Pg(3) User Contributed Perl Documentation Pg(3)NAMEDBD::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 2.15.1 of the DBD::Pg module
DESCRIPTIONDBD::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 sucessfully 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_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::PgDBI 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).
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 8.5 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 C<disconnect>)
-2 An unknown transaction status was returned (e.g. after forking)
-3 The handle exists, but no data was returned from a test query.
In practice, you should only ever see -1 and -2.
get_info
$value = $dbh->get_info($info_type);
Supports a very large set (> 250) of the information types, including
the minimum recommended by DBI.
table_info
$sth = $dbh->table_info(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::PgDBI 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_FLOAT4 PG_FLOAT4ARRAY PG_FLOAT8 PG_FLOAT8ARRAY PG_GTSVECTOR
PG_GTSVECTORARRAY PG_INET PG_INETARRAY PG_INT2 PG_INT2ARRAY PG_INT2VECTOR
PG_INT2VECTORARRAY PG_INT4 PG_INT4ARRAY PG_INT8 PG_INT8ARRAY PG_INTERNAL
PG_INTERVAL PG_INTERVALARRAY PG_LANGUAGE_HANDLER PG_LINE PG_LINEARRAY PG_LSEG
PG_LSEGARRAY PG_MACADDR PG_MACADDRARRAY PG_MONEY PG_MONEYARRAY PG_NAME
PG_NAMEARRAY PG_NUMERIC PG_NUMERICARRAY PG_OID PG_OIDARRAY PG_OIDVECTOR
PG_OIDVECTORARRAY PG_OPAQUE PG_PATH PG_PATHARRAY PG_PG_ATTRIBUTE PG_PG_CLASS
PG_PG_PROC PG_PG_TYPE PG_POINT PG_POINTARRAY PG_POLYGON PG_POLYGONARRAY
PG_RECORD PG_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 = $pg_result();
print "Result of do(): $res\n";
}
print "Query is still running...\n";
if (cancel_request_received) {
$dbh->pg_cancel();
}
sleep 1;
redo;
}
print "Async prepare/execute example:\n";
$sth = $dbh->prepare("SELECT long_running_query(1)", {pg_async => PG_ASYNC});
$sth->execute();
## Changed our mind, cancel and run again:
$sth = $dbh->prepare("SELECT 678", {pg_async => PG_ASYNC + PG_OLDQUERY_CANCEL});
$sth->execute();
do_something_else();
if (!$sth->pg_ready) {
do_another_thing();
}
## We wait until it is done, and get the result:
$res = $dbh->pg_result();
Asynchronous Constants
There are currently three asynchronous constants exported by DBD::Pg.
You can import all of them by putting either of these at the top of
your script:
use DBD::Pg;
use DBD::Pg ':async';
You may also use the numbers instead of the constants, but using the
constants is recommended as it makes your script more readable.
PG_ASYNC
This is a constant for the number 1. It is passed to either the
"do" or the "prepare" method as a value to the pg_async key and
indicates that the query should be sent asynchronously.
PG_OLDQUERY_CANCEL
This is a constant for the number 2. When passed to either the "do"
or the "prepare" method, it causes any currently running
asynchronous query to be cancelled and rolled back. It has no
effect if no asynchronous query is currently running.
PG_OLDQUERY_WAIT
This is a constant for the number 4. When passed to either the "do"
or the "prepare" method, it waits for any currently running
asynchronous query to complete. It has no effect if there is no
asynchronous query currently running.
Asynchronous Methods
pg_cancel
This database-level method attempts to cancel any currently running
asynchronous query. It returns true if the cancel succeeded, and
false otherwise. Note that a query that has finished before this
method is executed will also return false. WARNING: a successful
cancellation will leave the database in an unusable state, so
DBD::Pg will automatically clear out the error message and issue a
ROLLBACK.
$result = $dbh->pg_cancel();
pg_ready
This method can be called as a database handle method or (for
convenience) as a statement handle method. Both simply see if a
previously issued asynchronous query has completed yet. It returns
true if the statement has finished, in which case you should then
call the "pg_result" method. Calls to "pg_ready()" should only be
used when you have other things to do while the query is running.
If you simply want to wait until the query is done, do not call
pg_ready() over and over, but simply call the pg_result() method.
my $time = 0;
while (!$dbh->pg_ready) {
print "Query is still running. Seconds: $time\n";
$time++;
sleep 1;
}
$result = $dbh->pg_result;
pg_result
This database handle method returns the results of a previously
issued asynchronous query. If the query is still running, this
method will wait until it has finished. The result returned is the
number of rows: the same thing that would have been returned by the
asynchronous "do" or "execute" if it had been called without an
asynchronous flag.
$result = $dbh->pg_result;
Asynchronous Examples
Here are some working examples of asynchronous queries. Note that we'll
use the pg_sleep function to emulate a long-running query.
use strict;
use warnings;
use Time::HiRes 'sleep';
use DBD::Pg ':async';
my $dbh = DBI->connect('dbi:Pg:dbname=postgres', 'postgres', '', {AutoCommit=>0,RaiseError=>1});
## Kick off a long running query on the first database:
my $sth = $dbh->prepare("SELECT pg_sleep(?)", {pg_async => PG_ASYNC});
$sth->execute(5);
## While that is running, do some other things
print "Your query is processing. Thanks for waiting\n";
check_on_the_kids(); ## Expensive sub, takes at least three seconds.
while (!$dbh->pg_ready) {
check_on_the_kids();
## If the above function returns quickly for some reason, we add a small sleep
sleep 0.1;
}
print "The query has finished. Gathering results\n";
my $result = $sth->pg_result;
print "Result: $result\n";
my $info = $sth->fetchall_arrayref();
Without asynchronous queries, the above script would take about 8
seconds to run: five seconds waiting for the execute to finish, then
three for the check_on_the_kids() function to return. With asynchronous
queries, the script takes about 6 seconds to run, and gets in two
iterations of check_on_the_kids in the process.
Here's an example showing the ability to cancel a long-running query.
Imagine two slave databases in different geographic locations over a
slow network. You need information as quickly as possible, so you query
both at once. When you get an answer, you tell the other one to stop
working on your query, as you don't need it anymore.
use strict;
use warnings;
use Time::HiRes 'sleep';
use DBD::Pg ':async';
my $dbhslave1 = DBI->connect('dbi:Pg:dbname=postgres;host=slave1', 'postgres', '', {AutoCommit=>0,RaiseError=>1});
my $dbhslave2 = DBI->connect('dbi:Pg:dbname=postgres;host=slave2', 'postgres', '', {AutoCommit=>0,RaiseError=>1});
$SQL = "SELECT count(*) FROM largetable WHERE flavor='blueberry'";
my $sth1 = $dbhslave1->prepare($SQL, {pg_async => PG_ASYNC});
my $sth2 = $dbhslave2->prepare($SQL, {pg_async => PG_ASYNC});
$sth1->execute();
$sth2->execute();
my $winner;
while (!defined $winner) {
if ($sth1->pg_ready) {
$winner = 1;
}
elsif ($sth2->pg_ready) {
$winner = 2;
}
Time::HiRes::sleep 0.05;
}
my $count;
if ($winner == 1) {
$sth2->pg_cancel();
$sth1->pg_result();
$count = $sth1->fetchall_arrayref()->[0][0];
}
else {
$sth1->pg_cancel();
$sth2->pg_result();
$count = $sth2->fetchall_arrayref()->[0][0];
}
Array support
DBD::Pg allows arrays (as arrayrefs) to be passed in to both the
"quote" and the "execute" 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
"func" method. 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 ALSOBUGS
To report a bug, or view the current list of bugs, please visit
http://rt.cpan.org/Public/Dist/Display.html?Name=DBD-Pg
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
The DBD::Pg module is free software. You may distribute under the terms
of either the GNU General Public License or the Artistic License, as
specified in the Perl README file.
perl v5.10.1 2009-08-07 Pg(3)