9.26.1. Configuration Settings Functions

Table 9-63 shows the functions available to query and alter run-time configuration parameters.

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)

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 9-64 send control signals to other server processes. Use of these functions is usually restricted to superusers, with noted exceptions.

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 9-65 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).

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.) The function writes a backup label file (backup_label) 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.

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.

pg_stop_backup removes the label file created by pg_start_backup, and 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. 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. The write location is the end of what can be examined from outside the server, and is usually what you want if you are interested in archiving partially-complete transaction log files. The insertion point is made available primarily for server debugging purposes. These are both read-only operations and do not require superuser permissions.

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 9-65 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 9-66 provide information about the current status of the standby. These functions may be executed both during recovery and in normal running.

The functions shown in Table 9-67 control the progress of recovery. These functions may be executed only during recovery.

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 9-68, and imported with the SET TRANSACTION command.

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

PostgreSQL exposes a number of functions for controlling and interacting with replication features. See Section 25.2.5 and Section 25.2.6.

Many of these functions have equivalent commands in the replication protocol; see Section 49.3.

The sections Section 9.26.5, Section 9.26.4 and Section 9.26.3 are also relevant for replication.

9.26.7. Database Object Management Functions

The functions shown in Table 9-70 calculate the disk space usage of database objects.

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. Specifying 'main' or leaving out the second argument returns the size of the main data fork of the relation. Specifying 'fsm' returns the size of the Free Space Map (see Section 59.3) associated with the relation. Specifying 'vm' returns the size of the Visibility Map (see Section 59.4) associated with the relation. Specifying 'init' returns the size of the initialization fork, if any, associated with the relation. Note that this function shows the size of only one fork; for most purposes it is more convenient to use the higher-level functions pg_total_relation_size or pg_table_size.

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

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 9-71 assist in identifying the specific disk files associated with database objects.

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 59.1 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. Generic File Access Functions

The functions shown in Table 9-72 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.

pg_ls_dir returns all the names in the specified directory, except the special entries "." and "..".

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;

9.26.9. Advisory Lock Functions

The functions shown in Table 9-73 manage advisory locks. For details about proper use of these functions, see Section 13.3.4.

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.