PostgreSQL 8.4.2 Documentation
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# 9.24. System Administration Functions

Table 9-54 shows the functions available to query and alter run-time configuration parameters.

Table 9-54. Configuration Settings Functions

Name | Return Type | Description
--- | --- | ---
``current_setting`(`setting_name`)` | `text` | get current value of setting
``set_config(`setting_name`, `new_value`, `is_local`)`` | `text` | set parameter and return new value

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

    SELECT current_setting('datestyle');

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

`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)

The functions shown in Table 9-55 send control signals to other server processes. Use of these functions is restricted to superusers.

Table 9-55. Server Signalling Functions

Name | Return Type | Description
--- | --- | ---
``pg_cancel_backend`(`pid` `int`)` | `boolean` | Cancel a backend's current query
``pg_terminate_backend`(`pid` `int`)` | `boolean` | Terminate a backend
``pg_reload_conf`()` | `boolean` | Cause server processes to reload their configuration files
``pg_rotate_logfile`()` | `boolean` | Rotate server's log file

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 `procpid` 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).

`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.

The functions shown in Table 9-56 assist in making on-line backups. Use of the first three functions is restricted to superusers.

Table 9-56. Backup Control Functions

Name | Return Type | Description
--- | --- | ---
``pg_start_backup`(`label` `text` [, `fast` `boolean` ])` | `text` | Prepare for performing on-line backup
``pg_stop_backup`()` | `text` | Finish performing on-line backup
``pg_switch_xlog`()` | `text` | Force switch to a new transaction log file
``pg_current_xlog_location`()` | `text` | Get current transaction log write location
``pg_current_xlog_insert_location`()` | `text` | Get current transaction log insert location
``pg_xlogfile_name_offset`(`location` `text`)` | `text`, `integer` | Convert transaction log location string to file name and decimal byte offset within file
``pg_xlogfile_name`(`location` `text`)` | `text` | Convert transaction log location string to file name

`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 boolean second parameter. 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_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.

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

The functions shown in Table 9-57 calculate the disk space usage of database objects.

Table 9-57. 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_relation_size`(`relation` `regclass`, `fork` `text`)` | `bigint` | Disk space used by the specified fork (`'main'`, `'fsm'` or `'vm'`) of the table or index with the specified OID or name
``pg_relation_size`(`relation` `regclass`)` | `bigint` | Shorthand for `pg_relation_size(..., 'main')`
``pg_size_pretty`(`bigint`)` | `text` | Converts a size in bytes into a human-readable format with size units
``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 table with the specified OID or name, including indexes and TOAST data

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

`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.

`pg_relation_size` accepts the OID or name of a table, index or toast table, and returns the 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 53.3) associated with the relation. Specifying `'vm'` returns the size of the Visibility Map (see Section 53.4) 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 kB, MB, GB or TB as appropriate.

`pg_total_relation_size` accepts the OID or name of a table or toast table, and returns the size in bytes of the data and all associated indexes and toast tables.

The functions shown in Table 9-58 provide native access to files on the machine hosting the server. Only files within the database cluster directory and the `log_directory` can be accessed. Use a relative path for files in the cluster directory, and a path matching the `log_directory` configuration setting for log files. Use of these functions is restricted to superusers.

Table 9-58. Generic File Access Functions

Name | Return Type | Description
--- | --- | ---
``pg_ls_dir`(`dirname` `text`)` | `setof text` | List the contents of a directory
``pg_read_file`(`filename` `text`, `offset` `bigint`, `length` `bigint`)` | `text` | Return the contents of a text file
``pg_stat_file`(`filename` `text`)` | `record` | Return information about a file

`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.

`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;

The functions shown in Table 9-59 manage advisory locks. For details about proper use of these functions, see Section 13.3.4.

Table 9-59. Advisory Lock Functions

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

`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). The key type is specified in `pg_locks.objid`. If another session already holds a lock on the same resource, the 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 be also 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 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 raised by the server.

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

`pg_advisory_unlock_all` will release all advisory locks held by the current session. (This function is implicitly invoked at session end, even if the client disconnects ungracefully.)

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