NAME

DBD::Pg - PostgreSQL database driver for the DBI module

VERSION

This documents version 2.0.0_2 of the DBD::Pg module

SYNOPSIS

use DBI;

$dbh = DBI->connect("dbi:Pg:dbname=$dbname", "", "", {AutoCommit => 0});

# For some advanced uses you may need PostgreSQL type values:
use DBD::Pg qw(:pg_types);

# See the DBI module documentation for full details

DESCRIPTION

DBD::Pg is a Perl module that works with the DBI module to provide access to PostgreSQL databases.

MODULE DOCUMENTATION

This documentation describes driver specific behavior and restrictions. It is not supposed to be used as the only reference for the user. In any case consult the DBI documentation first!

THE DBI CLASS

DBI Class Methods

connect

To connect to a database with a minimum of parameters, use the following syntax:

$dbh = DBI->connect("dbi:Pg:dbname=$dbname", "", "");

This connects to the database $dbname on the default port (usually 5432) without any user authentication.

The following connect statement shows almost all possible parameters:

$dbh = DBI->connect("dbi:Pg:dbname=$dbname;host=$host;port=$port;" .
                    "options=$options", "$username", "$password",
                    {AutoCommit => 0});

If a parameter is not given, the PostgreSQL server will first look for specific environment variables, and then use hard-coded defaults:

parameter  environment variable  hard coded default
--------------------------------------------------
host       PGHOST                local domain socket
hostaddr   PGHOSTADDR            local domain socket
port       PGPORT                5432
dbname*    PGDATABASE            current userid
username   PGUSER                current userid
password   PGPASSWORD            (none)
options    PGOPTIONS             (none)
service    PGSERVICE             (none)
sslmode    PGSSLMODE             (none)

* Can also use "db" or "database"

The options parameter specifies runtime options for the Postgres backend. Common usage is to increase the number of buffers with the -B option. Also important is the -F option, which disables automatic fsync() call after each transaction. For further details please refer to the PostgreSQL documentation at http://www.postgresql.org/docs/.

For authentication with username and password, appropriate entries have to be made in the pg_hba.conf file. If the username and password entries passed via connect() are undefined (as opposed to being empty), DBI will use the environment variables DBI_USER and DBI_PASS if they exists.

You can also connect by using a service connection file, which is named "pg_service.conf." The location of this file can be controlled by setting the PGSYSCONFDIR environment variable. To use one of the named services within the file, set the name by using either the "service" parameter or the environment variable PGSERVICE. Note that when connecting this way, only the minimum parameters should be used. For example, to connect to a service named "zephyr", you could use:

$dbh = DBI->connect("dbi:Pg:service=zephyr", "", "");

You could also set $ENV{PGSERVICE} to "zephyr" and connect like this:

$dbh = DBI->connect("dbi:Pg:", "", "");

The format of the pg_service.conf file is simply a bracketed service name, followed by one parameter per line in the format name=value. For example:

[zephyr]
dbname=winds
user=wisp
password=W$2Hc00YSgP
port=6543

There are four valid arguments to the "sslmode" parameter, which controls whether to use SSL to connect to the database:

disable - SSL connections are never used
allow - try non-SSL, then SSL
prefer - try SSL, then non-SSL
require - connect only with SSL

connect_cached

Implemented by DBI, no driver-specific impact.

installed_drivers

Implemented by DBI, no driver-specific impact.

installed_versions

Implemented by DBI, no driver-specific impact.

available_drivers

@driver_names = DBI->available_drivers;

Implemented by DBI, no driver-specific impact.

data_sources

@data_sources = DBI->data_sources('Pg');

This driver supports this method. Note that the necessary database connection to the database "template1" will be made on the localhost without any user authentication. Other preferences can be set with the environment variables PGHOST, PGPORT, DBI_USER, DBI_PASS, and PGSERVICE.

You can also pass in options to add to the connection string as the second argument to data_sources. For example, to specify an alternate port and host:

@data_sources = DBI->data_sources('Pg', 'port=5824;host=example.com');

METHODS COMMON TO ALL HANDLES

err

$rv = $h->err;

Supported by this driver as proposed by DBI. For the connect method it returns PQstatus. In all other cases it returns PQresultStatus of the current handle.

errstr

$str = $h->errstr;

Supported by this driver as proposed by DBI. It returns the PQerrorMessage related to the current handle.

state

$str = $h->state;

Supported by this driver. Returns a five-character "SQLSTATE" code. Success is indicated by a "00000" code, which gets mapped to an empty string by DBI. A code of S8006 indicates a connection failure, usually because the connection to the PostgreSQL server has been lost.

Note that state can be called as either $sth->state or $dbh->state.

The list of codes used by PostgreSQL can be found at: http://www.postgresql.org/docs/current/static/errcodes-appendix.html

Note that these codes are part of the SQL standard and only a small number of them will be used by PostgreSQL.

Common ones to note:

00000 Successful completion
25P01 No active SQL transaction
25P02 In failed SQL transaction

trace

$h->trace($trace_level);
$h->trace($trace_level, $trace_filename);

Implemented by DBI, no driver-specific impact.

trace_msg

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

Implemented by DBI, no driver-specific impact.

func

This driver supports a variety of driver specific functions accessible via the func method. Note that the name of the function comes last, after the arguments.

table_attributes

$attrs = $dbh->func($table, 'table_attributes');

The table_attributes function is no longer recommended. Instead, you can use the more portable column_info and primary_key methods to access the same information.

The table_attributes method returns, for the given table argument, a reference to an array of hashes, each of which contains the following keys:

NAME        attribute name
TYPE        attribute type
SIZE        attribute size (-1 for variable size)
NULLABLE    flag nullable
DEFAULT     default value
CONSTRAINT  constraint
PRIMARY_KEY flag is_primary_key
REMARKS     attribute description

lo_creat

$lobjId = $dbh->func($mode, 'lo_creat');

Creates a new large object and returns the object-id. $mode is a bitmask describing different attributes of the new object. Use the following constants:

$dbh->{pg_INV_WRITE}
$dbh->{pg_INV_READ}

Upon failure it returns undef.

lo_open

$lobj_fd = $dbh->func($lobjId, $mode, 'lo_open');

Opens an existing large object and returns an object-descriptor for use in subsequent lo_* calls. For the mode bits see lo_creat. Returns undef upon failure. Note that 0 is a perfectly correct object descriptor!

lo_write

$nbytes = $dbh->func($lobj_fd, $buf, $len, 'lo_write');

Writes $len bytes of $buf into the large object $lobj_fd. Returns the number of bytes written and undef upon failure.

lo_read

$nbytes = $dbh->func($lobj_fd, $buf, $len, 'lo_read');

Reads $len bytes into $buf from large object $lobj_fd. Returns the number of bytes read and undef upon failure.

lo_lseek

$loc = $dbh->func($lobj_fd, $offset, $whence, 'lo_lseek');

Changes the current read or write location on the large object $obj_id. Currently $whence can only be 0 (L_SET). Returns the current location and undef upon failure.

lo_tell

$loc = $dbh->func($lobj_fd, 'lo_tell');

Returns the current read or write location on the large object $lobj_fd and undef upon failure.

lo_close

$lobj_fd = $dbh->func($lobj_fd, 'lo_close');

Closes an existing large object. Returns true upon success and false upon failure.

lo_unlink

$ret = $dbh->func($lobjId, 'lo_unlink');

Deletes an existing large object. Returns true upon success and false upon failure.

lo_import

$lobjId = $dbh->func($filename, 'lo_import');

Imports a Unix file as large object and returns the object id of the new object or undef upon failure.

lo_export

$ret = $dbh->func($lobjId, $filename, 'lo_export');

Exports a large object into a Unix file. Returns false upon failure, true otherwise.

pg_notifies

$ret = $dbh->func('pg_notifies');

Returns either undef or a reference to two-element array [ $table, $backend_pid ] of asynchronous notifications received. Note that this does not check if the connection to the database is still valid - for that, use the c<ping> method. Also note that you may need to commit if not in autocommit mode - new notices will not be picked up while in the middle of a transaction. An example:

$dbh->do("LISTEN abc");
$dbh->do("LISTEN def");

## Hang around until we get the message we want
LISTENLOOP: {
  while (my $notify = $dbh->func('pg_notifies')) {
    my ($name, $pid) = @$notify;
    print qq{I received notice "$name" from PID $pid\n};
    ## Do something based on the notice received
  }
  $dbh->ping() or die qq{Ping failed!};
  $dbh->commit();
  sleep(5);
  redo;
}

getfd

$fd = $dbh->func('getfd');

Returns fd of the actual connection to server. Can be used with select() and func('pg_notifies'). Deprecated in favor of $dbh->{pg_socket}.

private_attribute_info

$hashref = $dbh->private_attribute_info();
$hashref = $sth->private_attribute_info();

Supported by this driver as proposed by DBI.

ATTRIBUTES COMMON TO ALL HANDLES

Warn (boolean, inherited): Implemented by DBI, no driver-specific impact.
Active (boolean, read-only): Supported by this driver as proposed by DBI. A database handle is active while it is connected and statement handle is active until it is finished.
Executed (boolean, read-only): Implemented by DBI, no driver-specific impact. Requires DBI 1.41 or greater.
Kids (integer, read-only): Implemented by DBI, no driver-specific impact.
ActiveKids (integer, read-only): Implemented by DBI, no driver-specific impact.
CachedKids (hash ref): Implemented by DBI, no driver-specific impact.
Type (scalar): Implemented by DBI, no driver-specific impact.
ChildHandles (array ref): Implemented by DBI, no driver-specific impact.
CompatMode (boolean, inherited): Not used by this driver.
InactiveDestroy (boolean): Implemented by DBI, no driver-specific impact. If set to true, then the disconnect() method will not be automatically called when the database handle goes out of scope (e.g. when exiting after a fork).
PrintWarn (boolean, inherited): Implemented by DBI, no driver-specific impact.
PrintError (boolean, inherited): Implemented by DBI, no driver-specific impact.
RaiseError (boolean, inherited): Implemented by DBI, no driver-specific impact.
HandleError (boolean, inherited): Implemented by DBI, no driver-specific impact.
HandleSetErr (code ref, inherited): Implemented by DBI, no driver-specific impact.
ErrCount (unsigned integer): Implemented by DBI, no driver-specific impact.
ShowErrorStatement (boolean, inherited): Implemented by DBI, no driver-specific impact.
TraceLevel (integer, inherited): Implemented by DBI, no driver-specific impact.
FetchHashKeyName (string, inherited): Implemented by DBI, no driver-specific impact.
ChopBlanks (boolean, inherited): Supported by this driver as proposed by DBI. This method is similar to the SQL function RTRIM.
LongReadLen (integer, inherited): Not used by this driver.
LongTruncOk (boolean, inherited): Not used by this driver.
Taint (boolean, inherited): Implemented by DBI, no driver-specific impact.
TaintIn (boolean, inherited): Implemented by DBI, no driver-specific impact.
TaintOut (boolean, inherited): Implemented by DBI, no driver-specific impact.
Profile (inherited): Implemented by DBI, no driver-specific impact.

DBI DATABASE HANDLE OBJECTS

Database Handle Methods

selectrow_array

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

Implemented by DBI, no driver-specific impact.

selectrow_arrayref

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

Implemented by DBI, no driver-specific impact.

selectrow_hashref

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

Implemented by DBI, no driver-specific impact.

selectall_arrayref

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

Implemented by DBI, no driver-specific impact.

selectall_hashref

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

Implemented by DBI, no driver-specific impact.

selectcol_arrayref

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

Implemented by DBI, no driver-specific impact.

prepare

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

WARNING: DBD::Pg now uses true prepared statements by sending them to the backend to be prepared by the PostgreSQL server. Statements that were legal before may no longer work. See below for details.

Prepares a statement for later execution. PostgreSQL supports prepared statements, which enables DBD::Pg to only send the query once, and simply send the arguments for every subsequent call to execute(). DBD::Pg can use these server-side prepared statements, or it can just send the entire query to the server each time. The best way is automatically chosen for each query. This will be sufficient for most users: keep reading for a more detailed explanation and some optional flags.

Statements that do not begin with the word "SELECT", "INSERT", "UPDATE", or "DELETE" will not be sent to be server-side prepared.

Deciding whether or not to use prepared statements depends on many factors, but you can force them to be used or not used by passing the pg_server_prepare attribute to prepare(). A "0" means to never use prepared statements. Setting pg_server_prepare to "1" means that prepared statements should be used whenever possible. This is the default for servers version 8.0 or higher. Servers that are version 7.4 get a special default value of "2", because server-side statements were only partially supported in that version. In this case, it only uses server-side prepares if all parameters are specifically bound.

The pg_server_prepare attribute can also be set at connection time like so:

$dbh = DBI->connect($DBNAME, $DBUSER, $DBPASS,
                    { AutoCommit => 0,
                      RaiseError => 1,
                      pg_server_prepare => 0 });

or you may set it after your database handle is created:

$dbh->{pg_server_prepare} = 1;

To enable it for just one particular statement:

$sth = $dbh->prepare("SELECT id FROM mytable WHERE val = ?",
                     { pg_server_prepare => 1 });

You can even toggle between the two as you go:

$sth->{pg_server_prepare} = 1;
$sth->execute(22);
$sth->{pg_server_prepare} = 0;
$sth->execute(44);
$sth->{pg_server_prepare} = 1;
$sth->execute(66);

In the above example, the first execute will use the previously prepared statement. The second execute will not, but will build the query into a single string and send it to the server. The third one will act like the first and only send the arguments. Even if you toggle back and forth, a statement is only prepared once.

Using prepared statements is in theory quite a bit faster: not only does the PostgreSQL backend only have to prepare the query only once, but DBD::Pg no longer has to worry about quoting each value before sending it to the server.

However, there are some drawbacks. The server cannot always choose the ideal parse plan because it will not know the arguments before hand. But for most situations in which you will be executing similar data many times, the default plan will probably work out well. Programs such as PgBouncer which cache connections at a low level should not use prepared statements via DBD::Pg, or must take extra care in the application to account for the fact that prepared statements are not shared across database connections. Further discussion on this subject is beyond the scope of this documentation: please consult the pgsql-performance mailing list, http://archives.postgresql.org/pgsql-performance/

Only certain commands will be sent to a server-side prepare: currently these include SELECT, INSERT, UPDATE, and DELETE. DBD::Pg uses a simple naming scheme for the prepared statements: dbdpg_#, where "#" starts at 1 and increases. This number is tracked at the database handle level, so multiple statement handles will not collide. If you use your own prepare statements, do not name them "dbdpg_"!

You cannot send more than one command at a time in the same prepare command, by separating them with semi-colons, when using server-side prepares.

The actual PREPARE is not performed until the first execute is called, due to the fact that information on the data types (provided by bind_param) may be given after the prepare but before the execute.

A server-side prepare can also happen before the first execute. If the server can handle the server-side prepare and the statement has no placeholders, it will be prepared right away. It will also be prepared if the pg_prepare_now attribute is passed. Similarly, the <pg_prepare_now> attribute can be set to 0 to ensure that the statement is not prepared immediately, although cases in which you would want this may be rare. Finally, you can set the default behavior of all prepare statements by setting the pg_prepare_now attribute on the database handle:

$dbh->{pg_prepare_now} = 1;

The following two examples will be prepared right away:

$sth->prepare("SELECT 123"); ## no placeholders

$sth->prepare("SELECT 123, ?", {pg_prepare_now => 1});

The following two examples will NOT be prepared right away:

$sth->prepare("SELECT 123, ?"); ## has a placeholder

$sth->prepare("SELECT 123", {pg_prepare_now => 0});

There are times when you may want to prepare a statement yourself. To do this, simply send the PREPARE statement directly to the server (e.g. with "do"). Create a statement handle and set the prepared name via pg_prepare_name attribute. The statement handle can be created with a dummy statement, as it will not be executed. However, it should have the same number of placeholders as your prepared statement. Example:

$dbh->do("PREPARE mystat AS SELECT COUNT(*) FROM pg_class WHERE reltuples < ?");
$sth = $dbh->prepare("SELECT ?");
$sth->bind_param(1, 1, SQL_INTEGER);
$sth->{pg_prepare_name} = "mystat";
$sth->execute(123);

The above will run this query:

SELECT COUNT(*) FROM pg_class WHERE reltuples < 123;

Note: DBD::Pg will not escape your custom prepared statement name, so don't use a name that needs escaping! DBD::Pg uses the prepare names dbdpg_# internally, so please do not use those either.

You can force DBD::Pg to send your query directly to the server by adding the pg_direct attribute to your prepare call. This is not recommended, but is added just in case you need it.

Placeholders

There are three types of placeholders that can be used in DBD::Pg. The first is the question mark method, in which each placeholder is represented by a single question mark. This is the method recommended by the DBI specs and is the most portable. Each question mark is replaced by a "dollar sign number" in the order in which they appear in the query (important when using bind_param).

The second method is to use "dollar sign numbers" directly. This is the method that PostgreSQL uses internally and is overall probably the best method to use if you do not need compatibility with other database systems. DBD::Pg, like PostgreSQL, allows the same number to be used more than once in the query. Numbers must start with "1" and increment by one value. If the same number appears more than once in a query, it is treated as a single parameter and all instances are replaced at once. Examples:

Not legal:

$SQL = 'SELECT count(*) FROM pg_class WHERE relpages > $2';

$SQL = 'SELECT count(*) FROM pg_class WHERE relpages BETWEEN $1 AND $3';

Legal:

$SQL = 'SELECT count(*) FROM pg_class WHERE relpages > $1';

$SQL = 'SELECT count(*) FROM pg_class WHERE relpages BETWEEN $1 AND $2';

$SQL = 'SELECT count(*) FROM pg_class WHERE relpages BETWEEN $1 AND $2 AND reltuples > $1';

$SQL = 'SELECT count(*) FROM pg_class WHERE relpages > $1 AND reltuples > $1';

In the final statement above, DBI thinks there is only one placeholder, so this statement will replace both placeholders:

$sth->bind_param(1, 2045);

While execute requires only a single argument as well:

$sth->execute(2045);

The final placeholder method is the named parameters in the format ":foo". While this syntax is supported by DBD::Pg, its use is highly discouraged.

The different types of placeholders cannot be mixed within a statement, but you may use different ones for each statement handle you have. Again, this is not encouraged.

If your queries use operators that contain question marks (e.g. some of the native Postgres geometric operators) or array slices (e.g. data[100:300]), you can tell DBD::Pg to ignore any non-dollar sign placeholders by setting the "pg_placeholder_dollaronly" attribute at either the database handle or the statement handle level. Examples:

$dbh->{pg_placeholder_dollaronly} = 1;
$sth = $dbh->prepare(q{SELECT * FROM mytable WHERE lseg1 ?# lseg2 AND name = $1});
$sth->execute('segname');

Alternatively, you can set it at prepare time:

$sth = $dbh->prepare(q{SELECT * FROM mytable WHERE lseg1 ?-| lseg2 AND name = $1},
  {pg_placeholder_dollaronly = 1});
$sth->execute('segname');

prepare_cached

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

Implemented by DBI, no driver-specific impact. This method is most useful when using a server that supports server-side prepares, and you have asked the prepare to happen immediately via the pg_prepare_now attribute.

do

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

Prepare and execute a single statement. Note that an empty statement (string with no length) will not be passed to the server; if you want a simple test, use "SELECT 123" or the ping() function. If neither attr nor bind_values is given, the query will be sent directly to the server without the overhead of creating a statement handle and running prepare and execute.

last_insert_id

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

Attempts to return the id of the last value to be inserted into a table. You can either provide a sequence name (preferred) or provide a table name with optional schema. The $catalog and $field arguments are always ignored. The current value of the sequence is returned by a call to the CURRVAL() PostgreSQL function. This will fail if the sequence has not yet been used in the current database connection.

If you do not know the name of the sequence, you can provide a table name and DBD::Pg will attempt to return the correct value. To do this, there must be at least one column in the table with a NOT NULL constraint, that has a unique constraint, and which uses a sequence as a default value. If more than one column meets these conditions, the primary key will be used. This involves some looking up of things in the system table, so DBD::Pg will cache the sequence name for subsequent calls. If you need to disable this caching for some reason, (such as the sequence name changing), you can control it via the pg_cache attribute.

Please keep in mind that this method is far from foolproof, so make your script use it properly. Specifically, make sure that it is called immediately after the insert, and that the insert does not add a value to the column that is using the sequence as a default value.

Some examples:

$dbh->do("CREATE SEQUENCE lii_seq START 1");
$dbh->do("CREATE TABLE lii (
  foobar INTEGER NOT NULL UNIQUE DEFAULT nextval('lii_seq'),
  baz VARCHAR)");
$SQL = "INSERT INTO lii(baz) VALUES (?)";
$sth = $dbh->prepare($SQL);
for (qw(uno dos tres cuatro)) {
  $sth->execute($_);
  my $newid = $dbh->last_insert_id(C<undef>,undef,undef,undef,{sequence=>'lii_seq'});
  print "Last insert id was $newid\n";
}

If you did not want to worry about the sequence name:

$dbh->do("CREATE TABLE lii2 (
  foobar SERIAL UNIQUE,
  baz VARCHAR)");
$SQL = "INSERT INTO lii2(baz) VALUES (?)";
$sth = $dbh->prepare($SQL);
for (qw(uno dos tres cuatro)) {
  $sth->execute($_);
  my $newid = $dbh->last_insert_id(undef,undef,"lii2",undef);
  print "Last insert id was $newid\n";
}

commit

$rc  = $dbh->commit;

Supported by this driver as proposed by DBI. See also the notes about Transactions elsewhere in this document.

rollback

$rc  = $dbh->rollback;

Supported by this driver as proposed by DBI. See also the notes about Transactions elsewhere in this document.

begin_work

Supported by this driver as proposed by DBI. Note that this will not issue a "begin" until immediately before the next given command.

disconnect

$rc  = $dbh->disconnect;

Supported by this driver as proposed by DBI.

ping

$rc = $dbh->ping;

This driver supports the ping method, which can be used to check the validity of a database handle. The value returned is either 0, indicating that the connection is no longer valid, or a positive integer, indicating the following:

Value    Meaning
--------------------------------------------------
  1      Database is idle (not in a transaction)
  2      Database is active, there is a command in progress (usually seen after a COPY command)
  3      Database is idle within a transaction
  4      Database is idle, within a failed transaction

Additional information on why a handle is not valid can be obtained by using the pg_ping method.

pg_ping

$rc = $dbh->pg_ping;

This is a DBD::Pg-specific extension to the ping command. This will check the validity of a database handle in exactly the same way as ping, but instead of returning a 0 for an invalid connection, it will return a negative number. So in addition to returning the positive numbers documented for ping, it may also return the following:

Value    Meaning
--------------------------------------------------
 -1      There is no connection to the database at all (e.g. after C<disconnect>)
 -2      An unknown transaction status was returned (e.g. after forking)
 -3      The handle exists, but no data was returned from a test query.

In practice, you should only ever see -1 and -2.

get_info

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

Supports a very large set (> 250) of the information types, including the minimum recommended by DBI.

table_info

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

Supported by this driver as proposed by DBI. This method returns all tables and views visible to the current user. The $catalog argument is currently unused. The schema and table arguments will do a LIKE search if a percent sign (%) or an underscore (_) is detected in the argument. The $type argument accepts a value of either "TABLE" or "VIEW" (using both is the default action).

The TABLE_CAT field will always return NULL (undef).

If your database supports tablespaces (version 8.0 or greater), two additional columns are returned, "pg_tablespace_name" and "pg_tablespace_location", that contain the name and location of the tablespace associated with this table. Tables that have not been assigned to a particular tablespace will return NULL (undef) for both of these columns.

Three additional fields are returned:

pg_schema - the unquoted name of the schema
pg_table - the unquoted name of the table
pg_column - the unquoted name of the column

column_info

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

Supported by this driver as proposed by DBI with the follow exceptions. These fields are currently always returned with NULL (undef) values:

TABLE_CAT
BUFFER_LENGTH
DECIMAL_DIGITS
NUM_PREC_RADIX
SQL_DATA_TYPE
SQL_DATETIME_SUB
CHAR_OCTET_LENGTH

Also, five additional non-standard fields are returned:

pg_type - data type with additional info i.e. "character varying(20)"
pg_constraint - holds column constraint definition
pg_schema - the unquoted name of the schema
pg_table - the unquoted name of the table
pg_column - the unquoted name of the column

Note that the TABLE_SCHEM, TABLE_NAME, and COLUMN_NAME fields all return output wrapped in quote_ident(). If you need the unquoted version, use the pg_ fields above.

primary_key_info

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

Supported by this driver as proposed by DBI. The $catalog argument is currently unused. There are no search patterns allowed, but leaving the $schema argument blank will cause the first table found in the schema search path to be used. An additional field, "DATA_TYPE", is returned and shows the data type for each of the arguments in the "COLUMN_NAME" field.

This method will also return tablespace information for servers that support tablespaces. See the table_info entry for more information.

The five additional custom fields returned are:

pg_tablespace_name - Name of the tablespace, if any
pg_tablespace_location - Location of the tablespace
pg_schema - the unquoted name of the schema
pg_table - the unquoted name of the table
pg_column - the unquoted name of the column

In addition to the standard format of returning one row for each column found for the primary key, you can pass the pg_onerow attribute to force a single row to be used. If the primary key has multiple columns, the "KEY_SEQ", "COLUMN_NAME", and "DATA_TYPE" fields will return a comma-delimited string. If the pg_onerow attribute is set to "2", the fields will be returned as an arrayref, which can be useful when multiple columns are involved:

$sth = $dbh->primary_key_info('', '', 'dbd_pg_test', {pg_onerow => 2});
if (defined $sth) {
  my $pk = $sth->fetchall_arrayref()->[0];
  print "Table $pk->[2] has a primary key on these columns:\n";
  for (my $x=0; defined $pk->[3][$x]; $x++) {
    print "Column: $pk->[3][$x]  (data type: $pk->[6][$x])\n";
  }
}

primary_key

Supported by this driver as proposed by DBI.

foreign_key_info

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

Supported by this driver as proposed by DBI, using the SQL/CLI variant. There are no search patterns allowed, but leaving the $schema argument blank will cause the first table found in the schema search path to be used. Two additional fields, "UK_DATA_TYPE" and "FK_DATA_TYPE", are returned to show the data type for the unique and foreign key columns. Foreign keys that have no named constraint (where the referenced column only has an unique index) will return undef for the "UK_NAME" field.

tables

@names = $dbh->tables( $catalog, $schema, $table, $type, \%attr );

Supported by this driver as proposed by DBI. This method returns all tables and/or views which are visible to the current user: see table_info() for more information about the arguments. The name of the schema appears before the table or view name. This can be turned off by adding in the pg_noprefix attribute:

my @tables = $dbh->tables( '', '', 'dbd_pg_test', '', {pg_noprefix => 1} );

type_info_all

$type_info_all = $dbh->type_info_all;

Supported by this driver as proposed by DBI. Information is only provided for SQL datatypes and for frequently used datatypes. The mapping between the PostgreSQL typename and the SQL92 datatype (if possible) has been done according to the following table:

+---------------+------------------------------------+
| typname       | SQL92                              |
|---------------+------------------------------------|
| bool          | BOOL                               |
| text          | /                                  |
| bpchar        | CHAR(n)                            |
| varchar       | VARCHAR(n)                         |
| int2          | SMALLINT                           |
| int4          | INT                                |
| int8          | /                                  |
| money         | /                                  |
| float4        | FLOAT(p)   p<7=float4, p<16=float8 |
| float8        | REAL                               |
| abstime       | /                                  |
| reltime       | /                                  |
| tinterval     | /                                  |
| date          | /                                  |
| time          | /                                  |
| datetime      | /                                  |
| timespan      | TINTERVAL                          |
| timestamp     | TIMESTAMP                          |
+---------------+------------------------------------+

For further details concerning the PostgreSQL specific datatypes please read pgbuiltin.

type_info

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

Implemented by DBI, no driver-specific impact.

quote

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

This module implements its own quote method. In addition to the DBI method it also doubles the backslash, because PostgreSQL treats a backslash as an escape character. You may also quote arrayrefs and received a string suitable for passing into Postgres array columns.

NOTE: The undocumented (and invalid) support for the SQL_BINARY data type is officially deprecated. Use PG_BYTEA with bind_param() instead:

$rv = $sth->bind_param($param_num, $bind_value,
                       { pg_type => PG_BYTEA });

quote_identifier

Implemented by DBI, no driver-specific impact.

pg_server_trace

$dbh->pg_server_trace($filehandle);

Writes debugging information from the PostgreSQL backend to a file. This is not the same as the trace() method and you should not use this method unless you know what you are doing. If you do enable this, be aware that the file will grow very large, very quick. To stop logging to the file, use the pg_server_untrace function. The first argument must be a file handle, not a filename. Example:

my $pid = $dbh->{pg_pid};
my $file = "pgbackend.$pid.debug.log";
open(my $fh, ">$file") or die qq{Could not open "$file": $!\n};
$dbh->pg_server_trace($fh);
## Run code you want to trace here
$dbh->pg_server_untrace;
close($fh);

pg_server_untrace

$dbh->pg_server_untrace

Stop server logging to a previously opened file.

Database Handle Attributes

AutoCommit (boolean)

Supported by this driver as proposed by DBI. According to the classification of DBI, PostgreSQL is a database in which a transaction must be explicitly started. Without starting a transaction, every change to the database becomes immediately permanent. The default of AutoCommit is on, but this may change in the future, so it is highly recommended that you explicitly set it when calling connect(). For details see the notes about Transactions elsewhere in this document.

pg_bool_tf (boolean)

PostgreSQL specific attribute. If true, boolean values will be returned as the characters 't' and 'f' instead of '1' and '0'.

Driver (handle)

Implemented by DBI, no driver-specific impact.

Name (string, read-only)

The default DBI method is overridden by a driver specific method that returns only the database name. Anything else from the connection string is stripped off. Note that, in contrast to the DBI specs, the DBD::Pg implementation for this method is read-only.

RowCacheSize (integer)

Implemented by DBI, not used by this driver.

Username (string, read-only)

Supported by this driver as proposed by DBI.

pg_auto_escape (boolean)

PostgreSQL specific attribute. If true, then quotes and backslashes in all parameters will be escaped in the following way:

escape quote with a quote (SQL)
escape backslash with a backslash

The default is on. Note that PostgreSQL also accepts quotes that are escaped by a backslash. Any other ASCII character can be used directly in a string constant.

pg_enable_utf8 (boolean)

PostgreSQL specific attribute. If true, then the utf8 flag will be turned for returned character data (if the data is valid UTF-8). For details about the utf8 flag, see Encode. This attribute is only relevant under perl 5.8 and later.

NB: This attribute is experimental and may be subject to change.

pg_INV_READ (integer, read-only)

Constant to be used for the mode in lo_creat and lo_open.

pg_INV_WRITE (integer, read-only)

Constant to be used for the mode in lo_creat and lo_open.

pg_errorlevel (integer)

PostgreSQL specific attribute. Sets the amount of information returned by the server's error messages. Valid entries are 0, 1, and 2. Any other number will be forced to the default value of 1.

A value of 0 ("TERSE") will show severity, primary text, and position only and will usually fit on a single line. A value of 1 ("DEFAULT") will also show any detail, hint, or context fields. A value of 2 ("VERBOSE") will show all available information.

pg_protocol (integer, read-only)

PostgreSQL specific attribute. Returns the version of the PostgreSQL server. If DBD::Pg is unable to figure out the version, it will return a "0". Otherwise, a "3" is returned.

pg_lib_version (integer, read-only)

PostgreSQL specific attribute. Indicates which version of PostgreSQL that DBD::Pg was compiled against. In other words, which libraries were used. Returns a number with major, minor, and revision together; version 8.1.4 would be returned as 80104.

pg_server_version (integer, read-only)

PostgreSQL specific attribute. Indicates which version of PostgreSQL that the current database handle is connected to. Returns a number with major, minor, and revision together; version 8.0.1 would be 80001.

pg_db (string, read-only)

PostgreSQL specific attribute. Returns the name of the current database.

pg_user (string, read-only)

PostgreSQL specific attribute. Returns the name of the user that connected to the server.

pg_pass (string, read-only)

PostgreSQL specific attribute. Returns the password used to connect to the server.

pg_host (string, read-only)

PostgreSQL specific attribute. Returns the host of the current server connection. Locally connected hosts will return an empty string.

pg_port (integer, read-only)

PostgreSQL specific attribute. Returns the port of the connection to the server.

pg_default_port (integer, read-only)

PostgreSQL specific attribute. Returns the default port used if none is specifically given.

pg_options (string, read-only)

PostgreSQL specific attribute. Returns the command-line options passed to the server. May be an empty string.

pg_socket (number, read-only)

PostgreSQL specific attribute. Returns the file description number of the connection socket to the server.

pg_pid (number, read-only)

PostgreSQL specific attribute. Returns the process id (PID) of the backend server process handling the connection.

DBI STATEMENT HANDLE OBJECTS

Statement Handle Methods

bind_param

$rv = $sth->bind_param($param_num, $bind_value, \%attr);

Allows the user to bind a value and/or a data type to a placeholder. This is especially important when using server-side prepares. See the prepare() method for more information.

The value of $param_num is a number if using the '?' or '$1' style placeholders. If using ":foo" style placeholders, the complete name (e.g. ":foo") must be given. For numeric values, you can either use a number or use a literal '$1'. See the examples below.

The $bind_value argument is fairly self-explanatory. A value of undef will bind a NULL to the placeholder. Using undef is useful when you want to change just the type and will be overwriting the value later. (Any value is actually usable, but undef is easy and efficient).

The %attr hash is used to indicate the data type of the placeholder. The default value is "varchar". If you need something else, you must use one of the values provided by DBI or by DBD::Pg. To use a SQL value, modify your "use DBI" statement at the top of your script as follows:

use DBI qw(:sql_types);

This will import some constants into your script. You can plug those directly into the bind_param call. Some common ones that you will encounter are:

SQL_INTEGER

To use PostgreSQL data types, import the list of values like this:

use DBD::Pg qw(:pg_types);

You can then set the data types by setting the value of the pg_type key in the hash passed to bind_param. The current list of Postgres data types exported is:

PG_ABSTIME PG_ABSTIMEARRAY PG_ACLITEM PG_ACLITEMARRAY PG_ANY PG_ANYARRAY PG_ANYELEMENT PG_ANYENUM PG_ANYNONARRAY PG_BIT PG_BITARRAY PG_BOOL PG_BOOLARRAY PG_BOX PG_BOXARRAY PG_BPCHAR PG_BPCHARARRAY PG_BYTEA PG_BYTEAARRAY PG_CHAR PG_CHARARRAY PG_CID PG_CIDARRAY PG_CIDR PG_CIDRARRAY PG_CIRCLE PG_CIRCLEARRAY PG_CSTRING PG_CSTRINGARRAY PG_DATE PG_DATEARRAY PG_FLOAT4 PG_FLOAT4ARRAY PG_FLOAT8 PG_FLOAT8ARRAY PG_GTSVECTOR PG_GTSVECTORARRAY PG_INET PG_INETARRAY PG_INT2 PG_INT2ARRAY PG_INT2VECTOR PG_INT2VECTORARRAY PG_INT4 PG_INT4ARRAY PG_INT8 PG_INT8ARRAY PG_INTERNAL PG_INTERVAL PG_INTERVALARRAY PG_LANGUAGE_HANDLER PG_LINE PG_LINEARRAY PG_LSEG PG_LSEGARRAY PG_MACADDR PG_MACADDRARRAY PG_MONEY PG_MONEYARRAY PG_NAME PG_NAMEARRAY PG_NUMERIC PG_NUMERICARRAY PG_OID PG_OIDARRAY PG_OIDVECTOR PG_OIDVECTORARRAY PG_OPAQUE PG_PATH PG_PATHARRAY PG_PG_ATTRIBUTE PG_PG_CLASS PG_PG_PROC PG_PG_TYPE PG_POINT PG_POINTARRAY PG_POLYGON PG_POLYGONARRAY PG_RECORD PG_REFCURSOR PG_REFCURSORARRAY PG_REGCLASS PG_REGCLASSARRAY PG_REGCONFIG PG_REGCONFIGARRAY PG_REGDICTIONARY PG_REGDICTIONARYARRAY PG_REGOPER PG_REGOPERARRAY PG_REGOPERATOR PG_REGOPERATORARRAY PG_REGPROC PG_REGPROCARRAY PG_REGPROCEDURE PG_REGPROCEDUREARRAY PG_REGTYPE PG_REGTYPEARRAY PG_RELTIME PG_RELTIMEARRAY PG_SMGR PG_TEXT PG_TEXTARRAY PG_TID PG_TIDARRAY PG_TIME PG_TIMEARRAY PG_TIMESTAMP PG_TIMESTAMPARRAY PG_TIMESTAMPTZ PG_TIMESTAMPTZARRAY PG_TIMETZ PG_TIMETZARRAY PG_TINTERVAL PG_TINTERVALARRAY PG_TRIGGER PG_TSQUERY PG_TSQUERYARRAY PG_TSVECTOR PG_TSVECTORARRAY PG_TXID_SNAPSHOT PG_TXID_SNAPSHOTARRAY PG_UNKNOWN PG_UUID PG_UUIDARRAY PG_VARBIT PG_VARBITARRAY PG_VARCHAR PG_VARCHARARRAY PG_VOID PG_XID PG_XIDARRAY PG_XML PG_XMLARRAY

Data types are "sticky," in that once a data type is set to a certain placeholder, it will remain for that placeholder, unless it is explicitly set to something else afterwards. If the statement has already been prepared, and you switch the data type to something else, DBD::Pg will re-prepare the statement for you before doing the next execute.

Examples:

use DBI qw(:sql_types);
use DBD::Pg qw(:pg_types);

$SQL = "SELECT id FROM ptable WHERE size > ? AND title = ?";
$sth = $dbh->prepare($SQL);

## Both arguments below are bound to placeholders as "varchar"
$sth->execute(123, "Merk");

## Reset the datatype for the first placeholder to an integer
$sth->bind_param(1, undef, SQL_INTEGER);

## The "undef" bound above is not used, since we supply params to execute
$sth->execute(123, "Merk");

## Set the first placeholder's value and data type
$sth->bind_param(1, 234, { pg_type => PG_TIMESTAMP });

## Set the second placeholder's value and data type.
## We don't send a third argument, so the default "varchar" is used
$sth->bind_param('$2', "Zool");

## We realize that the wrong data type was set above, so we change it:
$sth->bind_param('$1', 234, { pg_type => SQL_INTEGER });

## We also got the wrong value, so we change that as well.
## Because the data type is sticky, we don't need to change it
$sth->bind_param(1, 567);

## This executes the statement with 567 (integer) and "Zool" (varchar)
$sth->execute();

bind_param_inout

Experimental support for this feature is provided. The first argument to bind_param_inout should be a placeholder number. The second argument should be a reference to a scalar variable in your script. The third argument is not used and should simply be set to 0. Note that what this really does is assign a returned column to the variable, in the order in which the column appears. For example:

my $foo = 123;
$sth = $dbh->prepare("SELECT 1+?::int");
$sth->bind_param_inout(1, \$foo, 0);
$foo = 222;
$sth->execute(444);
$sth->fetch;

The above will cause $foo to have a new value of "223" after the final fetch. Note that the variables bound in this manner are very sticky, and will trump any values passed in to execute. This is because the binding is done as late as possible, at the execute() stage, allowing the value to be changed between the time it was bound and the time the query is executed. Thus, the above execute is the same as:

$sth->execute();

bind_param_array

Supported by this driver as proposed by DBI.

execute

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

Executes a previously prepared statement. In addition to UPDATE, DELETE, INSERT statements, for which it returns always the number of affected rows, the execute method can also be used for SELECT ... INTO table statements.

The "prepare/bind/execute" process has changed significantly for PostgreSQL servers 7.4 and later: please see the prepare() and bind_param() entries for much more information.

Setting one of the bind_values to "undef" is the equivalent of setting the value to NULL in the database. Setting the bind_value to $DBDPG_DEFAULT is equivalent to sending the literal string 'DEFAULT' to the backend. Note that using this option will force server-side prepares off until such time as PostgreSQL supports using DEFAULT in prepared statements.

DBD::Pg also supports passing in arrays to execute: simply pass in an arrayref, and DBD::Pg will flatten it into a string suitable for input on the backend.

execute_array

Supported by this driver as proposed by DBI.

execute_for_fetch

Supported by this driver as proposed by DBI.

fetchrow_arrayref

$ary_ref = $sth->fetchrow_arrayref;

Supported by this driver as proposed by DBI.

fetchrow_array

@ary = $sth->fetchrow_array;

Supported by this driver as proposed by DBI.

fetchrow_hashref

$hash_ref = $sth->fetchrow_hashref;

Supported by this driver as proposed by DBI.

fetchall_arrayref

$tbl_ary_ref = $sth->fetchall_arrayref;

Implemented by DBI, no driver-specific impact.

finish

$rc = $sth->finish;

Supported by this driver as proposed by DBI.

rows

$rv = $sth->rows;

Supported by this driver as proposed by DBI. In contrast to many other drivers the number of rows is available immediately after executing the statement.

bind_col

$rc = $sth->bind_col($column_number, \$var_to_bind, \%attr);

Supported by this driver as proposed by DBI.

bind_columns

$rc = $sth->bind_columns(\%attr, @list_of_refs_to_vars_to_bind);

Supported by this driver as proposed by DBI.

dump_results

$rows = $sth->dump_results($maxlen, $lsep, $fsep, $fh);

Implemented by DBI, no driver-specific impact.

blob_read

$blob = $sth->blob_read($id, $offset, $len);

Supported by this driver as proposed by DBI. Implemented by DBI but not documented, so this method might change.

This method seems to be heavily influenced by the current implementation of blobs in Oracle. Nevertheless we try to be as compatible as possible. Whereas Oracle suffers from the limitation that blobs are related to tables and every table can have only one blob (datatype LONG), PostgreSQL handles its blobs independent of any table by using so-called object identifiers. This explains why the blob_read method is blessed into the STATEMENT package and not part of the DATABASE package. Here the field parameter has been used to handle this object identifier. The offset and len parameters may be set to zero, in which case the driver fetches the whole blob at once.

Starting with PostgreSQL 6.5, every access to a blob has to be put into a transaction. This holds even for a read-only access.

See also the PostgreSQL-specific functions concerning blobs, which are available via the func interface.

For further information and examples about blobs, please read the chapter about Large Objects in the PostgreSQL Programmer's Guide at http://www.postgresql.org/docs/current/static/largeobjects.html.

Statement Handle Attributes

NUM_OF_FIELDS (integer, read-only): Implemented by DBI, no driver-specific impact.
NUM_OF_PARAMS (integer, read-only): Implemented by DBI, no driver-specific impact.
NAME (array-ref, read-only): Supported by this driver as proposed by DBI.
NAME_lc (array-ref, read-only): Implemented by DBI, no driver-specific impact.
NAME_uc (array-ref, read-only): Implemented by DBI, no driver-specific impact.
NAME_hash (hash-ref, read-only): Implemented by DBI, no driver-specific impact.
NAME_lc_hash (hash-ref, read-only): Implemented by DBI, no driver-specific impact.
NAME_uc_hash (hash-ref, read-only): Implemented by DBI, no driver-specific impact.
TYPE (array-ref, read-only): Supported by this driver as proposed by DBI
PRECISION (array-ref, read-only): Supported by this driver. NUMERIC types will return the precision. Types of CHAR and VARCHAR will return their size (number of characters). Other types will return the number of bytes.
SCALE (array-ref, read-only): Supported by this driver as proposed by DBI. The only type that will return a value currently is NUMERIC.
NULLABLE (array-ref, read-only): Supported by this driver as proposed by DBI.
CursorName (string, read-only): Not supported by this driver. See the note about Cursors elsewhere in this document.
Database (dbh, read-only): Implemented by DBI, no driver-specific impact.
ParamValues (hash ref, read-only): Supported by this driver as proposed by DBI. If called before execute, the literal values passed in are returned. If called after execute, then the quoted versions of the values are shown.
ParamTypes (hash ref, read-only): Returns a hash of all current placeholders. The keys are the names of the placeholders, and the values are the types that have been bound to each one. Placeholders that have not yet been bound will return undef as the value.
Statement (string, read-only): Supported by this driver as proposed by DBI.
RowCache (integer, read-only): Not supported by this driver.
pg_size (array-ref, read-only): PostgreSQL specific attribute. It returns a reference to an array of integer values for each column. The integer shows the size of the column in bytes. Variable length columns are indicated by -1.
pg_type (array-ref, read-only): PostgreSQL specific attribute. It returns a reference to an array of strings for each column. The string shows the name of the data_type.
pg_oid_status (integer, read-only): PostgreSQL specific attribute. It returns the OID of the last INSERT command.
pg_cmd_status (integer, read-only): PostgreSQL specific attribute. It returns the type of the last command. Possible types are: "INSERT", "DELETE", "UPDATE", "SELECT".

FURTHER INFORMATION

Transactions

Transaction behavior is controlled via the AutoCommit attribute. For a complete definition of AutoCommit please refer to the DBI documentation.

According to the DBI specification the default for AutoCommit is a true value. In this mode, any change to the database becomes valid immediately. Any BEGIN, COMMIT or ROLLBACK statements will be rejected. DBD::Pg implements AutoCommit by issuing a BEGIN statement immediately before executing a statement, and a COMMIT afterwards.

Savepoints

PostgreSQL version 8.0 introduced the concept of savepoints, which allows transactions to be rolled back to a certain point without affecting the rest of the transaction. DBD::Pg encourages using the following methods to control savepoints:

pg_savepoint

Creates a savepoint. This will fail unless you are inside of a transaction. The only argument is the name of the savepoint. Note that PostgreSQL DOES allow multiple savepoints with the same name to exist.

$dbh->pg_savepoint("mysavepoint");

pg_rollback_to

Rolls the database back to a named savepoint, discarding any work performed after that point. If more than one savepoint with that name exists, rolls back to the most recently created one.

$dbh->pg_rollback_to("mysavepoint");

pg_release

Releases (or removes) a named savepoint. If more than one savepoint with that name exists, it will only destroy the most recently created one. Note that all savepoints created after the one being released are also destroyed.

$dbh->pg_release("mysavepoint");

Asynchronous Queries

It is possible to send a query to the backend and have your script do other work while the query is running on the backend. Both queries sent by the do() method, and by the execute() method can be sent asynchronously. (NOTE: This will only work if DBD::Pg has been compiled against Postgres libraries of version 8.0 or greater) The basic usage is as follows:

use DBD::Pg ':async';

print "Async do() example:\n";
$dbh->do("SELECT long_running_query()", {pg_async => PG_ASYNC});
do_something_else();
{
  if ($dbh->pg_ready()) {
    $res = $pg_result();
    print "Result of do(): $res\n";
  }
  print "Query is still running...\n";
  if (cancel_request_received) {
    $dbh->pg_cancel();
  }
  sleep 1;
  redo;
}

print "Async prepare/execute example:\n";
$sth = $dbh->prepare("SELECT long_running_query(1)", {pg_async => PG_ASYNC});
$sth->execute();

## Changed our mind, cancel and run again:
$sth = $dbh->prepare("SELECT 678", {pg_async => PG_ASYNC + PG_OLDQUERY_CANCEL});
$sth->execute();

do_something_else();

if (!$sth->pg_ready) {
  do_another_thing();
}

## We wait until it is done, and get the result:
$res = $dbh->pg_result();

Asynchronous Constants

There are currently three asynchronous constants exported by DBD::Pg. You can import all of them by putting either of these at the top of your script:

use DBD::Pg;

use DBD::Pg ':async';

You may also use the numbers instead of the constants, but using the constants is recommended as it makes your script more readable.

PG_ASYNC: This is a constant for the number 1. It is passed to either the do() or the prepare() method as a value to the pg_async key and indicates that the query should be sent asynchronously.
PG_OLDQUERY_CANCEL: This is a constant for the number 2. When passed to either the do() or the prepare method(), it causes any currently running asynchronous query to be cancelled and rolled back. It has no effect if no asynchronous query is currently running.
PG_OLDQUERY_WAIT: This is a constant for the number 4. When passed to either the do() or the prepare method(), it waits for any currently running asynchronous query to complete. It has no effect if there is no asynchronous query currently running.

Asynchronous Methods

pg_cancel

This database-level method attempts to cancel any currently running asynchronous query. It returns true if the cancel succeeded, and false otherwise. Note that a query that has finished before this method is executed will also return false. WARNING: a successful cancellation will leave the database in an unusable state, so DBD::Pg will automatically clear out the error message and issue a ROLLBACK.

$result = $dbh->pg_cancel();

pg_ready

This method can be called as a database handle method or (for convenience) as a statement handle method. Both simply see if a previously issued asynchronous query has completed yet. It returns true if the statement has finished, in which case you should then call the pg_result() method. Calls to pg_ready() should only be used when you have other things to do while the query is running. If you simply want to wait until the query is done, do not call pg_ready() over and over, but simply call the pg_result() method.

my $time = 0;
while (!$dbh->pg_ready) {
  print "Query is still running. Seconds: $time\n";
  $time++;
  sleep 1;
}
$result = $dbh->pg_result;

pg_result

This database handle method returns the results of a previously issued asynchronous query. If the query is still running, this method will wait until it has finished. The result returned is the number of rows: the same thing that would have been returned by the asynchronous do() or execute() if it had been called without an asynchronous flag.

$result = $dbh->pg_result;

Asynchronous Examples

Here are some working examples of asynchronous queries. Note that we'll use the pg_sleep function to emulate a long-running query.

use strict;
use warnings;
use Time::HiRes 'sleep';
use DBD::Pg ':async';

my $dbh = DBI->connect('dbi:Pg:dbname=postgres', 'postgres', '', {AutoCommit=>0,RaiseError=>1});

## Kick off a long running query on the first database:
my $sth = $dbh->prepare("SELECT pg_sleep(?)", {pg_async => PG_ASYNC});
$sth->execute(5);

## While that is running, do some other things
print "Your query is processing. Thanks for waiting\n";
check_on_the_kids(); ## Expensive sub, takes at least three seconds.

while (!$dbh->pg_ready) {
  check_on_the_kids();
  ## If the above function returns quickly for some reason, we add a small sleep
  sleep 0.1;
}

print "The query has finished. Gathering results\n";
my $result = $sth->pg_result;
print "Result: $result\n";
my $info = $sth->fetchall_arrayref();

Without asynchronous queries, the above script would take about 8 seconds to run: five seconds waiting for the execute to finish, then three for the check_on_the_kids() function to return. With asynchronous queries, the script takes about 6 seconds to run, and gets in two iterations of check_on_the_kids in the process.

Here's an example showing the ability to cancel a long-running query. Imagine two slave databases in different geographic locations over a slow network. You need information as quickly as possible, so you query both at once. When you get an answer, you tell the other one to stop working on your query, as you don't need it anymore.

use strict;
use warnings;
use Time::HiRes 'sleep';
use DBD::Pg ':async';

my $dbhslave1 = DBI->connect('dbi:Pg:dbname=postgres;host=slave1', 'postgres', '', {AutoCommit=>0,RaiseError=>1});
my $dbhslave2 = DBI->connect('dbi:Pg:dbname=postgres;host=slave2', 'postgres', '', {AutoCommit=>0,RaiseError=>1});

$SQL = "SELECT count(*) FROM largetable WHERE flavor='blueberry'";

my $sth1 = $dbhslave1->prepare($SQL, {pg_async => PG_ASYNC});
my $sth2 = $dbhslave2->prepare($SQL, {pg_async => PG_ASYNC});

$sth1->execute();
$sth2->execute();

my $winner;
while (!defined $winner) {
  if ($sth1->pg_ready) {
    $winner = 1;
  }
  elsif ($sth2->pg_ready) {
    $winner = 2;
  }
  Time::HiRes::sleep 0.05;
}

my $count;
if ($winner == 1) {
  $sth2->pg_cancel();
  $sth1->pg_result();
  $count = $sth1->fetchall_arrayref()->[0][0];
}
else {
  $sth1->pg_cancel();
  $sth2->pg_result();
  $count = $sth2->fetchall_arrayref()->[0][0];
}

Array support

DBD::Pg allows arrays (as arrayrefs) to be passed in to both the quote() and the execute() functions. In both cases, the array is flattened into a string representing a Postgres array.

When fetching rows from a table that contains a column with an array type, the result will be passed back to your script as an arrayref.

To turn off the automatic parsing of returned arrays into arrayrefs, you can set the variable "pg_expand_array", which is true by default.

$dbh->{pg_expand_array} = 0;

COPY support

DBD::Pg allows for the quick (bulk) reading and storing of data by using the COPY command. The basic process is to use $dbh->do to issue a COPY command, and then to either add rows using pg_putcopydata, or to read them by using pg_getcopydata.

The first step is to put the server into "COPY" mode. This is done by sending a complete COPY command to the server, by using the do() method. For example:

$dbh->do("COPY foobar FROM STDIN");

This would tell the server to enter a COPY OUT state. It is now ready to receive information via the pg_putcopydata method. The complete syntax of the COPY command is more complex and not documented here: the canonical PostgreSQL documentation for COPY can be found at:

http://www.postgresql.org/docs/current/static/sql-copy.html

Once the COPY command has been issued, no other SQL commands are allowed until pg_putcopyend() has been issued, or the final pg_getcopydata has been called.

Note: All other COPY methods (pg_putline, pg_getline, etc.) are now deprecated in favor of the pg_getcopydata, pg_putcopydata, and pg_putcopyend methods.

pg_getcopydata

Used to retrieve data from a table after the server has been put into COPY OUT mode by calling "COPY tablename TO STDOUT". The first argument to pg_getcopydata is the variable into which the data will be stored (this variable should not be undefined, or it may throw a warning, although it may be a reference). This argument returns a number greater than 1 indicating the new size of the variable, or a -1 when the COPY has finished. Once a -1 has been returned, no other action is necessary, as COPY mode will have already terminated. Example:

$dbh->do("COPY mytable TO STDOUT");
my @data;
my $x=0;
1 while $dbh->pg_getcopydata($data[$x++]) > 0;

There is also a variation of this function called pg_getcopydata_async, which, as the name suggests, returns immediately. The only difference from the original function is that this version may return a 0, indicating that the row is not ready to be delivered yet. When this happens, the variable has not been changed, and you will need to call the function again until you get a non-zero result.

pg_putcopydata

Used to put data into a table after the server has been put into COPY IN mode by calling "COPY tablename FROM STDIN". The only argument is the data you want inserted. Issue a pg_putcopyend() when you have added all your rows.

The default delimiter is a tab character, but this can be changed in the COPY statement. Returns a 1 on successful input. Examples:

## Simple example:
$dbh->do("COPY mytable FROM STDIN");
$dbh->pg_putcopydata("123\tPepperoni\t3\n");
$dbh->pg_putcopydata("314\tMushroom\t8\n");
$dbh->pg_putcopydata("6\tAnchovies\t100\n");
$dbh->pg_putcopyend();

## This example uses explicit columns and a custom delimiter
$dbh->do("COPY mytable(flavor, slices) FROM STDIN WITH DELIMITER '~'");
$dbh->pg_putcopydata("Pepperoni~123\n");
$dbh->pg_putcopydata("Mushroom~314\n");
$dbh->pg_putcopydata("Anchovies~6\n");
$dbh->pg_putcopyend();

pg_putcopyend

When you are finished with pg_putcopydata, call pg_putcopyend to let the server know that you are done, and it will return to a normal, non-COPY state. Returns a 1 on success. This method will fail if called when not in a COPY IN or COPY OUT state.

Large Objects

This driver supports all largeobject functions provided by libpq via the func method. Please note that, starting with PostgreSQL 6.5, any access to a large object -- even read-only large objects -- must be put into a transaction!

Cursors

Although PostgreSQL supports cursors, they have not been used in the current implementation. When DBD::Pg was defined, cursors in PostgreSQL could only be used inside a transaction block. Because only one transaction block at a time is allowed, this would have implied the restriction not to use any nested SELECT statements. Therefore the execute method fetches all data at once into data structures located in the front-end application. This fact must to be considered when selecting large amounts of data!

You can use cursors in your application, but you'll need to do a little work. First you must declare your cursor. Now you can issue queries against the cursor, then select against your queries. This typically results in a double loop, like this:

# WITH HOLD is not needed if AutoCommit is off
$dbh->do("DECLARE csr CURSOR WITH HOLD FOR $sql");
while (1) {
  my $sth = $dbh->prepare("fetch 1000 from csr");
  $sth->execute;
  last if 0 == $sth->rows;

  while (my $row = $sth->fetchrow_hashref) {
    # Do something with the data.
  }
}
$dbh->do("CLOSE csr");

Datatype bool

The current implementation of PostgreSQL returns 't' for true and 'f' for false. From the Perl point of view, this is a rather unfortunate choice. DBD::Pg therefore translates the result for the BOOL data type in a Perlish manner: 'f' -> '0' and 't' -> '1'. This way the application does not have to check the database-specific returned values for the data-type BOOL because Perl treats '0' as false and '1' as true. You may set the pg_bool_tf attribute to a true value to change the values back to 't' and 'f' if you wish.

Boolean values can be passed to PostgreSQL as TRUE, 't', 'true', 'y', 'yes' or '1' for true and FALSE, 'f', 'false', 'n', 'no' or '0' for false.

Schema support

PostgreSQL version 7.3 introduced schema support. Note that the PostgreSQL schema concept may differ from those of other databases. In a nutshell, a schema is a named collection of objects within a single database. Please refer to the PostgreSQL documentation for more details.

Currently, DBD::Pg does not provide explicit support for PostgreSQL schemas. However, schema functionality may be used without any restrictions by explicitly addressing schema objects, e.g.

my $res = $dbh->selectall_arrayref("SELECT * FROM my_schema.my_table");

or by manipulating the schema search path with SET search_path, e.g.

$dbh->do("SET search_path TO my_schema, public");

BUGS

Quoting will not work properly if standard_conforming_strings is set to 'on', so it is recommended that this be set to 'off' until full support for it can be built into DBD::Pg.

AUTHORS

DBI and DBD-Oracle by Tim Bunce (Tim.Bunce@ig.co.uk)

DBD-Pg by Edmund Mergl (E.Mergl@bawue.de) and Jeffrey W. Baker (jwbaker@acm.org). By David Wheeler <david@justatheory.com>, Jason Stewart <jason@openinformatics.com>, Bruce Momjian <pgman@candle.pha.pa.us>, Greg Sabino Mullane <greg@turnstep.com>, and others after v1.13.

Parts of this package have been copied from DBI and DBD-Oracle.

Mailing List

The current maintainers may be reached through the 'dbd-pg' mailing list: <dbd-pg@perl.org>

Bug Reports

If you feel certain you have found a bug, you can report it by sending an email to <bug-dbd-pg@rt.cpan.org>.

COPYRIGHT

The DBD::Pg module is free software. You may distribute under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.

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NAME

VERSION

SYNOPSIS

DESCRIPTION

MODULE DOCUMENTATION

THE DBI CLASS

DBI Class Methods

METHODS COMMON TO ALL HANDLES

ATTRIBUTES COMMON TO ALL HANDLES

DBI DATABASE HANDLE OBJECTS

Database Handle Methods

Database Handle Attributes

DBI STATEMENT HANDLE OBJECTS

Statement Handle Methods

Statement Handle Attributes

FURTHER INFORMATION

Transactions

Savepoints

Asynchronous Queries

Asynchronous Constants

Asynchronous Methods

Asynchronous Examples

Array support

COPY support

Large Objects

Cursors

Datatype bool

Schema support

SEE ALSO

BUGS

AUTHORS

COPYRIGHT

ACKNOWLEDGMENTS

NAME

VERSION

SYNOPSIS

DESCRIPTION

MODULE DOCUMENTATION

THE DBI CLASS

DBI Class Methods

METHODS COMMON TO ALL HANDLES

ATTRIBUTES COMMON TO ALL HANDLES

DBI DATABASE HANDLE OBJECTS

Database Handle Methods

Database Handle Attributes

DBI STATEMENT HANDLE OBJECTS

Statement Handle Methods

Statement Handle Attributes

FURTHER INFORMATION

Transactions

Savepoints

Asynchronous Queries

Asynchronous Constants

Asynchronous Methods

Asynchronous Examples

Array support

COPY support

Large Objects

Cursors

Datatype bool

Schema support

SEE ALSO

BUGS

AUTHORS

COPYRIGHT

ACKNOWLEDGMENTS

Module Install Instructions