote-like Operators'' for more details.
Perl versions 5.004_50 and later include optional experimental support for multiple threads on many platforms. If the DBI is built using a Perl that has threads enabled then it will use a per-driver mutex to ensure that only one thread is with a driver at any one time. Please note that support for threads in Perl is still experimental and is known to have some significant problems. It's use is not recommended.
The first thing to say is that signal handling in Perl is currently not safe. There is always a small risk of Perl crashing and/or core dumping when, or after, handling a signal. (The risk was reduced with 5.004_04 but is still present.)
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}.
To assist in implementing these operations, 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.
However, it must be stressed that: a) few drivers implement this at
the moment (the DBI provides a default method that just returns undef);
and b) even if implemented, there is still a possibility that the statement
handle, and possibly 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
engine does not have cancel implemented.
In addition to the trace method, you can enable the same trace
information by setting the DBI_TRACE environment variable before
starting Perl.
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
If DBI_TRACE is set to a non-numeric value, then it is assumed to
be a file name and the trace level will be set to 2 with all trace
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
See also the trace method.
$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.
$sth handle you're using to call execute is probably undefined because
the preceeding prepare failed. You should always check the return status of
DBI methods, or use the RaiseError attribute.
(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.)
$dbh handle went out of scope or the program ended before the handle
was disconnected from the database. You should always explicitly call
disconnect when you are finished using a database handle. If using
transactions then you should also explicitly call commit or rollback
before disconnect.
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.
Programming the Perl DBI, by Alligator Descartes and Tim Bunce.
Programming Perl 2nd Ed. by Larry Wall, Tom Christiansen & Randal Schwartz.
Learning Perl by Randal Schwartz.
Dr Dobb's Journal, November 1996.
The Perl Journal, April 1997.
perl(1), perlmod(1), perlbook(1)
The dbi-users mailing list is the primary means of communication among users of the DBI and its related modules. Subscribe and unsubscribe via:
http://www.isc.org/dbi-lists.html
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 are held at:
http://www.xray.mpe.mpg.de/mailing-lists/dbi/ http://www.egroups.com/list/dbi-users/info.html http://www.bitmechanic.com/mail-archives/dbi-users/
The DBI ``Home Page'':
http://www.symbolstone.org/technology/perl/DBI
Other DBI related links:
http://tegan.deltanet.com/~phlip/DBUIdoc.html http://dc.pm.org/perl_db.html http://wdvl.com/Authoring/DB/Intro/toc.html http://www.hotwired.com/webmonkey/backend/tutorials/tutorial1.html http://bumppo.net/lists/macperl/1999/06/msg00197.html
Other database related links:
http://www.jcc.com/sql_stnd.html http://cuiwww.unige.ch/OSG/info/FreeDB/FreeDB.home.html
Commercial and Data Warehouse Links
http://www.dwinfocenter.org http://www.datawarehouse.com http://www.datamining.org http://www.olapcouncil.org http://www.idwa.org http://www.knowledgecenters.org/dwcenter.asp
Recommended Perl Programming Links
http://language.perl.com/style/
Please also read the DBI FAQ which is installed as a DBI::FAQ module.
You can use perldoc to read it by executing the perldoc DBI::FAQ command.
DBI by Tim Bunce. This pod text by Tim Bunce, J. Douglas Dunlop,
Jonathan Leffler and others. Perl by Larry Wall and the
perl5-porters.
The DBI module is Copyright (c) 1994-2000 Tim Bunce. England. All rights reserved.
You may distribute under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.
I would like to acknowledge the valuable contributions of the many people I have worked with on the DBI project, especially in the early years (1992-1994). In no particular order: Kevin Stock, Buzz Moschetti, Kurt Andersen, Ted Lemon, William Hails, Garth Kennedy, Michael Peppler, Neil S. Briscoe, Jeff Urlwin, David J. Hughes, Jeff Stander, Forrest D Whitcher, Larry Wall, Jeff Fried, Roy Johnson, Paul Hudson, Georg Rehfeld, Steve Sizemore, Ron Pool, Jon Meek, Tom Christiansen, Steve Baumgarten, Randal Schwartz, and a whole lot more.
Then, of course, there are the poor souls who have struggled through untold and undocumented obstacles to actually implement DBI drivers. Among their ranks are Jochen Wiedmann, Alligator Descartes, Jonathan Leffler, Jeff Urlwin, Michael Peppler, Henrik Tougaard, Edwin Pratomo, Davide Migliavacca, Jan Pazdziora, Peter Haworth, Edmund Mergl, Steve Williams, Thomas Lowery, and Phlip Plumlee. Without them, the DBI would not be the practical reality it is today. I'm also especially grateful to Alligator Descartes for starting work on the ``Programming the Perl DBI'' book and letting me jump on board.
A German translation of this manual and other Perl module docs (all probably slightly out of date) is available, thanks to O'Reilly, at:
http://www.oreilly.de/catalog/perlmodger/
Some other translations:
http://cronopio.net/perl/ - Spanish http://member.nifty.ne.jp/hippo2000/dbimemo.htm - Japanese
The DBI is free software. IT COMES WITHOUT WARRANTY OF ANY KIND.
Commercial support for Perl and the DBI, DBD::Oracle and Oraperl modules can be arranged via The Perl Clinic. For more details visit:
http://www.perlclinic.com
References to DBI related training resources. No recommendation implied.
http://www.treepax.co.uk/ http://www.keller.com/dbweb/
See the DBI FAQ for a more comprehensive list of FAQs. Use the
perldoc DBI::FAQ command to read it.
To measure the speed of the DBI and DBD::Oracle code, I modified DBD::Oracle so you can set an attribute that will cause the same row to be fetched from the row cache over and over again (without involving Oracle code but exercising *all* the DBI and DBD::Oracle code in the code path for a fetch).
The results (on my lightly loaded old Sparc 10) fetching 50000 rows using:
1 while $csr->fetch;
| were: | |
| one field: 5300 fetches per cpu second (approx) | |
| ten fields: 4000 fetches per cpu second (approx) |
Obviously results will vary between platforms (newer faster platforms can reach around 50000 fetches per second), but it does give a feel for the maximum performance: fast. By way of comparison, using the code:
1 while @row = $csr->fetchrow_array;
(fetchrow_array is roughly the same as ora_fetch) gives:
one field: 3100 fetches per cpu second (approx)
ten fields: 1000 fetches per cpu second (approx)
Notice the slowdown and the more dramatic impact of extra fields. (The fields were all one char long. The impact would be even bigger for longer strings.)
Changing that slightly to represent actually doing something in Perl with the fetched data:
while(@row = $csr->fetchrow_array) {
$hash{++$i} = [ @row ];
}
gives: ten fields: 500 fetches per cpu second (approx)
That simple addition has *halved* the performance.
I therefore conclude that DBI and DBD::Oracle overheads are small compared with Perl language overheads (and probably database overheads).
So, if you think the DBI or your driver is slow, try replacing your fetch loop with just:
1 while $csr->fetch;
and time that. If that helps then point the finger at your own code. If that doesn't help much then point the finger at the database, the platform, the network etc. But think carefully before pointing it at the DBI or your driver.
(Having said all that, if anyone can show me how to make the DBI or drivers even more efficient, I'm all ears.)
Read the information in the references below. Please do not post CGI related questions to the dbi-users mailing list (or to me).
http://www.perl.com/cgi-bin/pace/pub/doc/FAQs/cgi/perl-cgi-faq.html http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html http://www.boutell.com/faq/ http://www.perl.com/perl/faq/
General problems and good ideas:
Use the CGI::ErrorWrap module. Remember that many env vars won't be set for CGI scripts.
For information on the Apache httpd server and the mod_perl module see
http://perl.apache.org/
A DBD::ODBC module is available.
No. The DBI has no knowledge or understanding of dates at all.
Individual drivers (DBD::*) may have some date handling code but are unlikely to have year 2000 related problems within their code. However, your application code which uses the DBI and DBD drivers may have year 2000 related problems if it has not been designed and written well.
See also the ``Does Perl have a year 2000 problem?'' section of the Perl FAQ:
http://www.perl.com/CPAN/doc/FAQs/FAQ/PerlFAQ.html
mod_perl. Establishes a database connection which
remains open for the lifetime of the HTTP daemon. This way the CGI
connect and disconnect for every database access becomes superfluous.
jdbc:dbi://host.domain.etc:999/Driver/DBName.
It seems to be very similar to some commercial products, such as jdbcKona.
Hugo van der Sanden <hv@crypt.compulink.co.uk>
Stephen Zander <stephen.zander@mckesson.com>
Based on the O'Reilly lex/yacc book examples and byacc.
See also the SQL::Statement module, a very simple SQL parser and engine, base of the DBD::CSV driver.