PostgreSQL
9.27. System Administration Functions
The functions described in this section are used to control and monitor a PostgreSQL installation.
9.27.1. Configuration Settings Functions
Table 9.83 shows the functions available to query and alter run-time configuration parameters.
Table 9.83. Configuration Settings Functions
| Function Description Example(s) |
|---|
`+current_setting+` ( _`+setting_name+`_ `+text+` [[.optional]#, _`+missing_ok+`_ `+boolean+`# ] ) → `+text+` Returns the current value of the setting `setting_name
|
`+set_config+` ( _`+setting_name+`_ `+text+`, _`+new_value+`_ `+text+`, _`+is_local+`_ `+boolean+` ) → `+text+` Sets the parameter `setting_name
|
+
9.27.2. Server Signaling Functions
The functions shown in Table 9.84 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.
Each of these functions returns true if successful and false otherwise.
Table 9.84. Server Signaling Functions
| Function Description |
|---|
`+pg_cancel_backend+` ( _`+pid+`_ `+integer+` ) → `+boolean+` Cancels the current query of the session whose backend process has the specified process ID. 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_reload_conf+` () → `+boolean+` Causes all processes of the PostgreSQL server to reload their configuration files. (This is initiated by sending a SIGHUP signal to the postmaster process, which in turn sends SIGHUP to each of its children.) |
`+pg_rotate_logfile+` () → `+boolean+` 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. |
`+pg_terminate_backend+` ( _`+pid+`_ `+integer+` ) → `+boolean+` Terminates the session whose backend process has the specified process ID. 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_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.
9.27.3. Backup Control Functions
The functions shown in Table 9.85 assist in making on-line backups. These functions cannot be executed during recovery (except non-exclusive pg_start_backup, non-exclusive pg_stop_backup, pg_is_in_backup, pg_backup_start_time and pg_wal_lsn_diff).
For details about proper usage of these functions, see Section 25.3.
Table 9.85. Backup Control Functions
| Function Description |
|---|
`+pg_create_restore_point+` ( _`+name+`_ `+text+` ) → `+pg_lsn+` Creates a named marker record in the write-ahead log that can later be used as a 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. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
`+pg_current_wal_flush_lsn+` () → `+pg_lsn+` Returns the current write-ahead log flush location (see notes below). |
`+pg_current_wal_insert_lsn+` () → `+pg_lsn+` Returns the current write-ahead log insert location (see notes below). |
`+pg_current_wal_lsn+` () → `+pg_lsn+` Returns the current write-ahead log write location (see notes below). |
`+pg_start_backup+` ( _`+label+`_ `+text+` [[.optional]#, _`+fast+`_ `+boolean+` [[.optional]#, _`+exclusive+`_ `+boolean+`# ]#] ) → `+pg_lsn+` Prepares the server to begin an on-line backup. The only required parameter is an arbitrary user-defined label for the backup. (Typically this would be the name under which the backup dump file will be stored.) If the optional second parameter is given as When used in exclusive mode, this function writes a backup label file ( This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
`+pg_stop_backup+` ( _`+exclusive+`_ `+boolean+` [[.optional]#, _`+wait_for_archive+`_ `+boolean+`# ] ) → `+setof record+` ( _`+lsn+`_ `+pg_lsn+`, _`+labelfile+`_ `+text+`, _`+spcmapfile+`_ `+text+` ) Finishes performing an exclusive or non-exclusive on-line backup. The `exclusive There is an optional second parameter of type When executed on a primary, this function also creates a backup history file in the write-ahead log archive area. The history file includes the label given to The result of the function is a single record. The `lsn This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
Finishes performing an exclusive on-line backup. This simplified version is equivalent to This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
`+pg_is_in_backup+` () → `+boolean+` Returns true if an on-line exclusive backup is in progress. |
`+pg_backup_start_time+` () → `+timestamp with time zone+` Returns the start time of the current on-line exclusive backup if one is in progress, otherwise |
`+pg_switch_wal+` () → `+pg_lsn+` Forces the server to switch to a new write-ahead log file, which allows the current file to be archived (assuming you are using continuous archiving). The result is the ending write-ahead log location plus 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, This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
`+pg_walfile_name+` ( _`+lsn+`_ `+pg_lsn+` ) → `+text+` Converts a write-ahead log location to the name of the WAL file holding that location. |
`+pg_walfile_name_offset+` ( _`+lsn+`_ `+pg_lsn+` ) → `+record+` ( _`+file_name+`_ `+text+`, _`+file_offset+`_ `+integer+` ) Converts a write-ahead log location to a WAL file name and byte offset within that file. |
`+pg_wal_lsn_diff+` ( _`+lsn1+`_ `+pg_lsn+`, _`+lsn2+`_ `+pg_lsn+` ) → `+numeric+` Calculates the difference in bytes (`lsn1 |
+
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 “[.quote]#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 the flush location is the last location known 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 all 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 a pg_lsn value. 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.
9.27.4. Recovery Control Functions
The functions shown in Table 9.86 provide information about the current status of a standby server. These functions may be executed both during recovery and in normal running.
Table 9.86. Recovery Information Functions
| Function Description |
|---|
`+pg_is_in_recovery+` () → `+boolean+` Returns true if recovery is still in progress. |
`+pg_last_wal_receive_lsn+` () → `+pg_lsn+` Returns the last write-ahead log location that has been received and synced to disk by streaming replication. While streaming replication is in progress this will increase monotonically. If recovery has completed then this will remain static at the location 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 |
`+pg_last_wal_replay_lsn+` () → `+pg_lsn+` Returns the last write-ahead log location that has been replayed during recovery. If recovery is still in progress this will increase monotonically. If recovery has completed then this will remain static at the location of the last WAL record applied during recovery. When the server has been started normally without recovery, the function returns |
`+pg_last_xact_replay_timestamp+` () → `+timestamp with time zone+` Returns the time stamp of the 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, the function returns |
+
The functions shown in Table 9.87 control the progress of recovery. These functions may be executed only during recovery.
Table 9.87. Recovery Control Functions
| Function Description |
|---|
`+pg_is_wal_replay_paused+` () → `+boolean+` Returns true if recovery is paused. |
`+pg_promote+` ( _`+wait+`_ `+boolean+` `+DEFAULT+` `+true+`, _`+wait_seconds+`_ `+integer+` `+DEFAULT+` `+60+` ) → `+boolean+` Promotes a standby server to primary status. With `wait This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
`+pg_wal_replay_pause+` () → `+void+` Pauses recovery. While recovery is paused, no further database changes are applied. If hot standby is active, all new queries will see the same consistent snapshot of the database, and no further query conflicts will be generated until recovery is resumed. This function is 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. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
+
pg_wal_replay_pause and pg_wal_replay_resume cannot be executed while a promotion is ongoing. If a promotion is triggered while recovery is paused, the paused state ends and promotion continues.
If streaming replication is disabled, the paused state may continue indefinitely without a problem. If streaming replication is in progress then 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.
9.27.5. 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 9.88, and imported with the SET TRANSACTION command.
Table 9.88. Snapshot Synchronization Functions
| Function Description |
|---|
`+pg_export_snapshot+` () → `+text+` Saves the transaction’s current snapshot and returns a A transaction can export more than one snapshot, if needed. Note that doing so is only useful in |
+
9.27.6. Replication Management Functions
The functions shown in Table 9.89 are for controlling and interacting with replication features. See Section 26.2.5, Section 26.2.6, and Chapter 49 for information about the underlying features. Use of functions for replication origin is restricted to superusers. Use of functions for replication slots is restricted to superusers and users having REPLICATION privilege.
Many of these functions have equivalent commands in the replication protocol; see Section 52.4.
The functions described in Section 9.27.3, Section 9.27.4, and Section 9.27.5 are also relevant for replication.
Table 9.89. Replication Management Functions
| Function Description |
|---|
`+pg_create_physical_replication_slot+` ( _`+slot_name+`_ `+name+` [[.optional]#, _`+immediately_reserve+`_ `+boolean+`, _`+temporary+`_ `+boolean+`# ] ) → `+record+` ( _`+slot_name+`_ `+name+`, _`+lsn+`_ `+pg_lsn+` ) Creates a new physical replication slot named `slot_name |
`+pg_drop_replication_slot+` ( _`+slot_name+`_ `+name+` ) → `+void+` Drops the physical or logical replication slot named `slot_name |
`+pg_create_logical_replication_slot+` ( _`+slot_name+`_ `+name+`, _`+plugin+`_ `+name+` [[.optional]#, _`+temporary+`_ `+boolean+`# ] ) → `+record+` ( _`+slot_name+`_ `+name+`, _`+lsn+`_ `+pg_lsn+` ) Creates a new logical (decoding) replication slot named `slot_name |
`+pg_copy_physical_replication_slot+` ( _`+src_slot_name+`_ `+name+`, _`+dst_slot_name+`_ `+name+` [[.optional]#, _`+temporary+`_ `+boolean+`# ] ) → `+record+` ( _`+slot_name+`_ `+name+`, _`+lsn+`_ `+pg_lsn+` ) Copies an existing physical replication slot named `src_slot_name |
`+pg_copy_logical_replication_slot+` ( _`+src_slot_name+`_ `+name+`, _`+dst_slot_name+`_ `+name+` [[.optional]#, _`+temporary+`_ `+boolean+` [[.optional]#, _`+plugin+`_ `+name+`# ]#] ) → `+record+` ( _`+slot_name+`_ `+name+`, _`+lsn+`_ `+pg_lsn+` ) Copies an existing logical replication slot named `src_slot_name |
`+pg_logical_slot_get_changes+` ( _`+slot_name+`_ `+name+`, _`+upto_lsn+`_ `+pg_lsn+`, _`+upto_nchanges+`_ `+integer+`, `+VARIADIC+` _`+options+`_ `+text[]+` ) → `+setof record+` ( _`+lsn+`_ `+pg_lsn+`, _`+xid+`_ `+xid+`, _`+data+`_ `+text+` ) Returns changes in the slot `slot_name |
`+pg_logical_slot_peek_changes+` ( _`+slot_name+`_ `+name+`, _`+upto_lsn+`_ `+pg_lsn+`, _`+upto_nchanges+`_ `+integer+`, `+VARIADIC+` _`+options+`_ `+text[]+` ) → `+setof record+` ( _`+lsn+`_ `+pg_lsn+`, _`+xid+`_ `+xid+`, _`+data+`_ `+text+` ) Behaves just like the |
`+pg_logical_slot_get_binary_changes+` ( _`+slot_name+`_ `+name+`, _`+upto_lsn+`_ `+pg_lsn+`, _`+upto_nchanges+`_ `+integer+`, `+VARIADIC+` _`+options+`_ `+text[]+` ) → `+setof record+` ( _`+lsn+`_ `+pg_lsn+`, _`+xid+`_ `+xid+`, _`+data+`_ `+bytea+` ) Behaves just like the |
`+pg_logical_slot_peek_binary_changes+` ( _`+slot_name+`_ `+name+`, _`+upto_lsn+`_ `+pg_lsn+`, _`+upto_nchanges+`_ `+integer+`, `+VARIADIC+` _`+options+`_ `+text[]+` ) → `+setof record+` ( _`+lsn+`_ `+pg_lsn+`, _`+xid+`_ `+xid+`, _`+data+`_ `+bytea+` ) Behaves just like the |
`+pg_replication_slot_advance+` ( _`+slot_name+`_ `+name+`, _`+upto_lsn+`_ `+pg_lsn+` ) → `+record+` ( _`+slot_name+`_ `+name+`, _`+end_lsn+`_ `+pg_lsn+` ) 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 the name of the slot and the actual position that it was advanced to. The updated slot position information is written out at the next checkpoint if any advancing is done. So in the event of a crash, the slot may return to an earlier position. |
`+pg_replication_origin_create+` ( _`+node_name+`_ `+text+` ) → `+oid+` Creates a replication origin with the given external name, and returns the internal ID assigned to it. |
`+pg_replication_origin_drop+` ( _`+node_name+`_ `+text+` ) → `+void+` Deletes a previously-created replication origin, including any associated replay progress. |
`+pg_replication_origin_oid+` ( _`+node_name+`_ `+text+` ) → `+oid+` Looks up a replication origin by name and returns the internal ID. If no such replication origin is found, |
`+pg_replication_origin_session_setup+` ( _`+node_name+`_ `+text+` ) → `+void+` Marks the current session as replaying from the given origin, allowing replay progress to be tracked. Can only be used if no origin is currently selected. Use |
`+pg_replication_origin_session_reset+` () → `+void+` Cancels the effects of |
`+pg_replication_origin_session_is_setup+` () → `+boolean+` Returns true if a replication origin has been selected in the current session. |
`+pg_replication_origin_session_progress+` ( _`+flush+`_ `+boolean+` ) → `+pg_lsn+` Returns the replay location for the replication origin selected 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+`_ `+timestamp with time zone+` ) → `+void+` Marks 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 been selected using |
`+pg_replication_origin_xact_reset+` () → `+void+` Cancels the effects of |
`+pg_replication_origin_advance+` ( _`+node_name+`_ `+text+`, _`+lsn+`_ `+pg_lsn+` ) → `+void+` Sets replication progress for the given node to the given location. This is primarily useful for setting up the initial location, or setting 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_name+`_ `+text+`, _`+flush+`_ `+boolean+` ) → `+pg_lsn+` Returns 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+`_ `+boolean+`, _`+prefix+`_ `+text+`, _`+content+`_ `+text+` ) → `+pg_lsn+`
Emits a logical decoding message. This can be used to pass generic messages to logical decoding plugins through WAL. The `transactional |
+
9.27.7. Database Object Management Functions
The functions shown in Table 9.90 calculate the disk space usage of database objects, or assist in presentation of usage results. All these functions return sizes measured in bytes. If an OID that does not represent an existing object is passed to one of these functions, NULL is returned.
Table 9.90. Database Object Size Functions
| Function Description |
|---|
`+pg_column_size+` ( `+"any"+` ) → `+integer+` Shows the number of bytes used to store any individual data value. If applied directly to a table column value, this reflects any compression that was done. |
`+pg_database_size+` ( `+name+` ) → `+bigint+`
Computes the total disk space used by the database with the specified name or OID. To use this function, you must have |
`+pg_indexes_size+` ( `+regclass+` ) → `+bigint+` Computes the total disk space used by indexes attached to the specified table. |
`+pg_relation_size+` ( _`+relation+`_ `+regclass+` [[.optional]#, _`+fork+`_ `+text+`# ] ) → `+bigint+` Computes the disk space used by one “[.quote]#fork”# of the specified relation. (Note that for most purposes it is more convenient to use the higher-level functions
|
`+pg_size_bytes+` ( `+text+` ) → `+bigint+` Converts a size in human-readable format (as returned by |
`+pg_size_pretty+` ( `+bigint+` ) → `+text+`
Converts a size in bytes into a more easily human-readable format with size units (bytes, kB, MB, GB or TB as appropriate). Note that the units are powers of 2 rather than powers of 10, so 1kB is 1024 bytes, 1MB is 10242 = 1048576 bytes, and so on. |
`+pg_table_size+` ( `+regclass+` ) → `+bigint+` Computes the disk space used by the specified table, excluding indexes (but including its TOAST table if any, free space map, and visibility map). |
`+pg_tablespace_size+` ( `+name+` ) → `+bigint+`
Computes the total disk space used in the tablespace with the specified name or OID. To use this function, you must have |
`+pg_total_relation_size+` ( `+regclass+` ) → `+bigint+` Computes the total disk space used by the specified table, including all indexes and TOAST data. The result is equivalent to |
+
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.
The functions shown in Table 9.91 assist in identifying the specific disk files associated with database objects.
Table 9.91. Database Object Location Functions
| Function Description |
|---|
`+pg_relation_filenode+` ( _`+relation+`_ `+regclass+` ) → `+oid+` Returns the “[.quote]#filenode”# number currently assigned to the specified relation. The filenode is the base component of the file name(s) used for the relation (see Section 69.1 for more information). For most relations the result is the same as |
`+pg_relation_filepath+` ( _`+relation+`_ `+regclass+` ) → `+text+` Returns the entire file path name (relative to the database cluster’s data directory, |
`+pg_filenode_relation+` ( _`+tablespace+`_ `+oid+`, _`+filenode+`_ `+oid+` ) → `+regclass+` Returns a relation’s OID given the tablespace OID and filenode it is stored under. This is essentially the inverse mapping of |
+
Table 9.92 lists functions used to manage collations.
Table 9.92. Collation Management Functions
| Function Description |
|---|
`+pg_collation_actual_version+` ( `+oid+` ) → `+text+` 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_import_system_collations+` ( _`+schema+`_ `+regnamespace+` ) → `+integer+` Adds collations to the system catalog |
+
Table 9.93 lists functions that provide information about the structure of partitioned tables.
Table 9.93. Partitioning Information Functions
| Function Description |
|---|
`+pg_partition_tree+` ( `+regclass+` ) → `+setof record+` ( _`+relid+`_ `+regclass+`, _`+parentrelid+`_ `+regclass+`, _`+isleaf+`_ `+boolean+`, _`+level+`_ `+integer+` ) Lists the tables or indexes in the partition tree of the given partitioned table or partitioned index, with one row for each partition. Information provided includes the OID of the partition, the OID of its immediate parent, a boolean value telling if the partition is a leaf, and an integer telling its level in the hierarchy. The level value is 0 for the input table or index, 1 for its immediate child partitions, 2 for their partitions, and so on. Returns no rows if the relation does not exist or is not a partition or partitioned table. |
`+pg_partition_ancestors+` ( `+regclass+` ) → `+setof regclass+` Lists the ancestor relations of the given partition, including the relation itself. Returns no rows if the relation does not exist or is not a partition or partitioned table. |
`+pg_partition_root+` ( `+regclass+` ) → `+regclass+` Returns the top-most parent of the partition tree to which the given relation belongs. Returns |
+
For example, to check the total size of the data contained in a partitioned table measurement, one could use the following query:
SELECT pg_size_pretty(sum(pg_relation_size(relid))) AS total_size
FROM pg_partition_tree('measurement');9.27.8. Index Maintenance Functions
Table 9.94 shows the functions available for index maintenance tasks. (Note that these maintenance tasks are normally done automatically by autovacuum; use of these functions is only required in special cases.) These functions cannot be executed during recovery. Use of these functions is restricted to superusers and the owner of the given index.
Table 9.94. Index Maintenance Functions
| Function Description |
|---|
`+brin_summarize_new_values+` ( _`+index+`_ `+regclass+` ) → `+integer+` Scans the specified BRIN 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 those table pages. Returns the number of new page range summaries that were inserted into the index. |
`+brin_summarize_range+` ( _`+index+`_ `+regclass+`, _`+blockNumber+`_ `+bigint+` ) → `+integer+` Summarizes the page range covering the given block, if not already summarized. This is like |
`+brin_desummarize_range+` ( _`+index+`_ `+regclass+`, _`+blockNumber+`_ `+bigint+` ) → `+void+` Removes the BRIN index tuple that summarizes the page range covering the given table block, if there is one. |
`+gin_clean_pending_list+` ( _`+index+`_ `+regclass+` ) → `+bigint+` Cleans up the “[.quote]#pending”# list of the specified GIN index by moving entries in it, in bulk, to the main GIN data structure. Returns the number of pages removed from the pending list. If the argument is a GIN index built with the |
+
9.27.9. Generic File Access Functions
The functions shown in Table 9.95 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 a superuser or 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 pg_read_file(), or related functions, allows them the ability to read any file on the server that the database server process can read; these functions bypass all in-database privilege checks. This means that, for example, a user with such access is able to read the contents of the pg_authid table where authentication information is stored, as well as read any table data in the database. Therefore, granting access to these functions should be carefully considered.
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 or an empty result set, as appropriate. If false, an error is raised. The default is false.
Table 9.95. Generic File Access Functions
| Function Description |
|---|
`+pg_ls_dir+` ( _`+dirname+`_ `+text+` [[.optional]#, _`+missing_ok+`_ `+boolean+`, _`+include_dot_dirs+`_ `+boolean+`# ] ) → `+setof text+` Returns the names of all files (and directories and other special files) in the specified directory. The `include_dot_dirs This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
`+pg_ls_logdir+` () → `+setof record+` ( _`+name+`_ `+text+`, _`+size+`_ `+bigint+`, _`+modification+`_ `+timestamp with time zone+` ) Returns the name, size, and last modification time (mtime) of each ordinary file in the server’s log directory. Filenames beginning with a dot, directories, and other special files are excluded. This function is restricted to superusers and members of the |
`+pg_ls_waldir+` () → `+setof record+` ( _`+name+`_ `+text+`, _`+size+`_ `+bigint+`, _`+modification+`_ `+timestamp with time zone+` ) Returns the name, size, and last modification time (mtime) of each ordinary file in the server’s write-ahead log (WAL) directory. Filenames beginning with a dot, directories, and other special files are excluded. This function is restricted to superusers and members of the |
`+pg_ls_archive_statusdir+` () → `+setof record+` ( _`+name+`_ `+text+`, _`+size+`_ `+bigint+`, _`+modification+`_ `+timestamp with time zone+` ) Returns the name, size, and last modification time (mtime) of each ordinary file in the server’s WAL archive status directory ( This function is restricted to superusers and members of the |
`+pg_ls_tmpdir+` ( [ [.optional]#_`+tablespace+`_ `+oid+`# ] ) → `+setof record+` ( _`+name+`_ `+text+`, _`+size+`_ `+bigint+`, _`+modification+`_ `+timestamp with time zone+` ) Returns the name, size, and last modification time (mtime) of each ordinary file in the temporary file directory for the specified `tablespace This function is restricted to superusers and members of the |
`+pg_read_file+` ( _`+filename+`_ `+text+` [[.optional]#, _`+offset+`_ `+bigint+`, _`+length+`_ `+bigint+` [[.optional]#, _`+missing_ok+`_ `+boolean+`# ]#] ) → `+text+` Returns all or part of a text file, starting at the given byte `offset This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
`+pg_read_binary_file+` ( _`+filename+`_ `+text+` [[.optional]#, _`+offset+`_ `+bigint+`, _`+length+`_ `+bigint+` [[.optional]#, _`+missing_ok+`_ `+boolean+`# ]#] ) → `+bytea+` Returns all or part of a file. This function is identical to This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. In combination with the |
`+pg_stat_file+` ( _`+filename+`_ `+text+` [[.optional]#, _`+missing_ok+`_ `+boolean+`# ] ) → `+record+` ( _`+size+`_ `+bigint+`, _`+access+`_ `+timestamp with time zone+`, _`+modification+`_ `+timestamp with time zone+`, _`+change+`_ `+timestamp with time zone+`, _`+creation+`_ `+timestamp with time zone+`, _`+isdir+`_ `+boolean+` ) Returns a record containing the file’s size, last access time stamp, last modification time stamp, last file status change time stamp (Unix platforms only), file creation time stamp (Windows only), and a flag indicating if it is a directory. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
+
9.27.10. Advisory Lock Functions
The functions shown in Table 9.96 manage advisory locks. For details about proper use of these functions, see Section 13.3.5.
All these functions are intended to be used to lock application-defined resources, 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 conflicting lock on the same resource identifier, the functions will either wait until the resource becomes available, or return a false result, as appropriate for the function. Locks can be either shared or exclusive: a shared lock does not conflict with other shared locks on the same resource, only with exclusive locks. Locks can be taken at session level (so that they are held until released or the session ends) or at transaction level (so that they are held until the current transaction ends; there is no provision for manual release). Multiple session-level lock requests stack, so that if the same resource identifier is locked three times there must then be three unlock requests to release the resource in advance of session end.
Table 9.96. Advisory Lock Functions
| Function Description |
|---|
`+pg_advisory_lock+` ( _`+key+`_ `+bigint+` ) → `+void+`
Obtains an exclusive session-level advisory lock, waiting if necessary. |
`+pg_advisory_lock_shared+` ( _`+key+`_ `+bigint+` ) → `+void+`
Obtains a shared session-level advisory lock, waiting if necessary. |
`+pg_advisory_unlock+` ( _`+key+`_ `+bigint+` ) → `+boolean+`
Releases a previously-acquired exclusive session-level advisory lock. Returns |
`+pg_advisory_unlock_all+` () → `+void+` Releases 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_unlock_shared+` ( _`+key+`_ `+bigint+` ) → `+boolean+`
Releases a previously-acquired shared session-level advisory lock. Returns |
`+pg_advisory_xact_lock+` ( _`+key+`_ `+bigint+` ) → `+void+`
Obtains an exclusive transaction-level advisory lock, waiting if necessary. |
`+pg_advisory_xact_lock_shared+` ( _`+key+`_ `+bigint+` ) → `+void+`
Obtains a shared transaction-level advisory lock, waiting if necessary. |
`+pg_try_advisory_lock+` ( _`+key+`_ `+bigint+` ) → `+boolean+`
Obtains an exclusive session-level advisory lock if available. This will either obtain the lock immediately and return |
`+pg_try_advisory_lock_shared+` ( _`+key+`_ `+bigint+` ) → `+boolean+`
Obtains a shared session-level advisory lock if available. This will either obtain the lock immediately and return |
`+pg_try_advisory_xact_lock+` ( _`+key+`_ `+bigint+` ) → `+boolean+`
Obtains an exclusive transaction-level advisory lock if available. This will either obtain the lock immediately and return |
`+pg_try_advisory_xact_lock_shared+` ( _`+key+`_ `+bigint+` ) → `+boolean+`
Obtains a shared transaction-level advisory lock if available. This will either obtain the lock immediately and return |
+
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9.26. System Information Functions and Operators |
9.28. Trigger Functions |
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