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PostgreSQL 8.2.6 Documentation | ||||
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Table 9-45 shows the functions available to query and alter run-time configuration parameters.
Table 9-45. 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 9-46 send control signals to other server processes. Use of these functions is restricted to superusers.
Table 9-46. 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
redirect_stderr is used for logging, since otherwise there
is no log-file manager subprocess.
The functions shown in Table 9-47 assist in making on-line backups. Use of the first three functions is restricted to superusers.
Table 9-47. 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 may 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 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 end location
of the previous transaction log file.
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 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 transction log location is exactly at an 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 23.3.
The functions shown in Table 9-48 calculate the actual disk space usage of database objects.
Table 9-48. 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 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_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 may 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 9-49 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 9-49. 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 9-50 manage advisory locks. For details about proper usage of these functions, see Section 12.3.4.
Table 9-50. 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 may 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.)