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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="FUNCTIONS-ADMIN"
>9.26. System Administration Functions</A
></H1
><P
> The functions described in this section are used to control and
monitor a <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> installation.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-ADMIN-SET"
>9.26.1. Configuration Settings Functions</A
></H2
><P
> <A
HREF="functions-admin.html#FUNCTIONS-ADMIN-SET-TABLE"
>Table 9-58</A
> shows the functions
available to query and alter run-time configuration parameters.
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-ADMIN-SET-TABLE"
></A
><P
><B
>Table 9-58. Configuration Settings Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
>
<TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>current_setting(<TT
CLASS="PARAMETER"
>setting_name</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>get current value of setting</TD
></TR
><TR
><TD
>
<TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>set_config(<TT
CLASS="PARAMETER"
>setting_name</TT
>,
<TT
CLASS="PARAMETER"
>new_value</TT
>,
<TT
CLASS="PARAMETER"
>is_local</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>set parameter and return new value</TD
></TR
></TBODY
></TABLE
></DIV
><P
> The function <CODE
CLASS="FUNCTION"
>current_setting</CODE
> yields the
current value of the setting <TT
CLASS="PARAMETER"
>setting_name</TT
>.
It corresponds to the <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> command
<TT
CLASS="COMMAND"
>SHOW</TT
>. An example:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT current_setting('datestyle');
current_setting
-----------------
ISO, MDY
(1 row)</PRE
><P>
</P
><P
> <CODE
CLASS="FUNCTION"
>set_config</CODE
> sets the parameter
<TT
CLASS="PARAMETER"
>setting_name</TT
> to
<TT
CLASS="PARAMETER"
>new_value</TT
>. If
<TT
CLASS="PARAMETER"
>is_local</TT
> is <TT
CLASS="LITERAL"
>true</TT
>, the
new value will only apply to the current transaction. If you want
the new value to apply for the current session, use
<TT
CLASS="LITERAL"
>false</TT
> instead. The function corresponds to the
SQL command <TT
CLASS="COMMAND"
>SET</TT
>. An example:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT set_config('log_statement_stats', 'off', false);
set_config
------------
off
(1 row)</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-ADMIN-SIGNAL"
>9.26.2. Server Signalling Functions</A
></H2
><P
> The functions shown in <A
HREF="functions-admin.html#FUNCTIONS-ADMIN-SIGNAL-TABLE"
>Table 9-59</A
> send control signals to
other server processes. Use of these functions is usually restricted
to superusers, with noted exceptions.
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-ADMIN-SIGNAL-TABLE"
></A
><P
><B
>Table 9-59. Server Signalling Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_cancel_backend(<TT
CLASS="PARAMETER"
>pid</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Cancel a backend's current query. You can execute this against
another backend that has exactly the same role as the user calling the
function. In all other cases, you must be a superuser.
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_reload_conf()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Cause server processes to reload their configuration files</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_rotate_logfile()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Rotate server's log file</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_terminate_backend(<TT
CLASS="PARAMETER"
>pid</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Terminate a backend. You can execute this against
another backend that has exactly the same role as the user
calling the function. In all other cases, you must be a
superuser.
</TD
></TR
></TBODY
></TABLE
></DIV
><P
> Each of these functions returns <TT
CLASS="LITERAL"
>true</TT
> if
successful and <TT
CLASS="LITERAL"
>false</TT
> otherwise.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_cancel_backend</CODE
> and <CODE
CLASS="FUNCTION"
>pg_terminate_backend</CODE
>
send signals (<SPAN
CLASS="SYSTEMITEM"
>SIGINT</SPAN
> or <SPAN
CLASS="SYSTEMITEM"
>SIGTERM</SPAN
>
respectively) to backend processes identified by process ID.
The process ID of an active backend can be found from
the <TT
CLASS="STRUCTFIELD"
>pid</TT
> column of the
<TT
CLASS="STRUCTNAME"
>pg_stat_activity</TT
> view, or by listing the
<TT
CLASS="COMMAND"
>postgres</TT
> processes on the server (using
<SPAN
CLASS="APPLICATION"
>ps</SPAN
> on Unix or the <SPAN
CLASS="APPLICATION"
>Task
Manager</SPAN
> on <SPAN
CLASS="PRODUCTNAME"
>Windows</SPAN
>).
The role of an active backend can be found from the
<TT
CLASS="STRUCTFIELD"
>usename</TT
> column of the
<TT
CLASS="STRUCTNAME"
>pg_stat_activity</TT
> view.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_reload_conf</CODE
> sends a <SPAN
CLASS="SYSTEMITEM"
>SIGHUP</SPAN
> signal
to the server, causing configuration files
to be reloaded by all server processes.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_rotate_logfile</CODE
> signals the log-file manager to switch
to a new output file immediately. This works only when the built-in
log collector is running, since otherwise there is no log-file manager
subprocess.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-ADMIN-BACKUP"
>9.26.3. Backup Control Functions</A
></H2
><P
> The functions shown in <A
HREF="functions-admin.html#FUNCTIONS-ADMIN-BACKUP-TABLE"
>Table 9-60</A
> assist in making on-line backups.
These functions cannot be executed during recovery (except
<CODE
CLASS="FUNCTION"
>pg_xlog_location_diff</CODE
>).
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-ADMIN-BACKUP-TABLE"
></A
><P
><B
>Table 9-60. Backup Control Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_create_restore_point(<TT
CLASS="PARAMETER"
>name</TT
> <TT
CLASS="TYPE"
>text</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>Create a named point for performing restore (restricted to superusers)</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_current_xlog_insert_location()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>Get current transaction log insert location</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_current_xlog_location()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>Get current transaction log write location</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_start_backup(<TT
CLASS="PARAMETER"
>label</TT
> <TT
CLASS="TYPE"
>text</TT
> [<SPAN
CLASS="OPTIONAL"
>, <TT
CLASS="PARAMETER"
>fast</TT
> <TT
CLASS="TYPE"
>boolean</TT
> </SPAN
>])</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>Prepare for performing on-line backup (restricted to superusers or replication roles)</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_stop_backup()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>Finish performing on-line backup (restricted to superusers or replication roles)</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_switch_xlog()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>Force switch to a new transaction log file (restricted to superusers)</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_xlogfile_name(<TT
CLASS="PARAMETER"
>location</TT
> <TT
CLASS="TYPE"
>text</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>Convert transaction log location string to file name</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_xlogfile_name_offset(<TT
CLASS="PARAMETER"
>location</TT
> <TT
CLASS="TYPE"
>text</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
>, <TT
CLASS="TYPE"
>integer</TT
></TD
><TD
>Convert transaction log location string to file name and decimal byte offset within file</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_xlog_location_diff(<TT
CLASS="PARAMETER"
>location</TT
> <TT
CLASS="TYPE"
>text</TT
>, <TT
CLASS="PARAMETER"
>location</TT
> <TT
CLASS="TYPE"
>text</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>numeric</TT
></TD
><TD
>Calculate the difference between two transaction log locations</TD
></TR
></TBODY
></TABLE
></DIV
><P
> <CODE
CLASS="FUNCTION"
>pg_start_backup</CODE
> accepts an
arbitrary user-defined label for the backup. (Typically this would be
the name under which the backup dump file will be stored.) The function
writes a backup label file (<TT
CLASS="FILENAME"
>backup_label</TT
>) into the
database cluster's data directory, performs a checkpoint,
and then returns the backup's starting transaction log location as text.
The user can ignore this result value, but it is
provided in case it is useful.
</P><PRE
CLASS="PROGRAMLISTING"
>postgres=# select pg_start_backup('label_goes_here');
pg_start_backup
-----------------
0/D4445B8
(1 row)</PRE
><P>
There is an optional second parameter of type <TT
CLASS="TYPE"
>boolean</TT
>. If <TT
CLASS="LITERAL"
>true</TT
>,
it specifies executing <CODE
CLASS="FUNCTION"
>pg_start_backup</CODE
> as quickly as
possible. This forces an immediate checkpoint which will cause a
spike in I/O operations, slowing any concurrently executing queries.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_stop_backup</CODE
> removes the label file created by
<CODE
CLASS="FUNCTION"
>pg_start_backup</CODE
>, and creates a backup history file in
the transaction log archive area. The history file includes the label given to
<CODE
CLASS="FUNCTION"
>pg_start_backup</CODE
>, the starting and ending transaction log locations for
the backup, and the starting and ending times of the backup. The return
value is the backup's ending transaction log location (which again
can be ignored). After recording the ending location, the current
transaction log insertion
point is automatically advanced to the next transaction log file, so that the
ending transaction log file can be archived immediately to complete the backup.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_switch_xlog</CODE
> moves to the next transaction log file, allowing the
current file to be archived (assuming you are using continuous archiving).
The return value is the ending transaction log location + 1 within the just-completed transaction log file.
If there has been no transaction log activity since the last transaction log switch,
<CODE
CLASS="FUNCTION"
>pg_switch_xlog</CODE
> does nothing and returns the start location
of the transaction log file currently in use.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_create_restore_point</CODE
> creates a named transaction log
record that can be used as recovery target, and returns the corresponding
transaction log location. The given name can then be used with
<A
HREF="recovery-target-settings.html#RECOVERY-TARGET-NAME"
>recovery_target_name</A
> to specify the point up to which
recovery will proceed. Avoid creating multiple restore points with the
same name, since recovery will stop at the first one whose name matches
the recovery target.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_current_xlog_location</CODE
> displays the current transaction log write
location in the same format used by the above functions. Similarly,
<CODE
CLASS="FUNCTION"
>pg_current_xlog_insert_location</CODE
> displays the current transaction log
insertion point. The insertion point is the <SPAN
CLASS="QUOTE"
>"logical"</SPAN
> end
of the transaction log
at any instant, while the write location is the end of what has actually
been written out from the server's internal buffers. The write location
is the end of what can be examined from outside the server, and is usually
what you want if you are interested in archiving partially-complete transaction log
files. The insertion point is made available primarily for server
debugging purposes. These are both read-only operations and do not
require superuser permissions.
</P
><P
> You can use <CODE
CLASS="FUNCTION"
>pg_xlogfile_name_offset</CODE
> to extract the
corresponding transaction log file name and byte offset from the results of any of the
above functions. For example:
</P><PRE
CLASS="PROGRAMLISTING"
>postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
file_name | file_offset
--------------------------+-------------
00000001000000000000000D | 4039624
(1 row)</PRE
><P>
Similarly, <CODE
CLASS="FUNCTION"
>pg_xlogfile_name</CODE
> extracts just the transaction log file name.
When the given transaction log location is exactly at a transaction log file boundary, both
these functions return the name of the preceding transaction log file.
This is usually the desired behavior for managing transaction log archiving
behavior, since the preceding file is the last one that currently
needs to be archived.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_xlog_location_diff</CODE
> calculates the difference in bytes
between two transaction log locations. It can be used with
<TT
CLASS="STRUCTNAME"
>pg_stat_replication</TT
> or some functions shown in
<A
HREF="functions-admin.html#FUNCTIONS-ADMIN-BACKUP-TABLE"
>Table 9-60</A
> to get the replication lag.
</P
><P
> For details about proper usage of these functions, see
<A
HREF="continuous-archiving.html"
>Section 24.3</A
>.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-RECOVERY-CONTROL"
>9.26.4. Recovery Control Functions</A
></H2
><P
> The functions shown in <A
HREF="functions-admin.html#FUNCTIONS-RECOVERY-INFO-TABLE"
>Table 9-61</A
> provide information
about the current status of the standby.
These functions may be executed both during recovery and in normal running.
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-RECOVERY-INFO-TABLE"
></A
><P
><B
>Table 9-61. Recovery Information Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_is_in_recovery()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
>True if recovery is still in progress.
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_last_xlog_receive_location()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>Get last transaction log location received and synced to disk by
streaming replication. While streaming replication is in progress
this will increase monotonically. If recovery has completed this will
remain static at
the value of the last WAL record received and synced to disk during
recovery. If streaming replication is disabled, or if it has not yet
started, the function returns NULL.
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_last_xlog_replay_location()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>Get last transaction log location replayed during recovery.
If recovery is still in progress this will increase monotonically.
If recovery has completed then this value will remain static at
the value of the last WAL record applied during that recovery.
When the server has been started normally without recovery
the function returns NULL.
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_last_xact_replay_timestamp()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>timestamp with time zone</TT
></TD
><TD
>Get time stamp of last transaction replayed during recovery.
This is the time at which the commit or abort WAL record for that
transaction was generated on the primary.
If no transactions have been replayed during recovery, this function
returns NULL. Otherwise, if recovery is still in progress this will
increase monotonically. If recovery has completed then this value will
remain static at the value of the last transaction applied during that
recovery. When the server has been started normally without recovery
the function returns NULL.
</TD
></TR
></TBODY
></TABLE
></DIV
><P
> The functions shown in <A
HREF="functions-admin.html#FUNCTIONS-RECOVERY-CONTROL-TABLE"
>Table 9-62</A
> control the progress of recovery.
These functions may be executed only during recovery.
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-RECOVERY-CONTROL-TABLE"
></A
><P
><B
>Table 9-62. Recovery Control Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_is_xlog_replay_paused()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
>True if recovery is paused.
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_xlog_replay_pause()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>void</TT
></TD
><TD
>Pauses recovery immediately.
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_xlog_replay_resume()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>void</TT
></TD
><TD
>Restarts recovery if it was paused.
</TD
></TR
></TBODY
></TABLE
></DIV
><P
> While recovery is paused no further database changes are applied.
If in hot standby, all new queries will see the same consistent snapshot
of the database, and no further query conflicts will be generated until
recovery is resumed.
</P
><P
> If streaming replication is disabled, the paused state may continue
indefinitely without problem. While streaming replication is in
progress WAL records will continue to be received, which will
eventually fill available disk space, depending upon the duration of
the pause, the rate of WAL generation and available disk space.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-SNAPSHOT-SYNCHRONIZATION"
>9.26.5. Snapshot Synchronization Functions</A
></H2
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> allows database sessions to synchronize their
snapshots. A <I
CLASS="FIRSTTERM"
>snapshot</I
> determines which data is visible to the
transaction that is using the snapshot. Synchronized snapshots are
necessary when two or more sessions need to see identical content in the
database. If two sessions just start their transactions independently,
there is always a possibility that some third transaction commits
between the executions of the two <TT
CLASS="COMMAND"
>START TRANSACTION</TT
> commands,
so that one session sees the effects of that transaction and the other
does not.
</P
><P
> To solve this problem, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> allows a transaction to
<I
CLASS="FIRSTTERM"
>export</I
> the snapshot it is using. As long as the exporting
transaction remains open, other transactions can <I
CLASS="FIRSTTERM"
>import</I
> its
snapshot, and thereby be guaranteed that they see exactly the same view
of the database that the first transaction sees. But note that any
database changes made by any one of these transactions remain invisible
to the other transactions, as is usual for changes made by uncommitted
transactions. So the transactions are synchronized with respect to
pre-existing data, but act normally for changes they make themselves.
</P
><P
> Snapshots are exported with the <CODE
CLASS="FUNCTION"
>pg_export_snapshot</CODE
> function,
shown in <A
HREF="functions-admin.html#FUNCTIONS-SNAPSHOT-SYNCHRONIZATION-TABLE"
>Table 9-63</A
>, and
imported with the <A
HREF="sql-set-transaction.html"
>SET TRANSACTION</A
> command.
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-SNAPSHOT-SYNCHRONIZATION-TABLE"
></A
><P
><B
>Table 9-63. Snapshot Synchronization Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_export_snapshot()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>Save the current snapshot and return its identifier</TD
></TR
></TBODY
></TABLE
></DIV
><P
> The function <CODE
CLASS="FUNCTION"
>pg_export_snapshot</CODE
> saves the current snapshot
and returns a <TT
CLASS="TYPE"
>text</TT
> string identifying the snapshot. This string
must be passed (outside the database) to clients that want to import the
snapshot. The snapshot is available for import only until the end of the
transaction that exported it. A transaction can export more than one
snapshot, if needed. Note that doing so is only useful in <TT
CLASS="LITERAL"
>READ
COMMITTED</TT
> transactions, since in <TT
CLASS="LITERAL"
>REPEATABLE READ</TT
> and
higher isolation levels, transactions use the same snapshot throughout
their lifetime. Once a transaction has exported any snapshots, it cannot
be prepared with <A
HREF="sql-prepare-transaction.html"
>PREPARE TRANSACTION</A
>.
</P
><P
> See <A
HREF="sql-set-transaction.html"
>SET TRANSACTION</A
> for details of how to use an
exported snapshot.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-ADMIN-DBOBJECT"
>9.26.6. Database Object Management Functions</A
></H2
><P
> The functions shown in <A
HREF="functions-admin.html#FUNCTIONS-ADMIN-DBSIZE"
>Table 9-64</A
> calculate
the disk space usage of database objects.
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-ADMIN-DBSIZE"
></A
><P
><B
>Table 9-64. Database Object Size Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_column_size(<TT
CLASS="TYPE"
>any</TT
>)</CODE
></TT
></TD
><TD
><TT
CLASS="TYPE"
>int</TT
></TD
><TD
>Number of bytes used to store a particular value (possibly compressed)</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_database_size(<TT
CLASS="TYPE"
>oid</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
>Disk space used by the database with the specified OID</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_database_size(<TT
CLASS="TYPE"
>name</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
>Disk space used by the database with the specified name</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_indexes_size(<TT
CLASS="TYPE"
>regclass</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
> Total disk space used by indexes attached to the specified table
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_relation_size(<TT
CLASS="PARAMETER"
>relation</TT
> <TT
CLASS="TYPE"
>regclass</TT
>, <TT
CLASS="PARAMETER"
>fork</TT
> <TT
CLASS="TYPE"
>text</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
> Disk space used by the specified fork (<TT
CLASS="LITERAL"
>'main'</TT
>,
<TT
CLASS="LITERAL"
>'fsm'</TT
>, <TT
CLASS="LITERAL"
>'vm'</TT
>, or <TT
CLASS="LITERAL"
>'init'</TT
>)
of the specified table or index
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_relation_size(<TT
CLASS="PARAMETER"
>relation</TT
> <TT
CLASS="TYPE"
>regclass</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
> Shorthand for <TT
CLASS="LITERAL"
>pg_relation_size(..., 'main')</TT
>
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_size_pretty(<TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
> Converts a size in bytes expressed as a 64-bit integer into a
human-readable format with size units
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_size_pretty(<TT
CLASS="TYPE"
>numeric</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
> Converts a size in bytes expressed as a numeric value into a
human-readable format with size units
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_table_size(<TT
CLASS="TYPE"
>regclass</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
> Disk space used by the specified table, excluding indexes
(but including TOAST, free space map, and visibility map)
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_tablespace_size(<TT
CLASS="TYPE"
>oid</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
>Disk space used by the tablespace with the specified OID</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_tablespace_size(<TT
CLASS="TYPE"
>name</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
>Disk space used by the tablespace with the specified name</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_total_relation_size(<TT
CLASS="TYPE"
>regclass</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
> Total disk space used by the specified table,
including all indexes and <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> data
</TD
></TR
></TBODY
></TABLE
></DIV
><P
> <CODE
CLASS="FUNCTION"
>pg_column_size</CODE
> shows the space used to store any individual
data value.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_total_relation_size</CODE
> accepts the OID or name of a
table or toast table, and returns the total on-disk space used for
that table, including all associated indexes. This function is
equivalent to <CODE
CLASS="FUNCTION"
>pg_table_size</CODE
>
<TT
CLASS="LITERAL"
>+</TT
> <CODE
CLASS="FUNCTION"
>pg_indexes_size</CODE
>.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_table_size</CODE
> accepts the OID or name of a table and
returns the disk space needed for that table, exclusive of indexes.
(TOAST space, free space map, and visibility map are included.)
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_indexes_size</CODE
> accepts the OID or name of a table and
returns the total disk space used by all the indexes attached to that
table.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_database_size</CODE
> and <CODE
CLASS="FUNCTION"
>pg_tablespace_size</CODE
>
accept the OID or name of a database or tablespace, and return the total
disk space used therein. To use <CODE
CLASS="FUNCTION"
>pg_database_size</CODE
>,
you must have <TT
CLASS="LITERAL"
>CONNECT</TT
> permission on the specified database
(which is granted by default). To use <CODE
CLASS="FUNCTION"
>pg_tablespace_size</CODE
>,
you must have <TT
CLASS="LITERAL"
>CREATE</TT
> permission on the specified tablespace,
unless it is the default tablespace for the current database.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_relation_size</CODE
> accepts the OID or name of a table, index
or toast table, and returns the on-disk size in bytes of one fork of
that relation. (Note that for most purposes it is more convenient to
use the higher-level functions <CODE
CLASS="FUNCTION"
>pg_total_relation_size</CODE
>
or <CODE
CLASS="FUNCTION"
>pg_table_size</CODE
>, which sum the sizes of all forks.)
With one argument, it returns the size of the main data fork of the
relation. The second argument can be provided to specify which fork
to examine:
<P
></P
></P><UL
COMPACT="COMPACT"
><LI
><P
> <TT
CLASS="LITERAL"
>'main'</TT
> returns the size of the main
data fork of the relation.
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>'fsm'</TT
> returns the size of the Free Space Map
(see <A
HREF="storage-fsm.html"
>Section 56.3</A
>) associated with the relation.
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>'vm'</TT
> returns the size of the Visibility Map
(see <A
HREF="storage-vm.html"
>Section 56.4</A
>) associated with the relation.
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>'init'</TT
> returns the size of the initialization fork,
if any, (see <A
HREF="storage-init.html"
>Section 56.5</A
>) associated with the relation.
</P
></LI
></UL
><P>
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_size_pretty</CODE
> can be used to format the result of one of
the other functions in a human-readable way, using kB, MB, GB or TB as
appropriate.
</P
><P
> The functions above that operate on tables or indexes accept a
<TT
CLASS="TYPE"
>regclass</TT
> argument, which is simply the OID of the table or index
in the <TT
CLASS="STRUCTNAME"
>pg_class</TT
> system catalog. You do not have to look up
the OID by hand, however, since the <TT
CLASS="TYPE"
>regclass</TT
> data type's input
converter will do the work for you. Just write the table name enclosed in
single quotes so that it looks like a literal constant. For compatibility
with the handling of ordinary <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> names, the string
will be converted to lower case unless it contains double quotes around
the table name.
</P
><P
> If an OID that does not represent an existing object is passed as
argument to one of the above functions, NULL is returned.
</P
><P
> The functions shown in <A
HREF="functions-admin.html#FUNCTIONS-ADMIN-DBLOCATION"
>Table 9-65</A
> assist
in identifying the specific disk files associated with database objects.
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-ADMIN-DBLOCATION"
></A
><P
><B
>Table 9-65. Database Object Location Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_relation_filenode(<TT
CLASS="PARAMETER"
>relation</TT
> <TT
CLASS="TYPE"
>regclass</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
> Filenode number of the specified relation
</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_relation_filepath(<TT
CLASS="PARAMETER"
>relation</TT
> <TT
CLASS="TYPE"
>regclass</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
> File path name of the specified relation
</TD
></TR
></TBODY
></TABLE
></DIV
><P
> <CODE
CLASS="FUNCTION"
>pg_relation_filenode</CODE
> accepts the OID or name of a table,
index, sequence, or toast table, and returns the <SPAN
CLASS="QUOTE"
>"filenode"</SPAN
> number
currently assigned to it. The filenode is the base component of the file
name(s) used for the relation (see <A
HREF="storage-file-layout.html"
>Section 56.1</A
>
for more information). For most tables the result is the same as
<TT
CLASS="STRUCTNAME"
>pg_class</TT
>.<TT
CLASS="STRUCTFIELD"
>relfilenode</TT
>, but for certain
system catalogs <TT
CLASS="STRUCTFIELD"
>relfilenode</TT
> is zero and this function must
be used to get the correct value. The function returns NULL if passed
a relation that does not have storage, such as a view.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_relation_filepath</CODE
> is similar to
<CODE
CLASS="FUNCTION"
>pg_relation_filenode</CODE
>, but it returns the entire file path name
(relative to the database cluster's data directory <TT
CLASS="VARNAME"
>PGDATA</TT
>) of
the relation.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-ADMIN-GENFILE"
>9.26.7. Generic File Access Functions</A
></H2
><P
> The functions shown in <A
HREF="functions-admin.html#FUNCTIONS-ADMIN-GENFILE-TABLE"
>Table 9-66</A
> provide native access to
files on the machine hosting the server. Only files within the
database cluster directory and the <TT
CLASS="VARNAME"
>log_directory</TT
> can be
accessed. Use a relative path for files in the cluster directory,
and a path matching the <TT
CLASS="VARNAME"
>log_directory</TT
> configuration setting
for log files. Use of these functions is restricted to superusers.
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-ADMIN-GENFILE-TABLE"
></A
><P
><B
>Table 9-66. Generic File Access Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_ls_dir(<TT
CLASS="PARAMETER"
>dirname</TT
> <TT
CLASS="TYPE"
>text</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>setof text</TT
></TD
><TD
>List the contents of a directory</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_read_file(<TT
CLASS="PARAMETER"
>filename</TT
> <TT
CLASS="TYPE"
>text</TT
> [, <TT
CLASS="PARAMETER"
>offset</TT
> <TT
CLASS="TYPE"
>bigint</TT
>, <TT
CLASS="PARAMETER"
>length</TT
> <TT
CLASS="TYPE"
>bigint</TT
>])</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
>Return the contents of a text file</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_read_binary_file(<TT
CLASS="PARAMETER"
>filename</TT
> <TT
CLASS="TYPE"
>text</TT
> [, <TT
CLASS="PARAMETER"
>offset</TT
> <TT
CLASS="TYPE"
>bigint</TT
>, <TT
CLASS="PARAMETER"
>length</TT
> <TT
CLASS="TYPE"
>bigint</TT
>])</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>bytea</TT
></TD
><TD
>Return the contents of a file</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_stat_file(<TT
CLASS="PARAMETER"
>filename</TT
> <TT
CLASS="TYPE"
>text</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>record</TT
></TD
><TD
>Return information about a file</TD
></TR
></TBODY
></TABLE
></DIV
><P
> <CODE
CLASS="FUNCTION"
>pg_ls_dir</CODE
> returns all the names in the specified
directory, except the special entries <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>.</TT
>"</SPAN
> and
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>..</TT
>"</SPAN
>.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_read_file</CODE
> returns part of a text file, starting
at the given <TT
CLASS="PARAMETER"
>offset</TT
>, returning at most <TT
CLASS="PARAMETER"
>length</TT
>
bytes (less if the end of file is reached first). If <TT
CLASS="PARAMETER"
>offset</TT
>
is negative, it is relative to the end of the file.
If <TT
CLASS="PARAMETER"
>offset</TT
> and <TT
CLASS="PARAMETER"
>length</TT
> are omitted, the entire
file is returned. The bytes read from the file are interpreted as a string
in the server encoding; an error is thrown if they are not valid in that
encoding.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_read_binary_file</CODE
> is similar to
<CODE
CLASS="FUNCTION"
>pg_read_file</CODE
>, except that the result is a <TT
CLASS="TYPE"
>bytea</TT
> value;
accordingly, no encoding checks are performed.
In combination with the <CODE
CLASS="FUNCTION"
>convert_from</CODE
> function, this function
can be used to read a file in a specified encoding:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT convert_from(pg_read_binary_file('file_in_utf8.txt'), 'UTF8');</PRE
><P>
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_stat_file</CODE
> returns a record containing the file
size, last accessed time stamp, last modified time stamp,
last file status change time stamp (Unix platforms only),
file creation time stamp (Windows only), and a <TT
CLASS="TYPE"
>boolean</TT
>
indicating if it is a directory. Typical usages include:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM pg_stat_file('filename');
SELECT (pg_stat_file('filename')).modification;</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-ADVISORY-LOCKS"
>9.26.8. Advisory Lock Functions</A
></H2
><P
> The functions shown in <A
HREF="functions-admin.html#FUNCTIONS-ADVISORY-LOCKS-TABLE"
>Table 9-67</A
>
manage advisory locks. For details about proper use of these functions,
see <A
HREF="explicit-locking.html#ADVISORY-LOCKS"
>Section 13.3.4</A
>.
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-ADVISORY-LOCKS-TABLE"
></A
><P
><B
>Table 9-67. Advisory Lock Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_lock(<TT
CLASS="PARAMETER"
>key</TT
> <TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>void</TT
></TD
><TD
>Obtain exclusive session level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_lock(<TT
CLASS="PARAMETER"
>key1</TT
> <TT
CLASS="TYPE"
>int</TT
>, <TT
CLASS="PARAMETER"
>key2</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>void</TT
></TD
><TD
>Obtain exclusive session level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_lock_shared(<TT
CLASS="PARAMETER"
>key</TT
> <TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>void</TT
></TD
><TD
>Obtain shared session level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_lock_shared(<TT
CLASS="PARAMETER"
>key1</TT
> <TT
CLASS="TYPE"
>int</TT
>, <TT
CLASS="PARAMETER"
>key2</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>void</TT
></TD
><TD
>Obtain shared session level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_unlock(<TT
CLASS="PARAMETER"
>key</TT
> <TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Release an exclusive session level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_unlock(<TT
CLASS="PARAMETER"
>key1</TT
> <TT
CLASS="TYPE"
>int</TT
>, <TT
CLASS="PARAMETER"
>key2</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Release an exclusive session level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_unlock_all()</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>void</TT
></TD
><TD
>Release all session level advisory locks held by the current session</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_unlock_shared(<TT
CLASS="PARAMETER"
>key</TT
> <TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Release a shared session level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_unlock_shared(<TT
CLASS="PARAMETER"
>key1</TT
> <TT
CLASS="TYPE"
>int</TT
>, <TT
CLASS="PARAMETER"
>key2</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Release a shared session level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_xact_lock(<TT
CLASS="PARAMETER"
>key</TT
> <TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>void</TT
></TD
><TD
>Obtain exclusive transaction level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_xact_lock(<TT
CLASS="PARAMETER"
>key1</TT
> <TT
CLASS="TYPE"
>int</TT
>, <TT
CLASS="PARAMETER"
>key2</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>void</TT
></TD
><TD
>Obtain exclusive transaction level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_xact_lock_shared(<TT
CLASS="PARAMETER"
>key</TT
> <TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>void</TT
></TD
><TD
>Obtain shared transaction level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_advisory_xact_lock_shared(<TT
CLASS="PARAMETER"
>key1</TT
> <TT
CLASS="TYPE"
>int</TT
>, <TT
CLASS="PARAMETER"
>key2</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>void</TT
></TD
><TD
>Obtain shared transaction level advisory lock</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_try_advisory_lock(<TT
CLASS="PARAMETER"
>key</TT
> <TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Obtain exclusive session level advisory lock if available</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_try_advisory_lock(<TT
CLASS="PARAMETER"
>key1</TT
> <TT
CLASS="TYPE"
>int</TT
>, <TT
CLASS="PARAMETER"
>key2</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Obtain exclusive session level advisory lock if available</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_try_advisory_lock_shared(<TT
CLASS="PARAMETER"
>key</TT
> <TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Obtain shared session level advisory lock if available</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_try_advisory_lock_shared(<TT
CLASS="PARAMETER"
>key1</TT
> <TT
CLASS="TYPE"
>int</TT
>, <TT
CLASS="PARAMETER"
>key2</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Obtain shared session level advisory lock if available</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_try_advisory_xact_lock(<TT
CLASS="PARAMETER"
>key</TT
> <TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Obtain exclusive transaction level advisory lock if available</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_try_advisory_xact_lock(<TT
CLASS="PARAMETER"
>key1</TT
> <TT
CLASS="TYPE"
>int</TT
>, <TT
CLASS="PARAMETER"
>key2</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Obtain exclusive transaction level advisory lock if available</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_try_advisory_xact_lock_shared(<TT
CLASS="PARAMETER"
>key</TT
> <TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Obtain shared transaction level advisory lock if available</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>pg_try_advisory_xact_lock_shared(<TT
CLASS="PARAMETER"
>key1</TT
> <TT
CLASS="TYPE"
>int</TT
>, <TT
CLASS="PARAMETER"
>key2</TT
> <TT
CLASS="TYPE"
>int</TT
>)</CODE
></TT
>
</TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
>Obtain shared transaction level advisory lock if available</TD
></TR
></TBODY
></TABLE
></DIV
><P
> <CODE
CLASS="FUNCTION"
>pg_advisory_lock</CODE
> locks an application-defined resource,
which can be identified either by a single 64-bit key value or two
32-bit key values (note that these two key spaces do not overlap).
If another session already holds a lock on the same resource identifier,
this function will wait until the resource becomes available. The lock
is exclusive. Multiple lock requests stack, so that if the same resource
is locked three times it must then be unlocked three times to be
released for other sessions' use.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_advisory_lock_shared</CODE
> works the same as
<CODE
CLASS="FUNCTION"
>pg_advisory_lock</CODE
>,
except the lock can be shared with other sessions requesting shared locks.
Only would-be exclusive lockers are locked out.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_try_advisory_lock</CODE
> is similar to
<CODE
CLASS="FUNCTION"
>pg_advisory_lock</CODE
>, except the function will not wait for the
lock to become available. It will either obtain the lock immediately and
return <TT
CLASS="LITERAL"
>true</TT
>, or return <TT
CLASS="LITERAL"
>false</TT
> if the lock cannot be
acquired immediately.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_try_advisory_lock_shared</CODE
> works the same as
<CODE
CLASS="FUNCTION"
>pg_try_advisory_lock</CODE
>, except it attempts to acquire
a shared rather than an exclusive lock.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_advisory_unlock</CODE
> will release a previously-acquired
exclusive session level advisory lock. It
returns <TT
CLASS="LITERAL"
>true</TT
> if the lock is successfully released.
If the lock was not held, it will return <TT
CLASS="LITERAL"
>false</TT
>,
and in addition, an SQL warning will be reported by the server.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_advisory_unlock_shared</CODE
> works the same as
<CODE
CLASS="FUNCTION"
>pg_advisory_unlock</CODE
>,
except it releases a shared session level advisory lock.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_advisory_unlock_all</CODE
> will release all session level advisory
locks held by the current session. (This function is implicitly invoked
at session end, even if the client disconnects ungracefully.)
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_advisory_xact_lock</CODE
> works the same as
<CODE
CLASS="FUNCTION"
>pg_advisory_lock</CODE
>, except the lock is automatically released
at the end of the current transaction and cannot be released explicitly.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_advisory_xact_lock_shared</CODE
> works the same as
<CODE
CLASS="FUNCTION"
>pg_advisory_lock_shared</CODE
>, except the lock is automatically released
at the end of the current transaction and cannot be released explicitly.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_try_advisory_xact_lock</CODE
> works the same as
<CODE
CLASS="FUNCTION"
>pg_try_advisory_lock</CODE
>, except the lock, if acquired,
is automatically released at the end of the current transaction and
cannot be released explicitly.
</P
><P
> <CODE
CLASS="FUNCTION"
>pg_try_advisory_xact_lock_shared</CODE
> works the same as
<CODE
CLASS="FUNCTION"
>pg_try_advisory_lock_shared</CODE
>, except the lock, if acquired,
is automatically released at the end of the current transaction and
cannot be released explicitly.
</P
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