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9.26. System Administration Functions

The functions described in this section are used to control and monitor a PostgreSQL installation.

9.26.1. Configuration Settings Functions

Table 9-76 shows the functions available to query and alter run-time configuration parameters.

Table 9-76. Configuration Settings Functions

Name Return Type Description
current_setting(setting_name [, missing_ok ]) text get current value of setting
set_config(setting_name, new_value, is_local) text set parameter and return new value

The function current_setting yields the current value of the setting setting_name. It corresponds to the SQL command SHOW. An example:

SELECT current_setting('datestyle');

 current_setting
-----------------
 ISO, MDY
(1 row)

If there is no setting named setting_name, current_setting throws an error unless missing_ok is supplied and is true.

set_config sets the parameter setting_name to new_value. If is_local is true, the new value will only apply to the current transaction. If you want the new value to apply for the current session, use false instead. The function corresponds to the SQL command SET. An example:

SELECT set_config('log_statement_stats', 'off', false);

 set_config
------------
 off
(1 row)

9.26.2. Server Signaling Functions

The functions shown in Table 9-77 send control signals to other server processes. Use of these functions is restricted to superusers by default but access may be granted to others with the GRANT, with noted exceptions.

Table 9-77. Server Signaling Functions

Name Return Type Description
pg_cancel_backend(pid int) boolean Cancel a backend's current query. This is also allowed if the calling role is a member of the role whose backend is being canceled or the calling role has been granted pg_signal_backend, however only superusers can cancel superuser backends.
pg_reload_conf() boolean Cause server processes to reload their configuration files
pg_rotate_logfile() boolean Rotate server's log file
pg_terminate_backend(pid int) boolean Terminate a backend. This is also allowed if the calling role is a member of the role whose backend is being terminated or the calling role has been granted pg_signal_backend, however only superusers can terminate superuser backends.

Each of these functions returns true if successful and false otherwise.

pg_cancel_backend and pg_terminate_backend send signals (SIGINT or SIGTERM respectively) to backend processes identified by process ID. The process ID of an active backend can be found from the pid column of the pg_stat_activity view, or by listing the postgres processes on the server (using ps on Unix or the Task Manager on Windows). The role of an active backend can be found from the usename column of the pg_stat_activity view.

pg_reload_conf sends a SIGHUP signal to the server, causing configuration files to be reloaded by all server processes.

pg_rotate_logfile 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.

9.26.3. Backup Control Functions

The functions shown in Table 9-78 assist in making on-line backups. These functions cannot be executed during recovery (except pg_is_in_backup, pg_backup_start_time and pg_xlog_location_diff).

Table 9-78. Backup Control Functions

Name Return Type Description
pg_create_restore_point(name text) pg_lsn Create a named point for performing restore (restricted to superusers by default, but other users can be granted EXECUTE to run the function)
pg_current_xlog_flush_location() pg_lsn Get current transaction log flush location
pg_current_xlog_insert_location() pg_lsn Get current transaction log insert location
pg_current_xlog_location() pg_lsn Get current transaction log write location
pg_start_backup(label text [, fast boolean [, exclusive boolean ]]) pg_lsn Prepare for performing on-line backup (restricted to superusers by default, but other users can be granted EXECUTE to run the function)
pg_stop_backup() pg_lsn Finish performing exclusive on-line backup (restricted to superusers by default, but other users can be granted EXECUTE to run the function)
pg_stop_backup(exclusive boolean) setof record Finish performing exclusive or non-exclusive on-line backup (restricted to superusers by default, but other users can be granted EXECUTE to run the function)
pg_is_in_backup() bool True if an on-line exclusive backup is still in progress.
pg_backup_start_time() timestamp with time zone Get start time of an on-line exclusive backup in progress.
pg_switch_xlog() pg_lsn Force switch to a new transaction log file (restricted to superusers by default, but other users can be granted EXECUTE to run the function)
pg_xlogfile_name(location pg_lsn) text Convert transaction log location string to file name
pg_xlogfile_name_offset(location pg_lsn) text, integer Convert transaction log location string to file name and decimal byte offset within file
pg_xlog_location_diff(location pg_lsn, location pg_lsn) numeric Calculate the difference between two transaction log locations

pg_start_backup accepts an arbitrary user-defined label for the backup. (Typically this would be the name under which the backup dump file will be stored.) When used in exclusive mode, the function writes a backup label file (backup_label) and, if there are any links in the pg_tblspc/ directory, a tablespace map file (tablespace_map) 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. When used in non-exclusive mode, the contents of these files are instead returned by the pg_stop_backup function, and should be written to the backup by the caller.

postgres=# select pg_start_backup('label_goes_here');
 pg_start_backup
-----------------
 0/D4445B8
(1 row)

There is an optional second parameter of type boolean. If true, it specifies executing pg_start_backup as quickly as possible. This forces an immediate checkpoint which will cause a spike in I/O operations, slowing any concurrently executing queries.

In an exclusive backup, pg_stop_backup removes the label file and, if it exists, the tablespace_map file created by pg_start_backup. In a non-exclusive backup, the contents of the backup_label and tablespace_map are returned in the result of the function, and should be written to files in the backup (and not in the data directory).

The function also creates a backup history file in the transaction log archive area. The history file includes the label given to pg_start_backup, 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.

pg_switch_xlog 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, pg_switch_xlog does nothing and returns the start location of the transaction log file currently in use.

pg_create_restore_point 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 recovery_target_name 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.

pg_current_xlog_location displays the current transaction log write location in the same format used by the above functions. Similarly, pg_current_xlog_insert_location displays the current transaction log insertion point and pg_current_xlog_flush_location displays the current transaction log flush point. The insertion point is the "logical" 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 and flush location is the location guaranteed to be written to durable storage. 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 and flush points are made available primarily for server debugging purposes. These are both read-only operations and do not require superuser permissions.

You can use pg_xlogfile_name_offset to extract the corresponding transaction log file name and byte offset from the results of any of the above functions. For example:

postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
        file_name         | file_offset 
--------------------------+-------------
 00000001000000000000000D |     4039624
(1 row)

Similarly, pg_xlogfile_name 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.

pg_xlog_location_diff calculates the difference in bytes between two transaction log locations. It can be used with pg_stat_replication or some functions shown in Table 9-78 to get the replication lag.

For details about proper usage of these functions, see Section 24.3.

9.26.4. Recovery Control Functions

The functions shown in Table 9-79 provide information about the current status of the standby. These functions may be executed both during recovery and in normal running.

Table 9-79. Recovery Information Functions

Name Return Type Description
pg_is_in_recovery() bool True if recovery is still in progress.
pg_last_xlog_receive_location() pg_lsn 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.
pg_last_xlog_replay_location() pg_lsn 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.
pg_last_xact_replay_timestamp() timestamp with time zone 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.

The functions shown in Table 9-80 control the progress of recovery. These functions may be executed only during recovery.

Table 9-80. Recovery Control Functions

Name Return Type Description
pg_is_xlog_replay_paused() bool True if recovery is paused.
pg_xlog_replay_pause() void Pauses recovery immediately (restricted to superusers by default, but other users can be granted EXECUTE to run the function).
pg_xlog_replay_resume() void Restarts recovery if it was paused (restricted to superusers by default, but other users can be granted EXECUTE to run the function).

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.

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.

9.26.5. Snapshot Synchronization Functions

PostgreSQL allows database sessions to synchronize their snapshots. A snapshot 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 START TRANSACTION commands, so that one session sees the effects of that transaction and the other does not.

To solve this problem, PostgreSQL allows a transaction to export the snapshot it is using. As long as the exporting transaction remains open, other transactions can import 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.

Snapshots are exported with the pg_export_snapshot function, shown in Table 9-81, and imported with the SET TRANSACTION command.

Table 9-81. Snapshot Synchronization Functions

Name Return Type Description
pg_export_snapshot() text Save the current snapshot and return its identifier

The function pg_export_snapshot saves the current snapshot and returns a text 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 READ COMMITTED transactions, since in REPEATABLE READ and higher isolation levels, transactions use the same snapshot throughout their lifetime. Once a transaction has exported any snapshots, it cannot be prepared with PREPARE TRANSACTION.

See SET TRANSACTION for details of how to use an exported snapshot.

9.26.6. Replication Functions

The functions shown in Table 9-82 are for controlling and interacting with replication features. See Section 25.2.5, Section 25.2.6, and Chapter 47 for information about the underlying features. Use of these functions is restricted to superusers.

Many of these functions have equivalent commands in the replication protocol; see Section 50.3.

The functions described in Section 9.26.3, Section 9.26.4, and Section 9.26.5 are also relevant for replication.

Table 9-82. Replication SQL Functions

Function Return Type Description
pg_create_physical_replication_slot(slot_name name [, immediately_reserve boolean ]) (slot_name name, xlog_position pg_lsn) Creates a new physical replication slot named slot_name. The optional second parameter, when true, specifies that the LSN for this replication slot be reserved immediately; otherwise the LSN is reserved on first connection from a streaming replication client. Streaming changes from a physical slot is only possible with the streaming-replication protocol — see Section 50.3. This function corresponds to the replication protocol command CREATE_REPLICATION_SLOT ... PHYSICAL.
pg_drop_replication_slot(slot_name name) void Drops the physical or logical replication slot named slot_name. Same as replication protocol command DROP_REPLICATION_SLOT.
pg_create_logical_replication_slot(slot_name name, plugin name) (slot_name name, xlog_position pg_lsn) Creates a new logical (decoding) replication slot named slot_name using the output plugin plugin. A call to this function has the same effect as the replication protocol command CREATE_REPLICATION_SLOT ... LOGICAL.
pg_logical_slot_get_changes(slot_name name, upto_lsn pg_lsn, upto_nchanges int, VARIADIC options text[]) (location pg_lsn, xid xid, data text) Returns changes in the slot slot_name, starting from the point at which since changes have been consumed last. If upto_lsn and upto_nchanges are NULL, logical decoding will continue until end of WAL. If upto_lsn is non-NULL, decoding will include only those transactions which commit prior to the specified LSN. If upto_nchanges is non-NULL, decoding will stop when the number of rows produced by decoding exceeds the specified value. Note, however, that the actual number of rows returned may be larger, since this limit is only checked after adding the rows produced when decoding each new transaction commit.
pg_logical_slot_peek_changes(slot_name name, upto_lsn pg_lsn, upto_nchanges int, VARIADIC options text[]) (location text, xid xid, data text) Behaves just like the pg_logical_slot_get_changes() function, except that changes are not consumed; that is, they will be returned again on future calls.
pg_logical_slot_get_binary_changes(slot_name name, upto_lsn pg_lsn, upto_nchanges int, VARIADIC options text[]) (location pg_lsn, xid xid, data bytea) Behaves just like the pg_logical_slot_get_changes() function, except that changes are returned as bytea.
pg_logical_slot_peek_binary_changes(slot_name name, upto_lsn pg_lsn, upto_nchanges int, VARIADIC options text[]) (location pg_lsn, xid xid, data bytea) Behaves just like the pg_logical_slot_get_changes() function, except that changes are returned as bytea and that changes are not consumed; that is, they will be returned again on future calls.
pg_replication_origin_create(node_name text) oid Create a replication origin with the given external name, and return the internal id assigned to it.
pg_replication_origin_drop(node_name text) void Delete a previously created replication origin, including any associated replay progress.
pg_replication_origin_oid(node_name text) oid Lookup a replication origin by name and return the internal id. If no corresponding replication origin is found an error is thrown.
pg_replication_origin_session_setup(node_name text) void Mark the current session as replaying from the given origin, allowing replay progress to be tracked. Use pg_replication_origin_session_reset to revert. Can only be used if no previous origin is configured.
pg_replication_origin_session_reset() void Cancel the effects of pg_replication_origin_session_setup().
pg_replication_origin_session_is_setup() bool Has a replication origin been configured in the current session?
pg_replication_origin_session_progress(flush bool) pg_lsn Return the replay position for the replication origin configured in the current session. The parameter flush determines whether the corresponding local transaction will be guaranteed to have been flushed to disk or not.
pg_replication_origin_xact_setup(origin_lsn pg_lsn, origin_timestamp timestamptz) void Mark the current transaction as replaying a transaction that has committed at the given LSN and timestamp. Can only be called when a replication origin has previously been configured using pg_replication_origin_session_setup().
pg_replication_origin_xact_reset() void Cancel the effects of pg_replication_origin_xact_setup().
pg_replication_origin_advance(node_name text, pos pg_lsn) void Set replication progress for the given node to the given position. This primarily is useful for setting up the initial position or a new position after configuration changes and similar. Be aware that careless use of this function can lead to inconsistently replicated data.
pg_replication_origin_progress(node_name text, flush bool) pg_lsn Return the replay position for the given replication origin. The parameter flush determines whether the corresponding local transaction will be guaranteed to have been flushed to disk or not.
pg_logical_emit_message(transactional bool, prefix text, content text) pg_lsn Emit text logical decoding message. This can be used to pass generic messages to logical decoding plugins through WAL. The parameter transactional specifies if the message should be part of current transaction or if it should be written immediately and decoded as soon as the logical decoding reads the record. The prefix is textual prefix used by the logical decoding plugins to easily recognize interesting messages for them. The content is the text of the message.
pg_logical_emit_message(transactional bool, prefix text, content bytea) pg_lsn Emit binary logical decoding message. This can be used to pass generic messages to logical decoding plugins through WAL. The parameter transactional specifies if the message should be part of current transaction or if it should be written immediately and decoded as soon as the logical decoding reads the record. The prefix is textual prefix used by the logical decoding plugins to easily recognize interesting messages for them. The content is the binary content of the message.

9.26.7. Database Object Management Functions

The functions shown in Table 9-83 calculate the disk space usage of database objects.

Table 9-83. Database Object Size Functions

Name Return Type Description
pg_column_size(any) int Number of bytes used to store a particular value (possibly compressed)
pg_database_size(oid) bigint Disk space used by the database with the specified OID
pg_database_size(name) bigint Disk space used by the database with the specified name
pg_indexes_size(regclass) bigint Total disk space used by indexes attached to the specified table
pg_relation_size(relation regclass, fork text) bigint Disk space used by the specified fork ('main', 'fsm', 'vm', or 'init') of the specified table or index
pg_relation_size(relation regclass) bigint Shorthand for pg_relation_size(..., 'main')
pg_size_bytes(text) bigint Converts a size in human-readable format with size units into bytes
pg_size_pretty(bigint) text Converts a size in bytes expressed as a 64-bit integer into a human-readable format with size units
pg_size_pretty(numeric) text Converts a size in bytes expressed as a numeric value into a human-readable format with size units
pg_table_size(regclass) bigint Disk space used by the specified table, excluding indexes (but including TOAST, free space map, and visibility map)
pg_tablespace_size(oid) bigint Disk space used by the tablespace with the specified OID
pg_tablespace_size(name) bigint Disk space used by the tablespace with the specified name
pg_total_relation_size(regclass) bigint Total disk space used by the specified table, including all indexes and TOAST data

pg_column_size shows the space used to store any individual data value.

pg_total_relation_size 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 pg_table_size + pg_indexes_size.

pg_table_size 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.)

pg_indexes_size accepts the OID or name of a table and returns the total disk space used by all the indexes attached to that table.

pg_database_size and pg_tablespace_size accept the OID or name of a database or tablespace, and return the total disk space used therein. To use pg_database_size, you must have CONNECT permission on the specified database (which is granted by default). To use pg_tablespace_size, you must have CREATE permission on the specified tablespace, unless it is the default tablespace for the current database.

pg_relation_size 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 pg_total_relation_size or pg_table_size, 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:

  • 'main' returns the size of the main data fork of the relation.

  • 'fsm' returns the size of the Free Space Map (see Section 64.3) associated with the relation.

  • 'vm' returns the size of the Visibility Map (see Section 64.4) associated with the relation.

  • 'init' returns the size of the initialization fork, if any, associated with the relation.

pg_size_pretty can be used to format the result of one of the other functions in a human-readable way, using bytes, kB, MB, GB or TB as appropriate.

pg_size_bytes can be used to get the size in bytes from a string in human-readable format. The input may have units of bytes, kB, MB, GB or TB, and is parsed case-insensitively. If no units are specified, bytes are assumed.

Note: The units kB, MB, GB and TB used by the functions pg_size_pretty and pg_size_bytes are defined using powers of 2 rather than powers of 10, so 1kB is 1024 bytes, 1MB is 10242 = 1048576 bytes, and so on.

The functions above that operate on tables or indexes accept a regclass argument, which is simply the OID of the table or index in the pg_class system catalog. You do not have to look up the OID by hand, however, since the regclass 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 SQL names, the string will be converted to lower case unless it contains double quotes around the table name.

If an OID that does not represent an existing object is passed as argument to one of the above functions, NULL is returned.

The functions shown in Table 9-84 assist in identifying the specific disk files associated with database objects.

Table 9-84. Database Object Location Functions

Name Return Type Description
pg_relation_filenode(relation regclass) oid Filenode number of the specified relation
pg_relation_filepath(relation regclass) text File path name of the specified relation
pg_filenode_relation(tablespace oid, filenode oid) regclass Find the relation associated with a given tablespace and filenode

pg_relation_filenode accepts the OID or name of a table, index, sequence, or toast table, and returns the "filenode" number currently assigned to it. The filenode is the base component of the file name(s) used for the relation (see Section 64.1 for more information). For most tables the result is the same as pg_class.relfilenode, but for certain system catalogs relfilenode 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.

pg_relation_filepath is similar to pg_relation_filenode, but it returns the entire file path name (relative to the database cluster's data directory PGDATA) of the relation.

pg_filenode_relation is the reverse of pg_relation_filenode. Given a "tablespace" OID and a "filenode", it returns the associated relation's OID. For a table in the database's default tablespace, the tablespace can be specified as 0.

9.26.8. Index Maintenance Functions

Table 9-85 shows the functions available for index maintenance tasks. These functions cannot be executed during recovery. Use of these functions is restricted to superusers and the owner of the given index.

Table 9-85. Index Maintenance Functions

Name Return Type Description
brin_summarize_new_values(index regclass) integer summarize page ranges not already summarized
gin_clean_pending_list(index regclass) bigint move GIN pending list entries into main index structure

brin_summarize_new_values accepts the OID or name of a BRIN index and inspects the index to find page ranges in the base table that are not currently summarized by the index; for any such range it creates a new summary index tuple by scanning the table pages. It returns the number of new page range summaries that were inserted into the index.

gin_clean_pending_list accepts the OID or name of a GIN index and cleans up the pending list of the specified index by moving entries in it to the main GIN data structure in bulk. It returns the number of pages removed from the pending list. Note that if the argument is a GIN index built with the fastupdate option disabled, no cleanup happens and the return value is 0, because the index doesn't have a pending list. Please see Section 62.4.1 and Section 62.5 for details of the pending list and fastupdate option.

9.26.9. Generic File Access Functions

The functions shown in Table 9-86 provide native access to files on the machine hosting the server. Only files within the database cluster directory and the log_directory can be accessed. Use a relative path for files in the cluster directory, and a path matching the log_directory configuration setting for log files. Use of these functions is restricted to superusers.

Table 9-86. Generic File Access Functions

Name Return Type Description
pg_ls_dir(dirname text [, missing_ok boolean, include_dot_dirs boolean]) setof text List the contents of a directory.
pg_read_file(filename text [, offset bigint, length bigint [, missing_ok boolean] ]) text Return the contents of a text file.
pg_read_binary_file(filename text [, offset bigint, length bigint [, missing_ok boolean] ]) bytea Return the contents of a file.
pg_stat_file(filename text[, missing_ok boolean]) record Return information about a file.

All of these functions take an optional missing_ok parameter, which specifies the behavior when the file or directory does not exist. If true, the function returns NULL (except pg_ls_dir, which returns an empty result set). If false, an error is raised. The default is false.

pg_ls_dir returns the names of all files (and directories and other special files) in the specified directory. The include_dot_dirs indicates whether "." and ".." are included in the result set. The default is to exclude them (false), but including them can be useful when missing_ok is true, to distinguish an empty directory from an non-existent directory.

pg_read_file returns part of a text file, starting at the given offset, returning at most length bytes (less if the end of file is reached first). If offset is negative, it is relative to the end of the file. If offset and length 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.

pg_read_binary_file is similar to pg_read_file, except that the result is a bytea value; accordingly, no encoding checks are performed. In combination with the convert_from function, this function can be used to read a file in a specified encoding:

SELECT convert_from(pg_read_binary_file('file_in_utf8.txt'), 'UTF8');

pg_stat_file 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 boolean indicating if it is a directory. Typical usages include:

SELECT * FROM pg_stat_file('filename');
SELECT (pg_stat_file('filename')).modification;

9.26.10. Advisory Lock Functions

The functions shown in Table 9-87 manage advisory locks. For details about proper use of these functions, see Section 13.3.5.

Table 9-87. Advisory Lock Functions

Name Return Type Description
pg_advisory_lock(key bigint) void Obtain exclusive session level advisory lock
pg_advisory_lock(key1 int, key2 int) void Obtain exclusive session level advisory lock
pg_advisory_lock_shared(key bigint) void Obtain shared session level advisory lock
pg_advisory_lock_shared(key1 int, key2 int) void Obtain shared session level advisory lock
pg_advisory_unlock(key bigint) boolean Release an exclusive session level advisory lock
pg_advisory_unlock(key1 int, key2 int) boolean Release an exclusive session level advisory lock
pg_advisory_unlock_all() void Release all session level advisory locks held by the current session
pg_advisory_unlock_shared(key bigint) boolean Release a shared session level advisory lock
pg_advisory_unlock_shared(key1 int, key2 int) boolean Release a shared session level advisory lock
pg_advisory_xact_lock(key bigint) void Obtain exclusive transaction level advisory lock
pg_advisory_xact_lock(key1 int, key2 int) void Obtain exclusive transaction level advisory lock
pg_advisory_xact_lock_shared(key bigint) void Obtain shared transaction level advisory lock
pg_advisory_xact_lock_shared(key1 int, key2 int) void Obtain shared transaction level advisory lock
pg_try_advisory_lock(key bigint) boolean Obtain exclusive session level advisory lock if available
pg_try_advisory_lock(key1 int, key2 int) boolean Obtain exclusive session level advisory lock if available
pg_try_advisory_lock_shared(key bigint) boolean Obtain shared session level advisory lock if available
pg_try_advisory_lock_shared(key1 int, key2 int) boolean Obtain shared session level advisory lock if available
pg_try_advisory_xact_lock(key bigint) boolean Obtain exclusive transaction level advisory lock if available
pg_try_advisory_xact_lock(key1 int, key2 int) boolean Obtain exclusive transaction level advisory lock if available
pg_try_advisory_xact_lock_shared(key bigint) boolean Obtain shared transaction level advisory lock if available
pg_try_advisory_xact_lock_shared(key1 int, key2 int) boolean Obtain shared transaction level advisory lock if available

pg_advisory_lock 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.

pg_advisory_lock_shared works the same as pg_advisory_lock, except the lock can be shared with other sessions requesting shared locks. Only would-be exclusive lockers are locked out.

pg_try_advisory_lock is similar to pg_advisory_lock, except the function will not wait for the lock to become available. It will either obtain the lock immediately and return true, or return false if the lock cannot be acquired immediately.

pg_try_advisory_lock_shared works the same as pg_try_advisory_lock, except it attempts to acquire a shared rather than an exclusive lock.

pg_advisory_unlock will release a previously-acquired exclusive session level advisory lock. It returns true if the lock is successfully released. If the lock was not held, it will return false, and in addition, an SQL warning will be reported by the server.

pg_advisory_unlock_shared works the same as pg_advisory_unlock, except it releases a shared session level advisory lock.

pg_advisory_unlock_all 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.)

pg_advisory_xact_lock works the same as pg_advisory_lock, except the lock is automatically released at the end of the current transaction and cannot be released explicitly.

pg_advisory_xact_lock_shared works the same as pg_advisory_lock_shared, except the lock is automatically released at the end of the current transaction and cannot be released explicitly.

pg_try_advisory_xact_lock works the same as pg_try_advisory_lock, except the lock, if acquired, is automatically released at the end of the current transaction and cannot be released explicitly.

pg_try_advisory_xact_lock_shared works the same as pg_try_advisory_lock_shared, except the lock, if acquired, is automatically released at the end of the current transaction and cannot be released explicitly.

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