9.20. System Administration Functions

Table 9-44 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)

The functions shown in Table 9-45 send control signals to other server processes. Use of these functions is restricted to superusers.

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 assist in making on-line backups. Use of these functions is restricted to superusers.

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.

The functions shown in Table 9-47 calculate the actual disk space usage of database objects.

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 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.

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;