9.26. 系統管理函式

本節中描述的函數用於控制和監控 PostgreSQL 環境。

9.26.1. 組態設定函數

Table 9.77 列出了可用於查詢和變更執行時組態參數的函數。

Table 9.77. Configuration Settings Functions

函數名稱	回傳型別	說明
current_setting(setting_name [, missing_ok ])	text	取得目前設定值
set_config(setting_name, new_value, is_local)	text	設定參數並回傳新值

函數 current_setting 會產生設定 setting_name 目前的值。它對應於 SQL 指令 SHOW。範例如下：

SELECT current_setting('datestyle');

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

如果沒有名為 setting_name 的設定，則 current_setting 會拋出錯誤，除非有設定了 missing_ok，並且為 true。

set_config 將參數 setting_name 設定為 new_value。如果 is_local 為true，則新值僅適用於目前的交易事務。如果要將新值套用於目前連線之中，請改用 false。此函數對應於 SQL 命令 SET。範例如下：

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

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

9.26.2. Server Signaling Functions

The functions shown in Table 9.78 send control signals to other server processes. Use of these functions is restricted to superusers by default but access may be granted to others using GRANT, with noted exceptions.

Table 9.78. Server Signaling Functions

Name	Return Type	Description
pg_cancel_backend(pidint)	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(pidint)	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 grantedpg_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 9.79 assist in making on-line backups. These functions cannot be executed during recovery (except pg_is_in_backup, pg_backup_start_time and pg_wal_lsn_diff).

Table 9.79. 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_wal_flush_lsn()	pg_lsn	Get current write-ahead log flush location
pg_current_wal_insert_lsn()	pg_lsn	Get current write-ahead log insert location
pg_current_wal_lsn()	pg_lsn	Get current write-ahead 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 [,wait_for_archive 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_wal()	pg_lsn	Force switch to a new write-ahead log file (restricted to superusers by default, but other users can be granted EXECUTE to run the function)
pg_walfile_name(lsn pg_lsn)	text	Convert write-ahead log location to file name
pg_walfile_name_offset(lsn pg_lsn)	text, integer	Convert write-ahead log location to file name and decimal byte offset within file
pg_wal_lsn_diff(lsn pg_lsn, lsn pg_lsn)	numeric	Calculate the difference between two write-ahead 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 write-ahead 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_labeland 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). There is an optional second parameter of type boolean. If false, the pg_stop_backup will return immediately after the backup is completed without waiting for WAL to be archived. This behavior is only useful for backup software which independently monitors WAL archiving. Otherwise, WAL required to make the backup consistent might be missing and make the backup useless. When this parameter is set to true, pg_stop_backup will wait for WAL to be archived when archiving is enabled; on the standby, this means that it will wait only when archive_mode = always. If write activity on the primary is low, it may be useful to run pg_switch_walon the primary in order to trigger an immediate segment switch.

When executed on a primary, the function also creates a backup history file in the write-ahead log archive area. The history file includes the label given to pg_start_backup, the starting and ending write-ahead log locations for the backup, and the starting and ending times of the backup. The return value is the backup's ending write-ahead log location (which again can be ignored). After recording the ending location, the current write-ahead log insertion point is automatically advanced to the next write-ahead log file, so that the ending write-ahead log file can be archived immediately to complete the backup.

pg_switch_wal moves to the next write-ahead log file, allowing the current file to be archived (assuming you are using continuous archiving). The return value is the ending write-ahead log location + 1 within the just-completed write-ahead log file. If there has been no write-ahead log activity since the last write-ahead log switch, pg_switch_wal does nothing and returns the start location of the write-ahead log file currently in use.

pg_create_restore_point creates a named write-ahead log record that can be used as recovery target, and returns the corresponding write-ahead 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_wal_lsn displays the current write-ahead log write location in the same format used by the above functions. Similarly, pg_current_wal_insert_lsn displays the current write-ahead log insertion location and pg_current_wal_flush_lsn displays the current write-ahead log flush location. The insertion location is the “logical” end of the write-ahead 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 write-ahead log files. The insertion and flush locations are made available primarily for server debugging purposes. These are both read-only operations and do not require superuser permissions.

You can use pg_walfile_name_offset to extract the corresponding write-ahead log file name and byte offset from the results of any of the above functions. For example:

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

Similarly, pg_walfile_name extracts just the write-ahead log file name. When the given write-ahead log location is exactly at a write-ahead log file boundary, both these functions return the name of the preceding write-ahead log file. This is usually the desired behavior for managing write-ahead log archiving behavior, since the preceding file is the last one that currently needs to be archived.

pg_wal_lsn_diff calculates the difference in bytes between two write-ahead log locations. It can be used with pg_stat_replication or some functions shown in Table 9.79 to get the replication lag.

For details about proper usage of these functions, see Section 25.3.

9.26.4. Recovery Control Functions

The functions shown in Table 9.80 provide information about the current status of the standby. These functions may be executed both during recovery and in normal running.

Table 9.80. Recovery Information Functions

Name	Return Type	Description
pg_is_in_recovery()	bool	True if recovery is still in progress.
pg_last_wal_receive_lsn()	pg_lsn	Get last write-ahead 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_wal_replay_lsn()	pg_lsn	Get last write-ahead 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 9.81 control the progress of recovery. These functions may be executed only during recovery.

Table 9.81. Recovery Control Functions

Name	Return Type	Description
pg_is_wal_replay_paused()	bool	True if recovery is paused.
pg_wal_replay_pause()	void	Pauses recovery immediately (restricted to superusers by default, but other users can be granted EXECUTE to run the function).
pg_wal_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 9.82, and imported with the SET TRANSACTION command.

Table 9.82. 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 9.83 are for controlling and interacting with replication features. See Section 26.2.5, Section 26.2.6, and Chapter 50 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 53.4.

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

Table 9.83. Replication SQL Functions

Function	Return Type	Description
pg_create_physical_replication_slot(slot_namename [, immediately_reserve boolean,temporary boolean])	(slot_namename, lsnpg_lsn)	Creates a new physical replication slot named slot_name. The optional second parameter, when true, specifies that the LSNfor 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 53.4. The optional third parameter, temporary, when set to true, specifies that the slot should not be permanently stored to disk and is only meant for use by current session. Temporary slots are also released upon any error. 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. For logical slots, this must be called when connected to the same database the slot was created on.
pg_create_logical_replication_slot(slot_namename, plugin name [, temporary boolean])	(slot_namename, lsnpg_lsn)	Creates a new logical (decoding) replication slot named slot_name using the output plugin plugin. The optional third parameter, temporary, when set to true, specifies that the slot should not be permanently stored to disk and is only meant for use by current session. Temporary slots are also released upon any error. 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, VARIADICoptions text[])	(lsnpg_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_lsnand 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, VARIADICoptions text[])	(lsnpg_lsn, 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_namename, upto_lsn pg_lsn, upto_nchanges int, VARIADIC options text[])	(lsnpg_lsn, xid xid, databytea)	Behaves just like the pg_logical_slot_get_changes() function, except that changes are returned as bytea.
pg_logical_slot_peek_binary_changes(slot_namename, upto_lsn pg_lsn, upto_nchanges int, VARIADIC options text[])	(lsnpg_lsn, xid xid, databytea)	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_slot_advance(slot_name name,upto_lsn pg_lsn)	(slot_namename, end_lsnpg_lsn) bool	Advances the current confirmed position of a replication slot named slot_name. The slot will not be moved backwards, and it will not be moved beyond the current insert location. Returns name of the slot and real position to which it was advanced to.
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_nametext)	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(flushbool)	pg_lsn	Return the replay location 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_lsnpg_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,lsn pg_lsn)	void	Set replication progress for the given node to the given location. This primarily is useful for setting up the initial location or a new location after configuration changes and similar. Be aware that careless use of this function can lead to inconsistently replicated data.
pg_replication_origin_progress(node_nametext, flush bool)	pg_lsn	Return the replay location 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 9.84 calculate the disk space usage of database objects.

Table 9.84. 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 CONNECTpermission on the specified database (which is granted by default), or be a member of the pg_read_all_stats role. To use pg_tablespace_size, you must have CREATE permission on the specified tablespace, or be a member of the pg_read_all_stats role 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 68.3) associated with the relation.
• 'vm' returns the size of the Visibility Map (see Section 68.4) 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 9.85 assist in identifying the specific disk files associated with database objects.

Table 9.85. 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 68.1 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.

Table 9.86 lists functions used to manage collations.

Table 9.86. Collation Management Functions

Name	Return Type	Description
pg_collation_actual_version(oid)	text	Return actual version of collation from operating system
pg_import_system_collations(schema regnamespace)	integer	Import operating system collations

pg_collation_actual_version returns the actual version of the collation object as it is currently installed in the operating system. If this is different from the value in pg_collation.collversion, then objects depending on the collation might need to be rebuilt. See also ALTER COLLATION.

pg_import_system_collations adds collations to the system catalog pg_collation based on all the locales it finds in the operating system. This is what initdb uses; see Section 23.2.2 for more details. If additional locales are installed into the operating system later on, this function can be run again to add collations for the new locales. Locales that match existing entries in pg_collation will be skipped. (But collation objects based on locales that are no longer present in the operating system are not removed by this function.) The schema parameter would typically be pg_catalog, but that is not a requirement; the collations could be installed into some other schema as well. The function returns the number of new collation objects it created.

9.26.8. Index Maintenance Functions

Table 9.87 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 9.87. Index Maintenance Functions

Name	Return Type	Description
brin_summarize_new_values(index regclass)	integer	summarize page ranges not already summarized
brin_summarize_range(index regclass, blockNumber bigint)	integer	summarize the page range covering the given block, if not already summarized
brin_desummarize_range(index regclass, blockNumber bigint)	integer	de-summarize the page range covering the given block, if 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. brin_summarize_range does the same, except it only summarizes the range that covers the given block number.

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 66.4.1 and Section 66.5 for details of the pending list and fastupdate option.

9.26.9. Generic File Access Functions

The functions shown in Table 9.88 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 unless the user is granted the role pg_read_server_files. Use a relative path for files in the cluster directory, and a path matching the log_directory configuration setting for log files.

Note that granting users the EXECUTE privilege on the pg_read_file(), or related, functions allows them the ability to read any file on the server which the database can read and that those reads bypass all in-database privilege checks. This means that, among other things, a user with this access is able to read the contents of the pg_authid table where authentication information is contained, as well as read any file in the database. Therefore, granting access to these functions should be carefully considered.

Table 9.88. 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. Restricted to superusers by default, but other users can be granted EXECUTE to run the function.
pg_ls_logdir()	setof record	List the name, size, and last modification time of files in the log directory. Access is granted to members of the pg_monitor role and may be granted to other non-superuser roles.
pg_ls_waldir()	setof record	List the name, size, and last modification time of files in the WAL directory. Access is granted to members of the pg_monitor role and may be granted to other non-superuser roles.
pg_read_file(filename text [, offset bigint, lengthbigint [, missing_ok boolean] ])	text	Return the contents of a text file. Restricted to superusers by default, but other users can be granted EXECUTE to run the function.
pg_read_binary_file(filename text [, offset bigint,length bigint [, missing_ok boolean] ])	bytea	Return the contents of a file. Restricted to superusers by default, but other users can be granted EXECUTE to run the function.
pg_stat_file(filename text[, missing_ok boolean])	record	Return information about a file. Restricted to superusers by default, but other users can be granted EXECUTE to run the function.

Some 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_ls_logdir returns the name, size, and last modified time (mtime) of each file in the log directory. By default, only superusers and members of the pg_monitor role can use this function. Access may be granted to others using GRANT.

pg_ls_waldir returns the name, size, and last modified time (mtime) of each file in the write ahead log (WAL) directory. By default only superusers and members of the pg_monitor role can use this function. Access may be granted to others using GRANT.

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 9.89 manage advisory locks. For details about proper use of these functions, see Section 13.3.5.

Table 9.89. 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 falseif 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.