| PostgreSQL 8.3.19 Documentation | ||||
|---|---|---|---|---|
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Table 9-51 shows the functions available to query and alter run-time configuration parameters.
Table 9-51. 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 9-52 send control signals to other server processes. Use of these functions is restricted to superusers.
Table 9-52. 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 server, 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 the built-in
    log collector is running, since otherwise there is no log-file manager 
    subprocess.
   
The functions shown in Table 9-53 assist in making on-line backups. Use of the first three functions is restricted to superusers.
Table 9-53. 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_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 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 transaction log location as text.  The user
    need not pay any attention to this result value, but it is provided in
    case it is of use. 
postgres=# select pg_start_backup('label_goes_here');
 pg_start_backup
-----------------
 0/D4445B8
(1 row)
    pg_stop_backup removes the label file created by
    pg_start_backup, and instead 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 might be of little
    interest).  After noting 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 result 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 9-54 calculate the actual disk space usage of database objects.
Table 9-54. 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(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 can 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_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(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 can be qualified with a schema name | 
    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.
   
    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 9-55 provide native file 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 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 9-55. 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 9-56 manage advisory locks. For details about proper usage of these functions, see Section 13.3.4.
Table 9-56. 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).  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 now.
   
    pg_try_advisory_lock_shared works the same as
    pg_try_advisory_lock, except it attempts to acquire
    shared rather than exclusive lock.
   
    pg_advisory_unlock will release a previously-acquired
    exclusive advisory lock.  It
    will return true if the lock is successfully released.
    If the lock was in fact 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 to release 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.)