Provided by: libdbd-pg-perl_2.19.3-2_amd64 
NAME
DBD::Pg - PostgreSQL database driver for the DBI module
SYNOPSIS
use DBI;
$dbh = DBI->connect("dbi:Pg:dbname=$dbname", '', '', {AutoCommit => 0});
# The AutoCommit attribute should always be explicitly set
# For some advanced uses you may need PostgreSQL type values:
use DBD::Pg qw(:pg_types);
# For asynchronous calls, import the async constants:
use DBD::Pg qw(:async);
$dbh->do('INSERT INTO mytable(a) VALUES (1)');
$sth = $dbh->prepare('INSERT INTO mytable(a) VALUES (?)');
$sth->execute();
VERSION
This documents version 2.19.3 of the DBD::Pg module
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
This method creates a database handle by connecting to a database, and is the DBI
equivalent of the "new" method. To connect to a Postgres database with a minimum of
parameters, use the following syntax:
$dbh = DBI->connect("dbi:Pg:dbname=$dbname", '', '', {AutoCommit => 0});
This connects to the database named in the $dbname variable 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, RaiseError => 1, PrintError => 0}
);
If a parameter is not given, the connect() method will first look for specific environment
variables, and then fall back to 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)
* May also use the aliases "db" or "database"
If the username and password values passed via "connect()" are undefined (as opposed to
merely being empty strings), DBI will use the environment variables DBI_USER and DBI_PASS
if they exist.
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
You can also connect using sockets in a specific directory. This may be needed if the
server you are connecting to has a different default socket directory from the one used to
compile DBD::Pg. Use the complete path to the socket directory as the name of the host,
like this:
$dbh = DBI->connect('dbi:Pg:dbname=foo;host=/var/tmp/socket',
$username,
$password,
{AutoCommit => 0, RaiseError => 1});
The attribute hash can also contain a key named "dbd_verbose", which simply calls
"$dbh->trace('DBD')" after the handle is created. This attribute is not recommended, as it
is clearer to simply explicitly call "trace" explicitly in your script.
connect_cached
$dbh = DBI->connect_cached("dbi:Pg:dbname=$dbname", $username, $password, \%options);
Implemented by DBI, no driver-specific impact.
data_sources
@data_sources = DBI->data_sources('Pg');
@data_sources = $dbh->data_sources();
Returns a list of available databases. Unless the environment variable "DBI_DSN" is set, a
connection will be attempted to the database "template1". The normal connection
environment variables also apply, such as "PGHOST", "PGPORT", "DBI_USER", "DBI_PASS", and
"PGSERVICE".
You can also pass in options to add to the connection string For example, to specify an
alternate port and host:
@data_sources = DBI->data_sources('Pg', 'port=5824;host=example.com');
or:
@data_sources = $dbh->data_sources('port=5824;host=example.com');
Methods Common To All Handles
For all of the methods below, $h can be either a database handle ($dbh) or a statement
handle ($sth). Note that $dbh and $sth can be replaced with any variable name you choose:
these are just the names most often used. Another common variable used in this
documentation is $rv, which stands for "return value".
err
$rv = $h->err;
Returns the error code from the last method called. For the connect method it returns
"PQstatus", which is a number used by libpq (the Postgres connection library). A value of
0 indicates no error (CONNECTION_OK), while any other number indicates a failed
connection. The only other number commonly seen is 1 (CONNECTION_BAD). See the libpq
documentation for the complete list of return codes.
In all other non-connect methods "$h->err" returns the "PQresultStatus" of the current
handle. This is a number used by libpq and is one of:
0 Empty query string
1 A command that returns no data successfully completed.
2 A command that returns data successfully completed.
3 A COPY OUT command is still in progress.
4 A COPY IN command is still in progress.
5 A bad response was received from the backend.
6 A nonfatal error occurred (a notice or warning message)
7 A fatal error was returned: the last query failed.
errstr
$str = $h->errstr;
Returns the last error that was reported by Postgres. This message is affected by the
"pg_errorlevel" setting.
state
$str = $h->state;
Returns a five-character "SQLSTATE" code. Success is indicated by a 00000 code, which gets
mapped to an empty string by DBI. A code of "S8006" indicates a connection failure,
usually because the connection to the Postgres server has been lost.
While this method can be called as either "$sth->state" or "$dbh->state", it is usually
clearer to always use "$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 codes:
00000 Successful completion
25P01 No active SQL transaction
25P02 In failed SQL transaction
S8006 Connection failure
trace
$h->trace($trace_settings);
$h->trace($trace_settings, $trace_filename);
$trace_settings = $h->trace;
Changes the trace settings on a database or statement handle. The optional second
argument specifies a file to write the trace information to. If no filename is given, the
information is written to STDERR. Note that tracing can be set globally as well by setting
"DBI->trace", or by using the environment variable DBI_TRACE.
The value is either a numeric level or a named flag. For the flags that DBD::Pg uses, see
parse_trace_flag.
trace_msg
$h->trace_msg($message_text);
$h->trace_msg($message_text, $min_level);
Writes a message to the current trace output (as set by the "trace" method). If a second
argument is given, the message is only written if the current tracing level is equal to or
greater than the $min_level.
parse_trace_flag and parse_trace_flags
$h->trace($h->parse_trace_flags('SQL|pglibpq'));
$h->trace($h->parse_trace_flags('1|pgstart'));
## Simpler:
$h->trace('SQL|pglibpq');
$h->trace('1|pgstart');
my $value = DBD::Pg->parse_trace_flag('pglibpq');
DBI->trace($value);
The parse_trace_flags method is used to convert one or more named flags to a number which
can passed to the "trace" method. DBD::Pg currently supports the DBI-specific flag,
"SQL", as well as the ones listed below.
Flags can be combined by using the parse_trace_flags method, which simply calls
"parse_trace_flag" on each item and combines them.
Sometimes you may wish to turn the tracing on before you connect to the database. The
second example above shows a way of doing this: the call to "DBD::Pg->parse_trace_flags"
provides a number than can be fed to "DBI->trace" before you create a database handle.
DBD::Pg supports the following trace flags:
SQL Outputs all SQL statements. Note that the output provided will not necessarily be in a
form suitable to passing directly to Postgres, as server-side prepared statements are
used extensively by DBD::Pg. For maximum portability of output (but with a potential
performance hit), use with "$dbh->{pg_server_prepare} = 0".
DBD Turns on all non-DBI flags, in other words, only the ones that are specific to DBD::Pg
(all those below which start with the letters 'pg').
pglibpq
Outputs the name of each libpq function (without arguments) immediately before running
it. This is a good way to trace the flow of your program at a low level. This
information is also output if the trace level is set to 4 or greater.
pgstart
Outputs the name of each internal DBD::Pg function, and other information such as the
function arguments or important global variables, as each function starts. This
information is also output if the trace level is set to 4 or greater.
pgend
Outputs a simple message at the very end of each internal DBD::Pg function. This is
also output if the trace level is set to 4 or greater.
pgprefix
Forces each line of trace output to begin with the string "dbdpg: ". This helps to
differentiate it from the normal DBI trace output.
pglogin
Outputs a message showing the connection string right before a new database connection
is attempted, a message when the connection was successful, and a message right after
the database has been disconnected. Also output if trace level is 5 or greater.
func
DBD::Pg uses the "func" method to support a variety of functions. Note that the name of
the function comes last, after the arguments.
table_attributes
$attrs = $dbh->func($table, 'table_attributes');
Use of the tables_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
pg_lo_creat
$lobjId = $dbh->pg_lo_creat($mode);
Creates a new large object and returns the object-id. $mode is a bitmask describing
read and write access to the new object. This setting is ignored since Postgres
version 8.1. For backwards compatibility, however, you should set a valid mode anyway
(see "pg_lo_open" for a list of valid modes).
Upon failure it returns "undef". This function cannot be used if AutoCommit is
enabled.
The old way of calling large objects functions is deprecated: $dbh->func(.., 'lo_);
lo_open
$lobj_fd = $dbh->pg_lo_open($lobjId, $mode);
Opens an existing large object and returns an object-descriptor for use in subsequent
"lo_*" calls. $mode is a bitmask describing read and write access to the opened
object. It may be one of:
$dbh->{pg_INV_READ}
$dbh->{pg_INV_WRITE}
$dbh->{pg_INV_READ} | $dbh->{pg_INV_WRITE}
"pg_INV_WRITE" and "pg_INV_WRITE | pg_INV_READ" modes are identical; in both modes,
the large object can be read from or written to. Reading from the object will provide
the object as written in other committed transactions, along with any writes performed
by the current transaction. Objects opened with "pg_INV_READ" cannot be written to.
Reading from this object will provide the stored data at the time of the transaction
snapshot which was active when "lo_write" was called.
Returns "undef" upon failure. Note that 0 is a perfectly correct (and common) object
descriptor! This function cannot be used if AutoCommit is enabled.
lo_write
$nbytes = $dbh->pg_lo_write($lobj_fd, $buffer, $len);
Writes $len bytes of c<$buffer> into the large object $lobj_fd. Returns the number of
bytes written and "undef" upon failure. This function cannot be used if AutoCommit is
enabled.
lo_read
$nbytes = $dbh->pg_lo_read($lobj_fd, $buffer, $len);
Reads $len bytes into c<$buffer> from large object $lobj_fd. Returns the number of
bytes read and "undef" upon failure. This function cannot be used if AutoCommit is
enabled.
lo_lseek
$loc = $dbh->pg_lo_lseek($lobj_fd, $offset, $whence);
Changes the current read or write location on the large object $obj_id. Currently
$whence can only be 0 (which is L_SET). Returns the current location and "undef" upon
failure. This function cannot be used if AutoCommit is enabled.
lo_tell
$loc = $dbh->pg_lo_tell($lobj_fd);
Returns the current read or write location on the large object $lobj_fd and "undef"
upon failure. This function cannot be used if AutoCommit is enabled.
lo_close
$lobj_fd = $dbh->pg_lo_close($lobj_fd);
Closes an existing large object. Returns true upon success and false upon failure.
This function cannot be used if AutoCommit is enabled.
lo_unlink
$ret = $dbh->pg_lo_unlink($lobjId);
Deletes an existing large object. Returns true upon success and false upon failure.
This function cannot be used if AutoCommit is enabled.
lo_import
$lobjId = $dbh->pg_lo_import($filename);
Imports a Unix file as a large object and returns the object id of the new object or
"undef" upon failure.
lo_import_with_oid
$lobjId = $dbh->pg_lo_import($filename, $OID);
Same as lo_import, but attempts to use the supplied OID as the large object number. If
this number is 0, it falls back to the behavior of lo_import (which assigns the next
available OID).
This is only available when DBD::Pg is compiled against a Postgres server version 8.4
or later.
lo_export
$ret = $dbh->pg_lo_export($lobjId, $filename);
Exports a large object into a Unix file. Returns false upon failure, true otherwise.
getfd
$fd = $dbh->func('getfd');
Deprecated, use $dbh->{pg_socket} instead.
private_attribute_info
$hashref = $dbh->private_attribute_info();
$hashref = $sth->private_attribute_info();
Returns a hash of all private attributes used by DBD::Pg, for either a database or a
statement handle. Currently, all the hash values are undef.
ATTRIBUTES COMMON TO ALL HANDLES
InactiveDestroy (boolean)
If set to true, then the "disconnect" method will not be automatically called when the
database handle goes out of scope. This is required if you are forking, and even then you
must tread carefully and ensure that either the parent or the child (but not both!)
handles all database calls from that point forwards, so that messages from the Postgres
backend are only handled by one of the processes. If you don't set things up properly, you
will see messages such as "server closed the connection unexpectedly", and "message type
0x32 arrived from server while idle". The best solution is to either have the child
process reconnect to the database with a fresh database handle, or to rewrite your
application not to use use forking. See the section on "Asynchronous Queries" for a way to
have your script continue to work while the database is processing a request.
RaiseError (boolean, inherited)
Forces errors to always raise an exception. Although it defaults to off, it is recommended
that this be turned on, as the alternative is to check the return value of every method
(prepare, execute, fetch, etc.) manually, which is easy to forget to do.
PrintError (boolean, inherited)
Forces database errors to also generate warnings, which can then be filtered with methods
such as locally redefining $SIG{__WARN__} or using modules such as "CGI::Carp". This
attribute is on by default.
ShowErrorStatement (boolean, inherited)
Appends information about the current statement to error messages. If placeholder
information is available, adds that as well. Defaults to false.
Warn (boolean, inherited)
Enables warnings. This is on by default, and should only be turned off in a local block
for a short a time only when absolutely needed.
Executed (boolean, read-only)
Indicates if a handle has been executed. For database handles, this value is true after
the "do" method has been called, or when one of the child statement handles has issued an
"execute". Issuing a "commit" or "rollback" always resets the attribute to false for
database handles. For statement handles, any call to "execute" or its variants will flip
the value to true for the lifetime of the statement handle.
TraceLevel (integer, inherited)
Sets the trace level, similar to the "trace" method. See the sections on "trace" and
"parse_trace_flag" for more details.
Active (boolean, read-only)
Indicates if a handle is active or not. For database handles, this indicates if the
database has been disconnected or not. For statement handles, it indicates if all the data
has been fetched yet or not. Use of this attribute is not encouraged.
Kids (integer, read-only)
Returns the number of child processes created for each handle type. For a driver handle,
indicates the number of database handles created. For a database handle, indicates the
number of statement handles created. For statement handles, it always returns zero,
because statement handles do not create kids.
ActiveKids (integer, read-only)
Same as "Kids", but only returns those that are active.
CachedKids (hash ref)
Returns a hashref of handles. If called on a database handle, returns all statement
handles created by use of the "prepare_cached" method. If called on a driver handle,
returns all database handles created by the "connect_cached" method.
ChildHandles (array ref)
Implemented by DBI, no driver-specific impact.
PrintWarn (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.
FetchHashKeyName (string, inherited)
Implemented by DBI, no driver-specific impact.
ChopBlanks (boolean, inherited)
Supported by DBD::Pg as proposed by DBI. This method is similar to the SQL function
"RTRIM".
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.
Type (scalar)
Returns "dr" for a driver handle, "db" for a database handle, and "st" for a statement
handle. Should be rarely needed.
LongReadLen
Not used by DBD::Pg
LongTruncOk
Not used by DBD::Pg
CompatMode
Not used by DBD::Pg
DBI DATABASE HANDLE OBJECTS
Database Handle Methods
selectall_arrayref
$ary_ref = $dbh->selectall_arrayref($sql);
$ary_ref = $dbh->selectall_arrayref($sql, \%attr);
$ary_ref = $dbh->selectall_arrayref($sql, \%attr, @bind_values);
Returns a reference to an array containing the rows returned by preparing and executing
the SQL string. See the DBI documentation for full details.
selectall_hashref
$hash_ref = $dbh->selectall_hashref($sql, $key_field);
Returns a reference to a hash containing the rows returned by preparing and executing the
SQL string. See the DBI documentation for full details.
selectcol_arrayref
$ary_ref = $dbh->selectcol_arrayref($sql, \%attr, @bind_values);
Returns a reference to an array containing the first column from each rows returned by
preparing and executing the SQL string. It is possible to specify exactly which columns to
return. See the DBI documentation for full details.
prepare
$sth = $dbh->prepare($statement, \%attr);
WARNING: DBD::Pg now (as of version 1.40) uses true prepared statements by sending them to
the backend to be prepared by the Postgres server. Statements that were legal before may
no longer work. See below for details.
The prepare method 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.
Queries that do not begin with the word "SELECT", "INSERT", "UPDATE", or "DELETE" are
never sent as server-side prepared statements.
Deciding whether or not to use prepared statements depends on many factors, but you can
force them to be used or not used by using the "pg_server_prepare" attribute when calling
"prepare". Setting this to "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 when connected to Postgres 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 themselves: dbdpg_XY_Z, where Y is the current PID, X is either 'p' or
'n' (depending on if the PID is a positive or negative number), and Z is a number that
starts at 1 and increases each time a new statement is prepared. This number is tracked at
the database handle level, so multiple statement handles will not collide.
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 usually not performed until the first execute is called, due to
the fact that information on the data types (provided by "bind_param") may be provided
after the prepare but before the execute.
A server-side prepare may happen before the first "execute", but only if the server can
handle the server-side prepare, and the statement contains no placeholders. It will also
be prepared if the "pg_prepare_now" attribute is passed in and set to a true value.
Similarly, the "pg_prepare_now" attribute can be set to 0 to ensure that the statement is
not prepared immediately, although the cases in which you would want this are very 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 the "do" method). Create a
statement handle and set the prepared name via the "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 the equivalent of this query on the backend:
EXECUTE mystat(123);
which is the equivalent of:
SELECT COUNT(*) FROM pg_class WHERE reltuples < 123;
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" type, in which each placeholder is represented by a single question mark
character. This is the method recommended by the DBI specs and is the most portable. Each
question mark is internally replaced by a "dollar sign number" in the order in which they
appear in the query (important when using "bind_param").
The method second type of placeholder is "dollar sign numbers". This is the method that
Postgres 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 (but can appear in any order within the query). 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'; # Does not start with 1
$SQL = 'SELECT count(*) FROM pg_class WHERE relpages BETWEEN $1 AND $3'; # Missing 2
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 $2 AND $1'; # legal but confusing
$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 a simple execute with no bind_param calls requires only a single argument as well:
$sth->execute(2045);
The final placeholder type is "named parameters" in the format ":foo". While this syntax
is supported by DBD::Pg, its use is discouraged in favor of dollar-sign numbers.
The different types of placeholders cannot be mixed within a statement, but you may use
different ones for each statement handle you have. This is confusing at best, so stick to
one style within your program.
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);
$rv = $dbh->do($statement, \%attr);
$rv = $dbh->do($statement, \%attr, @bind_values);
Prepare and execute a single statement. Returns the number of rows affected if the query
was successful, returns undef if an error occurred, and returns -1 if the number of rows
is unknown or not available. Note that this method will return 0E0 instead of 0 for 'no
rows were affected', in order to always return a true value if no error occurred.
If neither "\%attr" nor @bind_values is given, the query will be sent directly to the
server without the overhead of internally creating a statement handle and running prepare
and execute, for a measurable speed increase.
Note that an empty statement (a string with no length) will not be passed to the server;
if you want a simple test, use "SELECT 123" or the "ping" method.
last_insert_id
$rv = $dbh->last_insert_id(undef, $schema, $table, undef);
$rv = $dbh->last_insert_id(undef, $schema, $table, undef, {sequence => $seqname});
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, and
DBD::Pg will attempt to find the sequence itself. 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 by adding "pg_cache
=> 0" to the final (hashref) argument for last_insert_id.
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. However, because we are using sequences, you can be sure that the value you got
back has not been used by any other process.
Some examples:
$dbh->do('CREATE SEQUENCE lii_seq START 1');
$dbh->do(q{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(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
$rv = $dbh->commit;
Issues a COMMIT to the server, indicating that the current transaction is finished and
that all changes made will be visible to other processes. If AutoCommit is enabled, then a
warning is given and no COMMIT is issued. Returns true on success, false on error. See
also the the section on "Transactions".
rollback
$rv = $dbh->rollback;
Issues a ROLLBACK to the server, which discards any changes made in the current
transaction. If AutoCommit is enabled, then a warning is given and no ROLLBACK is issued.
Returns true on success, and false on error. See also the the section on "Transactions".
begin_work
This method turns on transactions until the next call to "commit" or "rollback", if
"AutoCommit" is currently enabled. If it is not enabled, calling begin_work will issue an
error. Note that the transaction will not actually begin until the first statement after
begin_work is called. Example:
$dbh->{AutoCommit} = 1;
$dbh->do('INSERT INTO foo VALUES (123)'); ## Changes committed immediately
$dbh->begin_work();
## Not in a transaction yet, but AutoCommit is set to 0
$dbh->do("INSERT INTO foo VALUES (345)");
## DBD::PG actually issues two statements here:
## BEGIN;
## INSERT INTO foo VALUES (345)
## We are now in a transaction
$dbh->commit();
## AutoCommit is now set to 1 again
disconnect
$rv = $dbh->disconnect;
Disconnects from the Postgres database. Any uncommitted changes will be rolled back upon
disconnection. It's good policy to always explicitly call commit or rollback at some point
before disconnecting, rather than relying on the default rollback behavior.
This method may give warnings about "disconnect invalidates X active statement handle(s)".
This means that you called "$sth->execute()" but did not finish fetching all the rows from
them. To avoid seeing this warning, either fetch all the rows or call "$sth->finish()" for
each executed statement handle.
If the script exits before disconnect is called (or, more precisely, if the database
handle is no longer referenced by anything), then the database handle's DESTROY method
will call the rollback() and disconnect() methods automatically. It is best to explicitly
disconnect rather than rely on this behavior.
quote
$rv = $dbh->quote($value, $data_type);
This module implements its own "quote" method. For simple string types, both backslashes
and single quotes are doubled. You may also quote arrayrefs and receive a string suitable
for passing into Postgres array columns.
If the value contains backslashes, and the server is version 8.1 or higher, then the
escaped string syntax will be used (which places a capital E before the first single
quote). This syntax is always used when quoting bytea values on servers 8.1 and higher.
The "data_type" argument is optional and should be one of the type constants exported by
DBD::Pg (such as PG_BYTEA). In addition to string, bytea, char, bool, and other standard
types, the following geometric types are supported: point, line, lseg, box, path, polygon,
and circle (PG_POINT, PG_LINE, PG_LSEG, PG_BOX, PG_PATH, PG_POLYGON, and PG_CIRCLE
respectively). To quote a Postgres-specific data type, you must use a 'hashref' argument
like so:
my $quotedval = $dbh->quote($value, { pg_type => PG_VARCHAR });
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
$string = $dbh->quote_identifier( $name );
$string = $dbh->quote_identifier( undef, $schema, $table);
Returns a quoted version of the supplied string, which is commonly a schema, table, or
column name. The three argument form will return the schema and the table together,
separated by a dot. Examples:
print $dbh->quote_identifier('grapefruit'); ## Prints: "grapefruit"
print $dbh->quote_identifier('juicy fruit'); ## Prints: "juicy fruit"
print $dbh->quote_identifier(undef, 'public', 'pg_proc');
## Prints: "public"."pg_proc"
pg_notifies
$ret = $dbh->pg_notifies;
Looks for any asynchronous notifications received and returns either "undef" or a
reference to a three-element array consisting of an event name, the PID of the backend
that sent the NOTIFY command, and the optional payload string. Note that this does not
check if the connection to the database is still valid first - for that, use the c<ping>
method. 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 = $dbh->pg_notifies) {
my ($name, $pid, $payload) = @$notify;
print qq{I received notice "$name" from PID $pid, payload was "$payload"\n};
## Do something based on the notice received
}
$dbh->ping() or die qq{Ping failed!};
$dbh->commit();
sleep(5);
redo;
}
Payloads will always be an empty string unless you are connecting to a Postgres server
version 9.0 or higher.
ping
$rv = $dbh->ping;
This "ping" method is 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
$rv = $dbh->pg_ping;
This is a DBD::Pg-specific extension to the "ping" method. 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 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(undef, $schema, $table, $type);
Returns all tables and views visible to the current user. 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). Note that a statement handle is returned, and not a direct list of
tables. See the examples below for ways to handle this.
The following fields are returned:
TABLE_CAT: Always NULL, as Postgres does not have the concept of catalogs.
TABLE_SCHEM: The name of the schema that the table or view is in.
TABLE_NAME: The name of the table or view.
TABLE_TYPE: The type of object returned. Will be one of "TABLE", "VIEW", or "SYSTEM
TABLE".
The TABLE_SCHEM and TABLE_NAME will be quoted via "quote_ident()".
Two additional fields specific to DBD::Pg are returned:
pg_schema: the unquoted name of the schema
pg_table: the unquoted name of the table
If your database supports tablespaces (version 8.0 or greater), two additional DBD::Pg
specific fields are returned:
pg_tablespace_name: the name of the tablespace the table is in
pg_tablespace_location: the location of the tablespace the table is in
Tables that have not been assigned to a particular tablespace (or views) will return NULL
("undef") for both of the above field.
Rows are returned alphabetically, with all tables first, and then all views.
Examples of use:
## Display all tables and views in the public schema:
$sth = $dbh->table_info('', 'public', undef, undef);
for my $rel (@{$sth->fetchall_arrayref({})}) {
print "$rel->{TABLE_TYPE} name is $rel->{TABLE_NAME}\n";
}
# Display the schema of all tables named 'foo':
$sth = $dbh->table_info('', undef, 'foo', 'TABLE');
for my $rel (@{$sth->fetchall_arrayref({})}) {
print "Table name is $rel->{TABLE_SCHEM}.$rel->{TABLE_NAME}\n";
}
column_info
$sth = $dbh->column_info( undef, $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, six 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
pg_enum_values: an array reference of allowed values for an enum 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( undef, $schema, $table, \%attr );
Supported by this driver as proposed by DBI. 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
@key_column_names = $dbh->primary_key(undef, $schema, $table);
Simple interface to the "primary_key_info" method. Returns a list of the column names that
comprise the primary key of the specified table. The list is in primary key column
sequence order. If there is no primary key then an empty list is returned.
foreign_key_info
$sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table,
$fk_catalog, $fk_schema, $fk_table );
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.
statistics_info
$sth = $dbh->statistics_info( undef, $schema, $table, $unique_only, $quick );
Returns a statement handle that can be fetched from to give statistics information on a
specific table and its indexes. The $table argument is mandatory. The $schema argument is
optional but recommended. The $unique_only argument, if true, causes only information
about unique indexes to be returned. The $quick argument is not used by DBD::Pg. For
information on the format of the rows returned, please see the DBI documentation.
tables
@names = $dbh->tables( undef, $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 |
+---------------+------------------------------------+
type_info
@type_info = $dbh->type_info($data_type);
Returns a list of hash references holding information about one or more variants of
$data_type. See the DBI documentation for more details.
pg_server_trace
$dbh->pg_server_trace($filehandle);
Writes debugging information from the PostgreSQL backend to a file. This is not related to
the DBI "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" method. 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.
selectrow_array
@row_ary = $dbh->selectrow_array($sql);
@row_ary = $dbh->selectrow_array($sql, \%attr);
@row_ary = $dbh->selectrow_array($sql, \%attr, @bind_values);
Returns an array of row information after preparing and executing the provided SQL string.
The rows are returned by calling "fetchrow_array". The string can also be a statement
handle generated by a previous prepare. Note that only the first row of data is returned.
If called in a scalar context, only the first column of the first row is returned. Because
this is not portable, it is not recommended that you use this method in that way.
selectrow_arrayref
$ary_ref = $dbh->selectrow_arrayref($statement);
$ary_ref = $dbh->selectrow_arrayref($statement, \%attr);
$ary_ref = $dbh->selectrow_arrayref($statement, \%attr, @bind_values);
Exactly the same as "selectrow_array", except that it returns a reference to an array, by
internal use of the "fetchrow_arrayref" method.
selectrow_hashref
$hash_ref = $dbh->selectrow_hashref($sql);
$hash_ref = $dbh->selectrow_hashref($sql, \%attr);
$hash_ref = $dbh->selectrow_hashref($sql, \%attr, @bind_values);
Exactly the same as "selectrow_array", except that it returns a reference to an hash, by
internal use of the "fetchrow_hashref" method.
clone
$other_dbh = $dbh->clone();
Creates a copy of the database handle by connecting with the same parameters as the
original handle, then trying to merge the attributes. See the DBI documentation for
complete usage.
Database Handle Attributes
AutoCommit (boolean)
Supported by DBD::Pg 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)
DBD::Pg specific attribute. If true, boolean values will be returned as the characters 't'
and 'f' instead of '1' and '0'.
ReadOnly (boolean)
$dbh->{ReadOnly} = 1;
Specifies if the current database connection should be in read-only mode or not. In this
mode, changes that change the database are not allowed and will throw an error. Note: this
method will not work if "AutoCommit" is true. The read-only effect is accomplished by
sending a SET TRANSACTION READ ONLY after every begin. For more details, please see:
http://www.postgresql.org/docs/current/interactive/sql-set-transaction.html
Please not that this method is not foolproof: there are still ways to update the database.
Consider this a safety net to catch applications that should not be issuing commands such
as INSERT, UPDATE, or DELETE.
This method method requires DBI version 1.55 or better.
pg_server_prepare (integer)
DBD::Pg specific attribute. Indicates if DBD::Pg should attempt to use server-side
prepared statements. The default value, 1, indicates that prepared statements should be
used whenever possible. See the section on the "prepare" method for more information.
pg_placeholder_dollaronly (boolean)
DBD::Pg specific attribute. Defaults to false. When true, question marks inside of
statements are not treated as placeholders. Useful for statements that contain unquoted
question marks, such as geometric operators.
pg_enable_utf8 (boolean)
DBD::Pg specific attribute. If true, then the "utf8" flag will be turned on for returned
character data (if the data is valid UTF-8). For details about the "utf8" flag, see the
"Encode" module. This attribute is only relevant under perl 5.8 and later.
pg_errorlevel (integer)
DBD::Pg 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_lib_version (integer, read-only)
DBD::Pg 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)
DBD::Pg 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.
Name (string, read-only)
Returns the name of the current database. This is the same as the DSN, without the
"dbi:Pg:" part. Before version 2.0.0, this only returned the bare database name (e.g.
'foo'). From version 2.0.0 onwards, it returns the more correct output (e.g. 'dbname=foo')
Username (string, read-only)
Returns the name of the user connected to the database.
pg_db (string, read-only)
DBD::Pg specific attribute. Returns the name of the current database.
pg_user (string, read-only)
DBD::Pg specific attribute. Returns the name of the user that connected to the server.
pg_host (string, read-only)
DBD::Pg specific attribute. Returns the host of the current server connection. Locally
connected hosts will return an empty string.
pg_port (integer, read-only)
DBD::Pg specific attribute. Returns the port of the connection to the server.
pg_socket (integer, read-only)
DBD::Pg specific attribute. Returns the file description number of the connection socket
to the server.
pg_pass (string, read-only)
DBD::Pg specific attribute. Returns the password used to connect to the server.
pg_options (string, read-only)
DBD::Pg specific attribute. Returns the command-line options passed to the server. May be
an empty string.
pg_default_port (integer, read-only)
DBD::Pg specific attribute. Returns the default port used if none is specifically given.
pg_pid (integer, read-only)
DBD::Pg specific attribute. Returns the process id (PID) of the backend server process
handling the connection.
pg_prepare_now (boolean)
DBD::Pg specific attribute. Default is off. If true, then the "prepare" method will
immediately prepare commands, rather than waiting until the first execute.
pg_expand_array (boolean)
DBD::Pg specific attribute. Defaults to true. If false, arrays returned from the server
will not be changed into a Perl arrayref, but remain as a string.
pg_async_status (integer, read-only)
DBD::Pg specific attribute. Returns the current status of an asynchronous command. 0
indicates no asynchronous command is in progress, 1 indicates that an asynchronous command
has started and -1 indicated that an asynchronous command has been cancelled.
pg_standard_conforming_strings (boolean, read-only)
DBD::Pg specific attribute. Returns true if the server is currently using standard
conforming strings. Only available if the target server is version 8.2 or better.
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".
Driver (handle, read-only)
Holds the handle of the parent driver. The only recommended use for this is to find the
name of the driver using:
$dbh->{Driver}->{Name}
pg_protocol (integer, read-only)
DBD::Pg 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.
RowCacheSize
Not used by DBD::Pg
DBI STATEMENT HANDLE OBJECTS
Statement Handle Methods
bind_param
$rv = $sth->bind_param($param_num, $bind_value);
$rv = $sth->bind_param($param_num, $bind_value, $bind_type);
$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_FDW_HANDLER 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_NODE_TREE PG_PG_PROC PG_PG_TYPE PG_POINT PG_POINTARRAY
PG_POLYGON PG_POLYGONARRAY PG_RECORD PG_RECORDARRAY 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
$rv = $sth->bind_param_inout($param_num, \$scalar, 0);
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
$rv = $sth->bind_param_array($param_num, $array_ref_or_value)
$rv = $sth->bind_param_array($param_num, $array_ref_or_value, $bind_type)
$rv = $sth->bind_param_array($param_num, $array_ref_or_value, \%attr)
Binds an array of values to a placeholder, so that each is used in turn by a call to the
"execute_array" method.
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.
If you are using Postgres version 8.2 or greater, you can also use any of the fetch
methods to retrieve the values of a "RETURNING" clause after you execute an "UPDATE",
"DELETE", or "INSERT". For example:
$dbh->do(q{CREATE TABLE abc (id SERIAL, country TEXT)});
$SQL = q{INSERT INTO abc (country) VALUES (?) RETURNING id};
$sth = $dbh->prepare($SQL);
$sth->execute('France');
$countryid = $sth->fetch()->[0];
$sth->execute('New Zealand');
$countryid = $sth->fetch()->[0];
execute_array
$tuples = $sth->execute_array() or die $sth->errstr;
$tuples = $sth->execute_array(\%attr) or die $sth->errstr;
$tuples = $sth->execute_array(\%attr, @bind_values) or die $sth->errstr;
($tuples, $rows) = $sth->execute_array(\%attr) or die $sth->errstr;
($tuples, $rows) = $sth->execute_array(\%attr, @bind_values) or die $sth->errstr;
Execute a prepared statement once for each item in a passed-in hashref, or items that were
previously bound via the "bind_param_array" method. See the DBI documentation for more
details.
execute_for_fetch
$tuples = $sth->execute_for_fetch($fetch_tuple_sub);
$tuples = $sth->execute_for_fetch($fetch_tuple_sub, \@tuple_status);
($tuples, $rows) = $sth->execute_for_fetch($fetch_tuple_sub);
($tuples, $rows) = $sth->execute_for_fetch($fetch_tuple_sub, \@tuple_status);
Used internally by the "execute_array" method, and rarely used directly. See the DBI
documentation for more details.
fetchrow_arrayref
$ary_ref = $sth->fetchrow_arrayref;
Fetches the next row of data from the statement handle, and returns a reference to an
array holding the column values. Any columns that are NULL are returned as undef within
the array.
If there are no more rows or if an error occurs, the this method return undef. You should
check "$sth->err" afterwards (or use the "RaiseError" attribute) to discover if the undef
returned was due to an error.
Note that the same array reference is returned for each fetch, so don't store the
reference and then use it after a later fetch. Also, the elements of the array are also
reused for each row, so take care if you want to take a reference to an element. See also
"bind_columns".
fetchrow_array
@ary = $sth->fetchrow_array;
Similar to the "fetchrow_arrayref" method, but returns a list of column information rather
than a reference to a list. Do not use this in a scalar context.
fetchrow_hashref
$hash_ref = $sth->fetchrow_hashref;
$hash_ref = $sth->fetchrow_hashref($name);
Fetches the next row of data and returns a hashref containing the name of the columns as
the keys and the data itself as the values. Any NULL value is returned as as undef value.
If there are no more rows or if an error occurs, the this method return 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 argument should be either "NAME", "NAME_lc" or "NAME_uc", and indicates
what sort of transformation to make to the keys in the hash.
fetchall_arrayref
$tbl_ary_ref = $sth->fetchall_arrayref();
$tbl_ary_ref = $sth->fetchall_arrayref( $slice );
$tbl_ary_ref = $sth->fetchall_arrayref( $slice, $max_rows );
Returns a reference to an array of arrays that contains all the remaining rows to be
fetched from the statement handle. If there are no more rows, an empty arrayref will be
returned. If an error occurs, the data read in so far will be returned. Because of this,
you should always check "$sth->err" after calling this method, unless "RaiseError" has
been enabled.
If $slice is an array reference, fetchall_arrayref uses the "fetchrow_arrayref" method to
fetch each row as an array ref. If the $slice array is not empty then it is used as a
slice to select individual columns by perl array index number (starting at 0, unlike
column and parameter numbers which start at 1).
With no parameters, or if $slice is undefined, fetchall_arrayref acts as if passed an
empty array ref.
If $slice is a hash reference, fetchall_arrayref uses "fetchrow_hashref" to fetch each row
as a hash reference.
See the DBI documentation for a complete discussion.
fetchall_hashref
$hash_ref = $sth->fetchall_hashref( $key_field );
Returns a hashref containing all rows to be fetched from the statement handle. See the DBI
documentation for a full discussion.
finish
$rv = $sth->finish;
Indicates to DBI that you are finished with the statement handle and are not going to use
it again. Only needed when you have not fetched all the possible rows.
rows
$rv = $sth->rows;
Returns the number of rows returned by the last query. In contrast to many other DBD
modules, the number of rows is available immediately after calling "$sth->execute". Note
that the "execute" method itself returns the number of rows itself, which means that this
method is rarely needed.
bind_col
$rv = $sth->bind_col($column_number, \$var_to_bind);
$rv = $sth->bind_col($column_number, \$var_to_bind, \%attr );
$rv = $sth->bind_col($column_number, \$var_to_bind, $bind_type );
Binds a Perl variable and/or some attributes to an output column of a SELECT statement.
Column numbers count up from 1. You do not need to bind output columns in order to fetch
data.
See the DBI documentation for a discussion of the optional parameters "\%attr" and
$bind_type
bind_columns
$rv = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
Calls the "bind_col" method for each column in the SELECT statement, using the supplied
list.
dump_results
$rows = $sth->dump_results($maxlen, $lsep, $fsep, $fh);
Fetches all the rows from the statement handle, calls "DBI::neat_list" for each row, and
prints the results to $fh (which defaults to STDOUT). Rows are separated by $lsep (which
defaults to a newline). Columns are separated by $fsep (which defaults to a comma). The
$maxlen controls how wide the output can be, and defaults to 35.
This method is designed as a handy utility for prototyping and testing queries. Since it
uses "neat_list" to format and edit the string for reading by humans, it is not
recommended for data transfer applications.
blob_read
$blob = $sth->blob_read($id, $offset, $len);
Supported by DBD::Pg. This method is implemented by DBI but not currently documented by
DBI, 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 whole blob is fetched at once.
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)
Returns the number of columns returned by the current statement. A number will only be
returned for SELECT statements, for SHOW statements (which always return 1), and for
INSERT, UPDATE, and DELETE statements which contain a RETURNING clause. This method
returns undef if called before "execute()".
NUM_OF_PARAMS (integer, read-only)
Returns the number of placeholders in the current statement.
NAME (arrayref, read-only)
Returns an arrayref of column names for the current statement. This method will only work
for SELECT statements, for SHOW statements, and for INSERT, UPDATE, and DELETE statements
which contain a RETURNING clause. This method returns undef if called before "execute()".
NAME_lc (arrayref, read-only)
The same as the "NAME" attribute, except that all column names are forced to lower case.
NAME_uc (arrayref, read-only)
The same as the "NAME" attribute, except that all column names are forced to upper case.
NAME_hash (hashref, read-only)
Similar to the "NAME" attribute, but returns a hashref of column names instead of an
arrayref. The names of the columns are the keys of the hash, and the values represent the
order in which the columns are returned, starting at 0. This method returns undef if
called before "execute()".
NAME_lc_hash (hashref, read-only)
The same as the "NAME_hash" attribute, except that all column names are forced to lower
case.
NAME_uc_hash (hashref, read-only)
The same as the "NAME_hash" attribute, except that all column names are forced to lower
case.
TYPE (arrayref, read-only)
Returns an arrayref indicating the data type for each column in the statement. This
method returns undef if called before "execute()".
PRECISION (arrayref, read-only)
Returns an arrayref of integer values for each column returned by the statement. The
number indicates the precision for "NUMERIC" columns, the size in number of characters for
"CHAR" and "VARCHAR" columns, and for all other types of columns it returns the number of
bytes. This method returns undef if called before "execute()".
SCALE (arrayref, read-only)
Returns an arrayref of integer values for each column returned by the statement. The
number indicates the scale of the that column. The only type that will return a value is
"NUMERIC". This method returns undef if called before "execute()".
NULLABLE (arrayref, read-only)
Returns an arrayref of integer values for each column returned by the statement. The
number indicates if the column is nullable or not. 0 = not nullable, 1 = nullable, 2 =
unknown. This method returns undef if called before "execute()".
Database (dbh, read-only)
Returns the database handle this statement handle was created from.
ParamValues (hash ref, read-only)
Returns a reference to a hash containing the values currently bound to placeholders. If
the "named parameters" type of placeholders are being used (such as ":foo"), then the keys
of the hash will be the names of the placeholders (without the colon). If the "dollar sign
numbers" type of placeholders are being used, the keys of the hash will be the numbers,
without the dollar signs. If the "question mark" type is used, integer numbers will be
returned, starting at one and increasing for every placeholder.
If this method is called before "execute", the literal values passed in are returned. If
called after "execute", then the quoted versions of the values are returned.
ParamTypes (hash ref, read-only)
Returns a reference to a hash containing the type names currently bound to placeholders.
The keys are the same as returned by the ParamValues method. The values are hashrefs
containing a single key value pair, in which the key is either 'TYPE' if the type has a
generic SQL equivalent, and 'pg_type' if the type can only be expressed by a Postgres
type. The value is the internal number corresponding to the type originally passed in.
(Placeholders that have not yet been bound will return undef as the value). This allows
the output of ParamTypes to be passed back to the "bind_param" method.
Statement (string, read-only)
Returns the statement string passed to the most recent "prepare" method called in this
database handle, even if that method failed. This is especially useful where "RaiseError"
is enabled and the exception handler checks $@ and sees that a "prepare" method call
failed.
pg_current_row (integer, read-only)
DBD::Pg specific attribute. Returns the number of the tuple (row) that was most recently
fetched. Returns zero before and after fetching is performed.
pg_numbound (integer, read-only)
DBD::Pg specific attribute. Returns the number of placeholders that are currently bound
(via bind_param).
pg_bound (hashref, read-only)
DBD::Pg specific attribute. Returns a hash of all named placeholders. The key is the name
of the placeholder, and the value is a 0 or a 1, indicating if the placeholder has been
bound yet (e.g. via bind_param)
pg_size (arrayref, read-only)
DBD::Pg 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 (arrayref, read-only)
DBD::Pg specific attribute. It returns a reference to an array of strings for each column.
The string shows the name of the data_type.
pg_segments (arrayref, read-only)
DBD::Pg specific attribute. Returns an arrayref of the query split on the placeholders.
pg_oid_status (integer, read-only)
DBD::Pg specific attribute. It returns the OID of the last INSERT command.
pg_cmd_status (integer, read-only)
DBD::Pg specific attribute. It returns the type of the last command. Possible types are:
"INSERT", "DELETE", "UPDATE", "SELECT".
pg_direct (boolean)
DBD::Pg specific attribute. Default is false. If true, the query is passed directly to the
backend without parsing for placeholders.
pg_prepare_now (boolean)
DBD::Pg specific attribute. Default is off. If true, the query will be immediately
prepared, rather than waiting for the "execute" call.
pg_prepare_name (string)
DBD::Pg specific attribute. Specifies the name of the prepared statement to use for this
statement handle. Not normally needed, see the section on the "prepare" method for more
information.
pg_server_prepare (integer)
DBD::Pg specific attribute. Indicates if DBD::Pg should attempt to use server-side
prepared statements for this statement handle. The default value, 1, indicates that
prepared statements should be used whenever possible. See the section on the "prepare"
method for more information.
pg_placeholder_dollaronly (boolean)
DBD::Pg specific attribute. Defaults to off. When true, question marks inside of the query
being prepared are not treated as placeholders. Useful for statements that contain
unquoted question marks, such as geometric operators.
pg_async (integer)
DBD::Pg specific attribute. Indicates the current behavior for asynchronous queries. See
the section on "Asynchronous Constants" for more information.
RowsInCache
Not used by DBD::Pg
RowCache
Not used by DBD::Pg
CursorName
Not used by DBD::Pg. See the note about "Cursors" elsewhere in this document.
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.
Note that preparing a statement is not always enough to trigger the first "BEGIN", as the
actual "PREPARE" is usually postponed until the first call to "execute".
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 = $dbh->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 may leave the database in an unusable state, so you may need
to ROLLBACK or ROLLBACK TO a savepoint. As of version 2.17.0 of DBD::Pg, rollbacks are
not done automatically.
$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"
methods. 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
attribute pg_expand_array, which is true by default.
$dbh->{pg_expand_array} = 0;
COPY support
DBD::Pg allows for quick (bulk) reading and storing of data by using the COPY command. The
basic process is to use "$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 IN mode (yes, that's confusing, but the mode is
COPY IN because of the command COPY FROM). 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 a COPY command has been issued, no other SQL commands are allowed until
"pg_putcopyend" has been issued (for COPY FROM), or the final "pg_getcopydata" has been
called (for COPY TO).
Note: All other COPY methods (pg_putline, pg_getline, etc.) are now heavily 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 a COPY OUT mode by
calling "COPY tablename TO STDOUT". Data is always returned one data row at a time. 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). The pg_gecopydata method 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 method called pg_getcopydata_async, which, as the name
suggests, returns immediately. The only difference from the original method 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 method
again until you get a non-zero result. (Data is still always returned one data row at a
time.)
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");
$dbh->pg_putcopyend();
pg_putcopyend
When you are finished with pg_putcopydata, call pg_putcopyend to let the server know that
you are done, and it will return to a normal, non-COPY state. Returns a 1 on success. This
method will fail if called when not in COPY IN mode.
Large Objects
DBD::Pg supports all largeobject functions provided by libpq via the "$dbh->pg_lo*"
methods. Please note that 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 created, 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' becomes the number 0 and 't'
becomes the number 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
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:
<http://www.postgresql.org/docs/current/static/ddl-schemas.html>
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");
SEE ALSO
BUGS
To report a bug, or view the current list of bugs, please visit
http://rt.cpan.org/Public/Dist/Display.html?Name=DBD-Pg
DEVELOPMENT
Patches can be submitted to rt.cpan.org. Detailed information on how to help out with this
module can be found in the README.dev file. The latest development version can be obtained
via: git clone git://bucardo.org/dbdpg.git
AUTHORS
DBI by Tim Bunce <http://www.tim.bunce.name>
The original DBD-Pg was by Edmund Mergl (E.Mergl@bawue.de) and Jeffrey W. Baker
(jwbaker@acm.org). Major developers include David Wheeler <david@justatheory.com>, Jason
Stewart <jason@openinformatics.com>, Bruce Momjian <pgman@candle.pha.pa.us>, and Greg
Sabino Mullane <greg@turnstep.com>, with help from many others: see the Changes file for a
complete list.
Parts of this package were originally copied from DBI and DBD-Oracle.
Mailing List
The current maintainers may be reached through the 'dbd-pg' mailing list:
<dbd-pg@perl.org>
COPYRIGHT AND LICENSE
Copyright (C) 1994-2012, Greg Sabino Mullane
This module (DBD::Pg) is free software; you can redistribute it and/or modify it under the
same terms as Perl 5.10.0. For more details, see the full text of the licenses in the
directory LICENSES.