9.27. System Administration Functions
The functions described in this section are used to control and
monitor a PostgreSQL installation.
9.27.1. Configuration Settings Functions
Table 9.85 shows the functions
available to query and alter run-time configuration parameters.
Table 9.85. Configuration Settings Functions
Function
Description
Example(s)
|
---|
current_setting ( setting_name text [, missing_ok boolean ] )
→ text
Returns the current value of the
setting setting_name . If there is no such
setting, current_setting throws an error
unless missing_ok is supplied and
is true (in which case NULL is returned).
This function corresponds to
the SQL command SHOW.
current_setting('datestyle')
→ ISO, MDY
|
set_config (
setting_name text ,
new_value text ,
is_local boolean )
→ text
Sets the parameter setting_name
to new_value , and returns that value.
If is_local is true , the new
value will only apply during the current transaction. If you want the
new value to apply for the rest of the current session,
use false instead. This function corresponds to
the SQL command SET.
set_config('log_statement_stats', 'off', false)
→ off
|
9.27.2. Server Signaling Functions
The functions shown in Table 9.86 send control signals to
other server processes. Use of these functions is restricted to
superusers by default but access may be granted to others using
GRANT
, with noted exceptions.
Each of these functions returns true
if
the signal was successfully sent and false
if sending the signal failed.
Table 9.86. Server Signaling Functions
Function
Description
|
---|
pg_cancel_backend ( pid integer )
→ boolean
Cancels the current query of the session whose backend process has the
specified process ID. This is also allowed if the
calling role is a member of the role whose backend is being canceled or
the calling role has been granted pg_signal_backend ,
however only superusers can cancel superuser backends.
|
pg_log_backend_memory_contexts ( pid integer )
→ boolean
Requests to log the memory contexts of the backend with the
specified process ID. These memory contexts will be logged at
LOG message level. They will appear in
the server log based on the log configuration set
(See Section 20.8 for more information),
but will not be sent to the client regardless of
client_min_messages.
Only superusers can request to log the memory contexts.
|
pg_reload_conf ()
→ boolean
Causes all processes of the PostgreSQL
server to reload their configuration files. (This is initiated by
sending a SIGHUP signal to the postmaster
process, which in turn sends SIGHUP to each
of its children.) You can use the
pg_file_settings and
pg_hba_file_rules views
to check the configuration files for possible errors, before reloading.
|
pg_rotate_logfile ()
→ boolean
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.
|
pg_terminate_backend ( pid integer , timeout bigint DEFAULT 0 )
→ boolean
Terminates the session whose backend process has the
specified process ID. This is also allowed if the calling role
is a member of the role whose backend is being terminated or the
calling role has been granted pg_signal_backend ,
however only superusers can terminate superuser backends.
If timeout is not specified or zero, this
function returns true whether the process actually
terminates or not, indicating only that the sending of the signal was
successful. If the timeout is specified (in
milliseconds) and greater than zero, the function waits until the
process is actually terminated or until the given time has passed. If
the process is terminated, the function
returns true . On timeout, a warning is emitted and
false is returned.
|
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 pid
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).
The role of an active backend can be found from the
usename
column of the
pg_stat_activity
view.
pg_log_backend_memory_contexts
can be used
to log the memory contexts of a backend process. For example:
postgres=# SELECT pg_log_backend_memory_contexts(pg_backend_pid());
pg_log_backend_memory_contexts
--------------------------------
t
(1 row)
One message for each memory context will be logged. For example:
LOG: logging memory contexts of PID 10377
STATEMENT: SELECT pg_log_backend_memory_contexts(pg_backend_pid());
LOG: level: 0; TopMemoryContext: 80800 total in 6 blocks; 14432 free (5 chunks); 66368 used
LOG: level: 1; pgstat TabStatusArray lookup hash table: 8192 total in 1 blocks; 1408 free (0 chunks); 6784 used
LOG: level: 1; TopTransactionContext: 8192 total in 1 blocks; 7720 free (1 chunks); 472 used
LOG: level: 1; RowDescriptionContext: 8192 total in 1 blocks; 6880 free (0 chunks); 1312 used
LOG: level: 1; MessageContext: 16384 total in 2 blocks; 5152 free (0 chunks); 11232 used
LOG: level: 1; Operator class cache: 8192 total in 1 blocks; 512 free (0 chunks); 7680 used
LOG: level: 1; smgr relation table: 16384 total in 2 blocks; 4544 free (3 chunks); 11840 used
LOG: level: 1; TransactionAbortContext: 32768 total in 1 blocks; 32504 free (0 chunks); 264 used
...
LOG: level: 1; ErrorContext: 8192 total in 1 blocks; 7928 free (3 chunks); 264 used
LOG: Grand total: 1651920 bytes in 201 blocks; 622360 free (88 chunks); 1029560 used
If there are more than 100 child contexts under the same parent, the first
100 child contexts are logged, along with a summary of the remaining contexts.
Note that frequent calls to this function could incur significant overhead,
because it may generate a large number of log messages.
9.27.3. Backup Control Functions
The functions shown in Table 9.87 assist in making on-line backups.
These functions cannot be executed during recovery (except
non-exclusive pg_start_backup
,
non-exclusive pg_stop_backup
,
pg_is_in_backup
, pg_backup_start_time
and pg_wal_lsn_diff
).
For details about proper usage of these functions, see
Section 26.3.
Table 9.87. Backup Control Functions
Function
Description
|
---|
pg_create_restore_point ( name text )
→ pg_lsn
Creates a named marker record in the write-ahead log that can later be
used as a recovery target, and returns the corresponding write-ahead
log location. The given name can then be used with
recovery_target_name to specify the point up to
which recovery will proceed. Avoid creating multiple restore points
with the same name, since recovery will stop at the first one whose
name matches the recovery target.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
|
pg_current_wal_flush_lsn ()
→ pg_lsn
Returns the current write-ahead log flush location (see notes below).
|
pg_current_wal_insert_lsn ()
→ pg_lsn
Returns the current write-ahead log insert location (see notes below).
|
pg_current_wal_lsn ()
→ pg_lsn
Returns the current write-ahead log write location (see notes below).
|
pg_start_backup (
label text
[, fast boolean
[, exclusive boolean
]] )
→ pg_lsn
Prepares the server to begin an on-line backup. The only required
parameter is an arbitrary user-defined label for the backup.
(Typically this would be the name under which the backup dump file
will be stored.)
If the optional second parameter is given as 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.
The optional third parameter specifies whether to perform an exclusive
or non-exclusive backup (default is exclusive).
When used in exclusive mode, this function writes a backup label file
(backup_label ) and, if there are any links in
the pg_tblspc/ directory, a tablespace map file
(tablespace_map ) into the database cluster's data
directory, then performs a checkpoint, and then returns the backup's
starting write-ahead log location. (The user can ignore this
result value, but it is provided in case it is useful.) When used in
non-exclusive mode, the contents of these files are instead returned
by the pg_stop_backup function, and should be
copied to the backup area by the user.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
|
pg_stop_backup (
exclusive boolean
[, wait_for_archive boolean
] )
→ setof record
( lsn pg_lsn ,
labelfile text ,
spcmapfile text )
Finishes performing an exclusive or non-exclusive on-line backup.
The exclusive parameter must match the
previous pg_start_backup call.
In an exclusive backup, pg_stop_backup removes
the backup label file and, if it exists, the tablespace map file
created by pg_start_backup . In a non-exclusive
backup, the desired contents of these files are returned as part of
the result of the function, and should be written to files in the
backup area (not in the data directory).
There is an optional second parameter of type boolean .
If false, the function will return immediately after the backup is
completed, without waiting for WAL to be archived. This behavior is
only useful with backup software that independently monitors WAL
archiving. Otherwise, WAL required to make the backup consistent might
be missing and make the backup useless. By default or when this
parameter is true, pg_stop_backup will wait for
WAL to be archived when archiving is enabled. (On a standby, this
means that it will wait only when archive_mode =
always . If write activity on the primary is low,
it may be useful to run pg_switch_wal on the
primary in order to trigger an immediate segment switch.)
When executed on a primary, this function also creates a backup
history file in the write-ahead log archive area. The history file
includes the label given to pg_start_backup , the
starting and ending write-ahead log locations for the backup, and the
starting and ending times of the backup. After recording the ending
location, the current write-ahead log insertion point is automatically
advanced to the next write-ahead log file, so that the ending
write-ahead log file can be archived immediately to complete the
backup.
The result of the function is a single record.
The lsn column holds the backup's ending
write-ahead log location (which again can be ignored). The second and
third columns are NULL when ending an exclusive
backup; after a non-exclusive backup they hold the desired contents of
the label and tablespace map files.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
|
pg_stop_backup ()
→ pg_lsn
Finishes performing an exclusive on-line backup. This simplified
version is equivalent to pg_stop_backup(true,
true) , except that it only returns the pg_lsn
result.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
|
pg_is_in_backup ()
→ boolean
Returns true if an on-line exclusive backup is in progress.
|
pg_backup_start_time ()
→ timestamp with time zone
Returns the start time of the current on-line exclusive backup if one
is in progress, otherwise NULL .
|
pg_switch_wal ()
→ pg_lsn
Forces the server to switch to a new write-ahead log file, which
allows the current file to be archived (assuming you are using
continuous archiving). The result is the ending write-ahead log
location plus 1 within the just-completed write-ahead log file. If
there has been no write-ahead log activity since the last write-ahead
log switch, pg_switch_wal does nothing and
returns the start location of the write-ahead log file currently in
use.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
|
pg_walfile_name ( lsn pg_lsn )
→ text
Converts a write-ahead log location to the name of the WAL file
holding that location.
|
pg_walfile_name_offset ( lsn pg_lsn )
→ record
( file_name text ,
file_offset integer )
Converts a write-ahead log location to a WAL file name and byte offset
within that file.
|
pg_wal_lsn_diff ( lsn1 pg_lsn , lsn2 pg_lsn )
→ numeric
Calculates the difference in bytes (lsn1 - lsn2 ) between two write-ahead log
locations. This can be used
with pg_stat_replication or some of the
functions shown in Table 9.87 to
get the replication lag.
|
pg_current_wal_lsn
displays the current write-ahead
log write location in the same format used by the above functions.
Similarly, pg_current_wal_insert_lsn
displays the
current write-ahead log insertion location
and pg_current_wal_flush_lsn
displays the current
write-ahead log flush location. The insertion location is
the “logical” end of the write-ahead log at any instant,
while the write location is the end of what has actually been written out
from the server's internal buffers, and the flush location is the last
location known to be written to durable storage. 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 write-ahead
log files. The insertion and flush locations are made available primarily
for server debugging purposes. These are all read-only operations and do
not require superuser permissions.
You can use pg_walfile_name_offset
to extract the
corresponding write-ahead log file name and byte offset from
a pg_lsn
value. For example:
postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
file_name | file_offset
--------------------------+-------------
00000001000000000000000D | 4039624
(1 row)
Similarly, pg_walfile_name
extracts just the write-ahead log file name.
When the given write-ahead log location is exactly at a write-ahead log file boundary, both
these functions return the name of the preceding write-ahead log file.
This is usually the desired behavior for managing write-ahead log archiving
behavior, since the preceding file is the last one that currently
needs to be archived.
9.27.4. Recovery Control Functions
The functions shown in Table 9.88 provide information
about the current status of a standby server.
These functions may be executed both during recovery and in normal running.
Table 9.88. Recovery Information Functions
Function
Description
|
---|
pg_is_in_recovery ()
→ boolean
Returns true if recovery is still in progress.
|
pg_last_wal_receive_lsn ()
→ pg_lsn
Returns the last write-ahead log location that has been received and
synced to disk by streaming replication. While streaming replication
is in progress this will increase monotonically. If recovery has
completed then this will remain static at the location of the last WAL
record received and synced to disk during recovery. If streaming
replication is disabled, or if it has not yet started, the function
returns NULL .
|
pg_last_wal_replay_lsn ()
→ pg_lsn
Returns the last write-ahead log location that has been replayed
during recovery. If recovery is still in progress this will increase
monotonically. If recovery has completed then this will remain
static at the location of the last WAL record applied during recovery.
When the server has been started normally without recovery, the
function returns NULL .
|
pg_last_xact_replay_timestamp ()
→ timestamp with time zone
Returns the time stamp of the last transaction replayed during
recovery. This is the time at which the commit or abort WAL record
for that transaction was generated on the primary. If no transactions
have been replayed during recovery, the function
returns NULL . Otherwise, if recovery is still in
progress this will increase monotonically. If recovery has completed
then this will remain static at the time of the last transaction
applied during recovery. When the server has been started normally
without recovery, the function returns NULL .
|
The functions shown in Table 9.89 control the progress of recovery.
These functions may be executed only during recovery.
Table 9.89. Recovery Control Functions
Function
Description
|
---|
pg_is_wal_replay_paused ()
→ boolean
Returns true if recovery pause is requested.
|
pg_get_wal_replay_pause_state ()
→ text
Returns recovery pause state. The return values are
not paused if pause is not requested,
pause requested if pause is requested but recovery is
not yet paused, and paused if the recovery is
actually paused.
|
pg_promote ( wait boolean DEFAULT true , wait_seconds integer DEFAULT 60 )
→ boolean
Promotes a standby server to primary status.
With wait set to true (the
default), the function waits until promotion is completed
or wait_seconds seconds have passed, and
returns true if promotion is successful
and false otherwise.
If wait is set to false , the
function returns true immediately after sending a
SIGUSR1 signal to the postmaster to trigger
promotion.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
|
pg_wal_replay_pause ()
→ void
Request to pause recovery. A request doesn't mean that recovery stops
right away. If you want a guarantee that recovery is actually paused,
you need to check for the recovery pause state returned by
pg_get_wal_replay_pause_state() . Note that
pg_is_wal_replay_paused() returns whether a request
is made. While recovery is paused, no further database changes are applied.
If hot standby is active, all new queries will see the same consistent
snapshot of the database, and no further query conflicts will be generated
until recovery is resumed.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
|
pg_wal_replay_resume ()
→ void
Restarts recovery if it was paused.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
|
pg_wal_replay_pause
and
pg_wal_replay_resume
cannot be executed while
a promotion is ongoing. If a promotion is triggered while recovery
is paused, the paused state ends and promotion continues.
If streaming replication is disabled, the paused state may continue
indefinitely without a problem. If streaming replication is in
progress then WAL records will continue to be received, which will
eventually fill available disk space, depending upon the duration of
the pause, the rate of WAL generation and available disk space.
9.27.5. Snapshot Synchronization Functions
PostgreSQL allows database sessions to synchronize their
snapshots. A snapshot determines which data is visible to the
transaction that is using the snapshot. Synchronized snapshots are
necessary when two or more sessions need to see identical content in the
database. If two sessions just start their transactions independently,
there is always a possibility that some third transaction commits
between the executions of the two START TRANSACTION
commands,
so that one session sees the effects of that transaction and the other
does not.
To solve this problem, PostgreSQL allows a transaction to
export the snapshot it is using. As long as the exporting
transaction remains open, other transactions can import its
snapshot, and thereby be guaranteed that they see exactly the same view
of the database that the first transaction sees. But note that any
database changes made by any one of these transactions remain invisible
to the other transactions, as is usual for changes made by uncommitted
transactions. So the transactions are synchronized with respect to
pre-existing data, but act normally for changes they make themselves.
Snapshots are exported with the pg_export_snapshot
function,
shown in Table 9.90, and
imported with the SET TRANSACTION command.
Table 9.90. Snapshot Synchronization Functions
Function
Description
|
---|
pg_export_snapshot ()
→ text
Saves the transaction's current snapshot and returns
a text string identifying the snapshot. This string must
be passed (outside the database) to clients that want to import the
snapshot. The snapshot is available for import only until the end of
the transaction that exported it.
A transaction can export more than one snapshot, if needed. Note that
doing so is only useful in READ COMMITTED
transactions, since in REPEATABLE READ and higher
isolation levels, transactions use the same snapshot throughout their
lifetime. Once a transaction has exported any snapshots, it cannot be
prepared with PREPARE TRANSACTION.
|
9.27.6. Replication Management Functions
The functions shown
in Table 9.91 are for
controlling and interacting with replication features.
See Section 27.2.5,
Section 27.2.6, and
Chapter 50
for information about the underlying features.
Use of functions for replication origin is only allowed to the
superuser by default, but may be allowed to other users by using the
GRANT
command.
Use of functions for replication slots is restricted to superusers
and users having REPLICATION
privilege.
Many of these functions have equivalent commands in the replication
protocol; see Section 53.4.
The functions described in
Section 9.27.3,
Section 9.27.4, and
Section 9.27.5
are also relevant for replication.
Table 9.91. Replication Management Functions
Function
Description
|
---|
pg_create_physical_replication_slot ( slot_name name [, immediately_reserve boolean , temporary boolean ] )
→ record
( slot_name name ,
lsn pg_lsn )
Creates a new physical replication slot named
slot_name . The optional second parameter,
when true , specifies that the LSN for this
replication slot be reserved immediately; otherwise
the LSN is reserved on first connection from a streaming
replication client. Streaming changes from a physical slot is only
possible with the streaming-replication protocol —
see Section 53.4. The optional third
parameter, temporary , when set to true, specifies that
the slot should not be permanently stored to disk and is only meant
for use by the current session. Temporary slots are also
released upon any error. This function corresponds
to the replication protocol command CREATE_REPLICATION_SLOT
... PHYSICAL .
|
pg_drop_replication_slot ( slot_name name )
→ void
Drops the physical or logical replication slot
named slot_name . Same as replication protocol
command DROP_REPLICATION_SLOT . For logical slots, this must
be called while connected to the same database the slot was created on.
|
pg_create_logical_replication_slot ( slot_name name , plugin name [, temporary boolean , twophase boolean ] )
→ record
( slot_name name ,
lsn pg_lsn )
Creates a new logical (decoding) replication slot named
slot_name using the output plugin
plugin . The optional third
parameter, temporary , when set to true, specifies that
the slot should not be permanently stored to disk and is only meant
for use by the current session. Temporary slots are also
released upon any error. The optional fourth parameter,
twophase , when set to true, specifies
that the decoding of prepared transactions is enabled for this
slot. A call to this function has the same effect as the replication
protocol command CREATE_REPLICATION_SLOT ... LOGICAL .
|
pg_copy_physical_replication_slot ( src_slot_name name , dst_slot_name name [, temporary boolean ] )
→ record
( slot_name name ,
lsn pg_lsn )
Copies an existing physical replication slot named src_slot_name
to a physical replication slot named dst_slot_name .
The copied physical slot starts to reserve WAL from the same LSN as the
source slot.
temporary is optional. If temporary
is omitted, the same value as the source slot is used.
|
pg_copy_logical_replication_slot ( src_slot_name name , dst_slot_name name [, temporary boolean [, plugin name ]] )
→ record
( slot_name name ,
lsn pg_lsn )
Copies an existing logical replication slot
named src_slot_name to a logical replication
slot named dst_slot_name , optionally changing
the output plugin and persistence. The copied logical slot starts
from the same LSN as the source logical slot. Both
temporary and plugin are
optional; if they are omitted, the values of the source slot are used.
|
pg_logical_slot_get_changes ( slot_name name , upto_lsn pg_lsn , upto_nchanges integer , VARIADIC options text[] )
→ setof record
( lsn pg_lsn ,
xid xid ,
data text )
Returns changes in the slot slot_name , starting
from the point from which changes have been consumed last. If
upto_lsn
and upto_nchanges are NULL,
logical decoding will continue until end of WAL. If
upto_lsn is non-NULL, decoding will include only
those transactions which commit prior to the specified LSN. If
upto_nchanges is non-NULL, decoding will
stop when the number of rows produced by decoding exceeds
the specified value. Note, however, that the actual number of
rows returned may be larger, since this limit is only checked after
adding the rows produced when decoding each new transaction commit.
|
pg_logical_slot_peek_changes ( slot_name name , upto_lsn pg_lsn , upto_nchanges integer , VARIADIC options text[] )
→ setof record
( lsn pg_lsn ,
xid xid ,
data text )
Behaves just like
the pg_logical_slot_get_changes() function,
except that changes are not consumed; that is, they will be returned
again on future calls.
|
pg_logical_slot_get_binary_changes ( slot_name name , upto_lsn pg_lsn , upto_nchanges integer , VARIADIC options text[] )
→ setof record
( lsn pg_lsn ,
xid xid ,
data bytea )
Behaves just like
the pg_logical_slot_get_changes() function,
except that changes are returned as bytea .
|
pg_logical_slot_peek_binary_changes ( slot_name name , upto_lsn pg_lsn , upto_nchanges integer , VARIADIC options text[] )
→ setof record
( lsn pg_lsn ,
xid xid ,
data bytea )
Behaves just like
the pg_logical_slot_peek_changes() function,
except that changes are returned as bytea .
|
pg_replication_slot_advance ( slot_name name , upto_lsn pg_lsn )
→ record
( slot_name name ,
end_lsn pg_lsn )
Advances the current confirmed position of a replication slot named
slot_name . The slot will not be moved backwards,
and it will not be moved beyond the current insert location. Returns
the name of the slot and the actual position that it was advanced to.
The updated slot position information is written out at the next
checkpoint if any advancing is done. So in the event of a crash, the
slot may return to an earlier position.
|
pg_replication_origin_create ( node_name text )
→ oid
Creates a replication origin with the given external
name, and returns the internal ID assigned to it.
|
pg_replication_origin_drop ( node_name text )
→ void
Deletes a previously-created replication origin, including any
associated replay progress.
|
pg_replication_origin_oid ( node_name text )
→ oid
Looks up a replication origin by name and returns the internal ID. If
no such replication origin is found, NULL is
returned.
|
pg_replication_origin_session_setup ( node_name text )
→ void
Marks the current session as replaying from the given
origin, allowing replay progress to be tracked.
Can only be used if no origin is currently selected.
Use pg_replication_origin_session_reset to undo.
|
pg_replication_origin_session_reset ()
→ void
Cancels the effects
of pg_replication_origin_session_setup() .
|
pg_replication_origin_session_is_setup ()
→ boolean
Returns true if a replication origin has been selected in the
current session.
|
pg_replication_origin_session_progress ( flush boolean )
→ pg_lsn
Returns the replay location for the replication origin selected in
the current session. The parameter flush
determines whether the corresponding local transaction will be
guaranteed to have been flushed to disk or not.
|
pg_replication_origin_xact_setup ( origin_lsn pg_lsn , origin_timestamp timestamp with time zone )
→ void
Marks the current transaction as replaying a transaction that has
committed at the given LSN and timestamp. Can
only be called when a replication origin has been selected
using pg_replication_origin_session_setup .
|
pg_replication_origin_xact_reset ()
→ void
Cancels the effects of
pg_replication_origin_xact_setup() .
|
pg_replication_origin_advance ( node_name text , lsn pg_lsn )
→ void
Sets replication progress for the given node to the given
location. This is primarily useful for setting up the initial
location, or setting a new location after configuration changes and
similar. Be aware that careless use of this function can lead to
inconsistently replicated data.
|
pg_replication_origin_progress ( node_name text , flush boolean )
→ pg_lsn
Returns the replay location for the given replication origin. The
parameter flush determines whether the
corresponding local transaction will be guaranteed to have been
flushed to disk or not.
|
pg_logical_emit_message ( transactional boolean , prefix text , content text )
→ pg_lsn
pg_logical_emit_message ( transactional boolean , prefix text , content bytea )
→ pg_lsn
Emits a logical decoding message. This can be used to pass generic
messages to logical decoding plugins through
WAL. The transactional parameter specifies if
the message should be part of the current transaction, or if it should
be written immediately and decoded as soon as the logical decoder
reads the record. The prefix parameter is a
textual prefix that can be used by logical decoding plugins to easily
recognize messages that are interesting for them.
The content parameter is the content of the
message, given either in text or binary form.
|
9.27.7. Database Object Management Functions
The functions shown in Table 9.92 calculate
the disk space usage of database objects, or assist in presentation
or understanding of usage results. bigint
results
are measured in bytes. If an OID that does
not represent an existing object is passed to one of these
functions, NULL
is returned.
Table 9.92. Database Object Size Functions
Function
Description
|
---|
pg_column_size ( "any" )
→ integer
Shows the number of bytes used to store any individual data value. If
applied directly to a table column value, this reflects any
compression that was done.
|
pg_column_compression ( "any" )
→ text
Shows the compression algorithm that was used to compress
an individual variable-length value. Returns NULL
if the value is not compressed.
|
pg_database_size ( name )
→ bigint
pg_database_size ( oid )
→ bigint
Computes the total disk space used by the database with the specified
name or OID. To use this function, you must
have CONNECT privilege on the specified database
(which is granted by default) or be a member of
the pg_read_all_stats role.
|
pg_indexes_size ( regclass )
→ bigint
Computes the total disk space used by indexes attached to the
specified table.
|
pg_relation_size ( relation regclass [, fork text ] )
→ bigint
Computes the disk space used by one “fork” of the
specified relation. (Note that for most purposes it is more
convenient to use the higher-level
functions pg_total_relation_size
or pg_table_size , which sum the sizes of all
forks.) With one argument, this returns the size of the main data
fork of the relation. The second argument can be provided to specify
which fork to examine:
main returns the size of the main
data fork of the relation.
fsm returns the size of the Free Space Map
(see Section 70.3) associated with the relation.
vm returns the size of the Visibility Map
(see Section 70.4) associated with the relation.
init returns the size of the initialization
fork, if any, associated with the relation.
|
pg_size_bytes ( text )
→ bigint
Converts a size in human-readable format (as returned
by pg_size_pretty ) into bytes.
|
pg_size_pretty ( bigint )
→ text
pg_size_pretty ( numeric )
→ text
Converts a size in bytes into a more easily human-readable format with
size units (bytes, kB, MB, GB or TB as appropriate). Note that the
units are powers of 2 rather than powers of 10, so 1kB is 1024 bytes,
1MB is 10242 = 1048576 bytes, and so on.
|
pg_table_size ( regclass )
→ bigint
Computes the disk space used by the specified table, excluding indexes
(but including its TOAST table if any, free space map, and visibility
map).
|
pg_tablespace_size ( name )
→ bigint
pg_tablespace_size ( oid )
→ bigint
Computes the total disk space used in the tablespace with the
specified name or OID. To use this function, you must
have CREATE privilege on the specified tablespace
or be a member of the pg_read_all_stats role,
unless it is the default tablespace for the current database.
|
pg_total_relation_size ( regclass )
→ bigint
Computes the total disk space used by the specified table, including
all indexes and TOAST data. The result is
equivalent to pg_table_size
+ pg_indexes_size .
|
The functions above that operate on tables or indexes accept a
regclass
argument, which is simply the OID of the table or index
in the pg_class
system catalog. You do not have to look up
the OID by hand, however, since the regclass
data type's input
converter will do the work for you. See Section 8.19
for details.
The functions shown in Table 9.93 assist
in identifying the specific disk files associated with database objects.
Table 9.93. Database Object Location Functions
Function
Description
|
---|
pg_relation_filenode ( relation regclass )
→ oid
Returns the “filenode” number currently assigned to the
specified relation. The filenode is the base component of the file
name(s) used for the relation (see
Section 70.1 for more information).
For most relations the result is the same as
pg_class .relfilenode ,
but for certain system catalogs relfilenode
is zero and this function must be used to get the correct value. The
function returns NULL if passed a relation that does not have storage,
such as a view.
|
pg_relation_filepath ( relation regclass )
→ text
Returns the entire file path name (relative to the database cluster's
data directory, PGDATA ) of the relation.
|
pg_filenode_relation ( tablespace oid , filenode oid )
→ regclass
Returns a relation's OID given the tablespace OID and filenode it is
stored under. This is essentially the inverse mapping of
pg_relation_filepath . For a relation in the
database's default tablespace, the tablespace can be specified as zero.
Returns NULL if no relation in the current database
is associated with the given values.
|
Table 9.94 lists functions used to manage
collations.
Table 9.94. Collation Management Functions
Function
Description
|
---|
pg_collation_actual_version ( oid )
→ text
Returns the actual version of the collation object as it is currently
installed in the operating system. If this is different from the
value in
pg_collation .collversion ,
then objects depending on the collation might need to be rebuilt. See
also ALTER COLLATION.
|
pg_import_system_collations ( schema regnamespace )
→ integer
Adds collations to the system
catalog pg_collation based on all the locales
it finds in the operating system. This is
what initdb uses; see
Section 24.2.2 for more details. If additional
locales are installed into the operating system later on, this
function can be run again to add collations for the new locales.
Locales that match existing entries
in pg_collation will be skipped. (But
collation objects based on locales that are no longer present in the
operating system are not removed by this function.)
The schema parameter would typically
be pg_catalog , but that is not a requirement; the
collations could be installed into some other schema as well. The
function returns the number of new collation objects it created.
Use of this function is restricted to superusers.
|
Table 9.95 lists functions that provide
information about the structure of partitioned tables.
Table 9.95. Partitioning Information Functions
Function
Description
|
---|
pg_partition_tree ( regclass )
→ setof record
( relid regclass ,
parentrelid regclass ,
isleaf boolean ,
level integer )
Lists the tables or indexes in the partition tree of the
given partitioned table or partitioned index, with one row for each
partition. Information provided includes the OID of the partition,
the OID of its immediate parent, a boolean value telling if the
partition is a leaf, and an integer telling its level in the hierarchy.
The level value is 0 for the input table or index, 1 for its
immediate child partitions, 2 for their partitions, and so on.
Returns no rows if the relation does not exist or is not a partition
or partitioned table.
|
pg_partition_ancestors ( regclass )
→ setof regclass
Lists the ancestor relations of the given partition,
including the relation itself. Returns no rows if the relation
does not exist or is not a partition or partitioned table.
|
pg_partition_root ( regclass )
→ regclass
Returns the top-most parent of the partition tree to which the given
relation belongs. Returns NULL if the relation
does not exist or is not a partition or partitioned table.
|
For example, to check the total size of the data contained in a
partitioned table measurement
, one could use the
following query:
SELECT pg_size_pretty(sum(pg_relation_size(relid))) AS total_size
FROM pg_partition_tree('measurement');
9.27.8. Index Maintenance Functions
Table 9.96 shows the functions
available for index maintenance tasks. (Note that these maintenance
tasks are normally done automatically by autovacuum; use of these
functions is only required in special cases.)
These functions cannot be executed during recovery.
Use of these functions is restricted to superusers and the owner
of the given index.
Table 9.96. Index Maintenance Functions
Function
Description
|
---|
brin_summarize_new_values ( index regclass )
→ integer
Scans the specified BRIN index to find page ranges in the base table
that are not currently summarized by the index; for any such range it
creates a new summary index tuple by scanning those table pages.
Returns the number of new page range summaries that were inserted
into the index.
|
brin_summarize_range ( index regclass , blockNumber bigint )
→ integer
Summarizes the page range covering the given block, if not already
summarized. This is
like brin_summarize_new_values except that it
only processes the page range that covers the given table block number.
|
brin_desummarize_range ( index regclass , blockNumber bigint )
→ void
Removes the BRIN index tuple that summarizes the page range covering
the given table block, if there is one.
|
gin_clean_pending_list ( index regclass )
→ bigint
Cleans up the “pending” list of the specified GIN index
by moving entries in it, in bulk, to the main GIN data structure.
Returns the number of pages removed from the pending list.
If the argument is a GIN index built with
the fastupdate option disabled, no cleanup happens
and the result is zero, because the index doesn't have a pending list.
See Section 67.4.1 and Section 67.5
for details about the pending list and fastupdate
option.
|
9.27.9. Generic File Access Functions
The functions shown in Table 9.97 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, unless the user is a superuser or is granted the role
pg_read_server_files
. Use a relative path for files in
the cluster directory, and a path matching the log_directory
configuration setting for log files.
Note that granting users the EXECUTE privilege on
pg_read_file()
, or related functions, allows them the
ability to read any file on the server that the database server process can
read; these functions bypass all in-database privilege checks. This means
that, for example, a user with such access is able to read the contents of
the pg_authid
table where authentication
information is stored, as well as read any table data in the database.
Therefore, granting access to these functions should be carefully
considered.
Some of these functions take an optional missing_ok
parameter, which specifies the behavior when the file or directory does
not exist. If true
, the function
returns NULL
or an empty result set, as appropriate.
If false
, an error is raised. The default
is false
.
Table 9.97. Generic File Access Functions
Function
Description
|
---|
pg_ls_dir ( dirname text [, missing_ok boolean , include_dot_dirs boolean ] )
→ setof text
Returns the names of all files (and directories and other special
files) in the specified
directory. The include_dot_dirs parameter
indicates whether “.” and “..” are to be
included in the result set; the default is to exclude them. Including
them can be useful when missing_ok
is true , to distinguish an empty directory from a
non-existent directory.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
|
pg_ls_logdir ()
→ setof record
( name text ,
size bigint ,
modification timestamp with time zone )
Returns the name, size, and last modification time (mtime) of each
ordinary file in the server's log directory. Filenames beginning with
a dot, directories, and other special files are excluded.
This function is restricted to superusers and members of
the pg_monitor role by default, but other users can
be granted EXECUTE to run the function.
|
pg_ls_waldir ()
→ setof record
( name text ,
size bigint ,
modification timestamp with time zone )
Returns the name, size, and last modification time (mtime) of each
ordinary file in the server's write-ahead log (WAL) directory.
Filenames beginning with a dot, directories, and other special files
are excluded.
This function is restricted to superusers and members of
the pg_monitor role by default, but other users can
be granted EXECUTE to run the function.
|
pg_ls_archive_statusdir ()
→ setof record
( name text ,
size bigint ,
modification timestamp with time zone )
Returns the name, size, and last modification time (mtime) of each
ordinary file in the server's WAL archive status directory
(pg_wal/archive_status ). Filenames beginning
with a dot, directories, and other special files are excluded.
This function is restricted to superusers and members of
the pg_monitor role by default, but other users can
be granted EXECUTE to run the function.
|
pg_ls_tmpdir ( [ tablespace oid ] )
→ setof record
( name text ,
size bigint ,
modification timestamp with time zone )
Returns the name, size, and last modification time (mtime) of each
ordinary file in the temporary file directory for the
specified tablespace .
If tablespace is not provided,
the pg_default tablespace is examined. Filenames
beginning with a dot, directories, and other special files are
excluded.
This function is restricted to superusers and members of
the pg_monitor role by default, but other users can
be granted EXECUTE to run the function.
|
pg_read_file ( filename text [, offset bigint , length bigint [, missing_ok boolean ]] )
→ text
Returns all or part of a text file, starting at the
given byte 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. If offset
and length are omitted, the entire file is
returned. The bytes read from the file are interpreted as a string in
the database's encoding; an error is thrown if they are not valid in
that encoding.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
|
pg_read_binary_file ( filename text [, offset bigint , length bigint [, missing_ok boolean ]] )
→ bytea
Returns all or part of a file. This function is identical to
pg_read_file except that it can read arbitrary
binary data, returning the result as bytea
not text ; accordingly, no encoding checks are performed.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
In combination with the convert_from function,
this function can be used to read a text file in a specified encoding
and convert to the database's encoding:
SELECT convert_from(pg_read_binary_file('file_in_utf8.txt'), 'UTF8');
|
pg_stat_file ( filename text [, missing_ok boolean ] )
→ record
( size bigint ,
access timestamp with time zone ,
modification timestamp with time zone ,
change timestamp with time zone ,
creation timestamp with time zone ,
isdir boolean )
Returns a record containing the file's size, last access time stamp,
last modification time stamp, last file status change time stamp (Unix
platforms only), file creation time stamp (Windows only), and a flag
indicating if it is a directory.
This function is restricted to superusers by default, but other users
can be granted EXECUTE to run the function.
|
9.27.10. Advisory Lock Functions
The functions shown in Table 9.98
manage advisory locks. For details about proper use of these functions,
see Section 13.3.5.
All these functions are intended to be used to lock application-defined
resources, 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 conflicting lock on the same resource
identifier, the functions will either wait until the resource becomes
available, or return a false
result, as appropriate for
the function.
Locks can be either shared or exclusive: a shared lock does not conflict
with other shared locks on the same resource, only with exclusive locks.
Locks can be taken at session level (so that they are held until released
or the session ends) or at transaction level (so that they are held until
the current transaction ends; there is no provision for manual release).
Multiple session-level lock requests stack, so that if the same resource
identifier is locked three times there must then be three unlock requests
to release the resource in advance of session end.
Table 9.98. Advisory Lock Functions
Function
Description
|
---|
pg_advisory_lock ( key bigint )
→ void
pg_advisory_lock ( key1 integer , key2 integer )
→ void
Obtains an exclusive session-level advisory lock, waiting if necessary.
|
pg_advisory_lock_shared ( key bigint )
→ void
pg_advisory_lock_shared ( key1 integer , key2 integer )
→ void
Obtains a shared session-level advisory lock, waiting if necessary.
|
pg_advisory_unlock ( key bigint )
→ boolean
pg_advisory_unlock ( key1 integer , key2 integer )
→ boolean
Releases a previously-acquired exclusive session-level advisory lock.
Returns true if the lock is successfully released.
If the lock was not held, false is returned, and in
addition, an SQL warning will be reported by the server.
|
pg_advisory_unlock_all ()
→ void
Releases all session-level advisory locks held by the current session.
(This function is implicitly invoked at session end, even if the
client disconnects ungracefully.)
|
pg_advisory_unlock_shared ( key bigint )
→ boolean
pg_advisory_unlock_shared ( key1 integer , key2 integer )
→ boolean
Releases a previously-acquired shared session-level advisory lock.
Returns true if the lock is successfully released.
If the lock was not held, false is returned, and in
addition, an SQL warning will be reported by the server.
|
pg_advisory_xact_lock ( key bigint )
→ void
pg_advisory_xact_lock ( key1 integer , key2 integer )
→ void
Obtains an exclusive transaction-level advisory lock, waiting if
necessary.
|
pg_advisory_xact_lock_shared ( key bigint )
→ void
pg_advisory_xact_lock_shared ( key1 integer , key2 integer )
→ void
Obtains a shared transaction-level advisory lock, waiting if
necessary.
|
pg_try_advisory_lock ( key bigint )
→ boolean
pg_try_advisory_lock ( key1 integer , key2 integer )
→ boolean
Obtains an exclusive session-level advisory lock if available.
This will either obtain the lock immediately and
return true , or return false
without waiting if the lock cannot be acquired immediately.
|
pg_try_advisory_lock_shared ( key bigint )
→ boolean
pg_try_advisory_lock_shared ( key1 integer , key2 integer )
→ boolean
Obtains a shared session-level advisory lock if available.
This will either obtain the lock immediately and
return true , or return false
without waiting if the lock cannot be acquired immediately.
|
pg_try_advisory_xact_lock ( key bigint )
→ boolean
pg_try_advisory_xact_lock ( key1 integer , key2 integer )
→ boolean
Obtains an exclusive transaction-level advisory lock if available.
This will either obtain the lock immediately and
return true , or return false
without waiting if the lock cannot be acquired immediately.
|
pg_try_advisory_xact_lock_shared ( key bigint )
→ boolean
pg_try_advisory_xact_lock_shared ( key1 integer , key2 integer )
→ boolean
Obtains a shared transaction-level advisory lock if available.
This will either obtain the lock immediately and
return true , or return false
without waiting if the lock cannot be acquired immediately.
|