## 9.20. System Administration Functions

Table 9.44, “Configuration Settings Functions” shows the functions available to query and alter run-time configuration parameters.

Table 9.44. Configuration Settings Functions

Name | Return Type | Description
--- | --- | ---
``current_setting`(`setting_name`)` | `text` | 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 9.45, “Server Signalling Functions” send control signals to other server processes. Use of these functions is restricted to superusers.

Table 9.45. Server Signalling Functions

Name | Return Type | Description
--- | --- | ---
``pg_cancel_backend`(`pid` `int`)` | `boolean` | Cancel a backend's current query
``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` sends a query cancel (SIGINT) signal to a backend process identified by process ID. The process ID of an active backend can be found from the `procpid` column in the `pg_stat_activity` view, or by listing the `postgres` processes on the server with ps.

`pg_reload_conf` sends a SIGHUP signal to the postmaster, causing the 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 `redirect_stderr` is used for logging, since otherwise there is no log-file manager subprocess.

The functions shown in Table 9.46, “Backup Control Functions” assist in making on-line backups. Use of these functions is restricted to superusers.

Table 9.46. Backup Control Functions

Name | Return Type | Description
--- | --- | ---
``pg_start_backup`(`label` `text`)` | `text` | Set up for performing on-line backup
``pg_stop_backup`()` | `text` | Finish performing on-line backup

`pg_start_backup` accepts a single parameter which is 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 into the database cluster's data directory, and then returns the backup's starting WAL offset as text. (The user need not pay any attention to this result value, but it is provided in case it is of use.)

`pg_stop_backup` removes the label file created by `pg_start_backup`, and instead creates a backup history file in the WAL archive area. The history file includes the label given to `pg_start_backup`, the starting and ending WAL offsets for the backup, and the starting and ending times of the backup. The return value is the backup's ending WAL offset (which again may be of little interest).

For details about proper usage of these functions, see Section 23.3, “On-line backup and point-in-time recovery (PITR)”.

The functions shown in Table 9.47, “Database Object Size Functions” calculate the actual disk space usage of database objects.

Table 9.47. 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_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_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`(`oid`)` | `bigint` | Disk space used by the table or index with the specified OID
``pg_relation_size`(`text`)` | `bigint` | Disk space used by the table or index with the specified name. The table name may be qualified with a schema name
``pg_total_relation_size`(`oid`)` | `bigint` | Total disk space used by the table with the specified OID, including indexes and toasted data
``pg_total_relation_size`(`text`)` | `bigint` | Total disk space used by the table with the specified name, including indexes and toasted data. The table name may be qualified with a schema name
``pg_size_pretty`(`bigint`)` | `text` | Converts a size in bytes into a human-readable format with size units

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

`pg_tablespace_size` and `pg_database_size` accept the OID or name of a tablespace or database, 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.

`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.

`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 shown in Table 9.48, “Generic File Access Functions” provide native file access to files on the machine hosting the server. Only files within the database cluster directory and the `log_directory` may be accessed. Use a relative path for files within the cluster directory, and a path matching the `log_directory` configuration setting for log files. Use of these functions is restricted to superusers.

Table 9.48. 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 timestamp (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;