Postgres-XC 1.1 Documentation | ||||
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The functions described in this section are used to control and monitor a PostgreSQL installation.
Table 9-57 shows the functions available to query and alter run-time configuration parameters.
Table 9-57. Configuration Settings Functions
Name | Return Type | Description |
---|---|---|
current_setting(setting_name)
| text | get 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)
Note: The following description applies only to Postgres-XC
Similar to the SET command, the set_config() function makes sure that as long as the session/transaction is active, the new parameter value is set across all the nodes on which the SQL queries are being run as part of that particular session or transaction.
The functions shown in Table 9-58 send control signals to other server processes. Use of these functions is usually restricted to superusers, with noted exceptions.
Table 9-58. Server Signalling Functions
Name | Return Type | Description |
---|---|---|
pg_cancel_backend(pid int)
| boolean | Cancel a backend's current query. You can execute this against another backend that has exactly the same role as the user calling the function. In all other cases, you must be a superuser. |
pg_reload_conf()
| boolean | Cause server processes to reload their configuration files |
pg_rotate_logfile()
| boolean | Rotate server's log file |
pg_terminate_backend(pid int)
| boolean | Terminate a backend. You can execute this against another backend that has exactly the same role as the user calling the function. In all other cases, you must be a superuser. |
Each of these functions returns true if successful and false otherwise.
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_reload_conf
sends a SIGHUP signal
to the server, causing 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.
Note: The following description applies only to Postgres-XC
Please note that these functions works just locally. To issue these functions to another Coordinators or Datanodes, you should issue these functions through EXECUTE DIRECT statement.
The functions shown in Table 9-59 assist in making on-line backups.
These functions cannot be executed during recovery (except
pg_xlog_location_diff
).
Table 9-59. Backup Control Functions
Name | Return Type | Description |
---|---|---|
pg_create_restore_point(name text)
| text | Create a named point for performing restore (restricted to superusers) |
pg_current_xlog_insert_location()
| text | Get current transaction log insert location |
pg_current_xlog_location()
| text | Get current transaction log write location |
pg_start_backup(label text [, fast boolean ])
| text | Prepare for performing on-line backup (restricted to superusers or replication roles) |
pg_stop_backup()
| text | Finish performing on-line backup (restricted to superusers or replication roles) |
pg_switch_xlog()
| text | Force switch to a new transaction log file (restricted to superusers) |
pg_xlogfile_name(location text)
| text | Convert transaction log location string to file name |
pg_xlogfile_name_offset(location text)
| text, integer | Convert transaction log location string to file name and decimal byte offset within file |
pg_xlog_location_diff(location text, location text)
| numeric | Calculate the difference between two transaction log locations |
pg_start_backup
accepts 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 (backup_label) into the
database cluster's data directory, performs a checkpoint,
and then returns the backup's starting transaction log location as text.
The user can ignore this result value, but it is
provided in case it is useful.
postgres=# select pg_start_backup('label_goes_here'); pg_start_backup ----------------- 0/D4445B8 (1 row)
There is an optional second parameter of type boolean. If 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.
pg_stop_backup
removes the label file created by
pg_start_backup
, and 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
can be ignored). After recording 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 return value 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_create_restore_point
creates a named transaction log
record that can be used as recovery target, and returns the corresponding
transaction 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.
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.
pg_xlog_location_diff
calculates the difference in bytes
between two transaction log locations. It can be used with
pg_stat_replication or some functions shown in
Table 9-59 to get the replication lag.
For details about proper usage of these functions, see Section 23.3.
Note: The following description applies only to Postgres-XC
Please note that these functions works just locally. To issue these functions to another Coordinators or Datanodes, you should issue these functions through EXECUTE DIRECT statement.
The functions shown in Table 9-60 provide information about the current status of the standby. These functions may be executed both during recovery and in normal running.
Table 9-60. Recovery Information Functions
Name | Return Type | Description |
---|---|---|
pg_is_in_recovery()
| bool | True if recovery is still in progress. |
pg_last_xlog_receive_location()
| text | Get last transaction log location received and synced to disk by streaming replication. While streaming replication is in progress this will increase monotonically. If recovery has completed this will remain static at the value 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_xlog_replay_location()
| text | Get last transaction log location replayed during recovery. If recovery is still in progress this will increase monotonically. If recovery has completed then this value will remain static at the value of the last WAL record applied during that recovery. When the server has been started normally without recovery the function returns NULL. |
pg_last_xact_replay_timestamp()
| timestamp with time zone | Get time stamp of 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, this function returns NULL. Otherwise, if recovery is still in progress this will increase monotonically. If recovery has completed then this value will remain static at the value of the last transaction applied during that recovery. When the server has been started normally without recovery the function returns NULL. |
The functions shown in Table 9-61 control the progress of recovery. These functions may be executed only during recovery.
Table 9-61. Recovery Control Functions
Name | Return Type | Description |
---|---|---|
pg_is_xlog_replay_paused()
| bool | True if recovery is paused. |
pg_xlog_replay_pause()
| void | Pauses recovery immediately. |
pg_xlog_replay_resume()
| void | Restarts recovery if it was paused. |
Note: The following description applies only to Postgres-XC
Please note that these functions works just locally. To issue these functions to another Coordinators or Datanodes, you should issue these functions through EXECUTE DIRECT statement.
While recovery is paused no further database changes are applied. If in hot standby, all new queries will see the same consistent snapshot of the database, and no further query conflicts will be generated until recovery is resumed.
If streaming replication is disabled, the paused state may continue indefinitely without problem. While streaming replication is in progress 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.
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 9-62, and
imported with the SET TRANSACTION command.
Table 9-62. Snapshot Synchronization Functions
Name | Return Type | Description |
---|---|---|
pg_export_snapshot()
| text | Save the current snapshot and return its identifier |
The function pg_export_snapshot
saves the 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.
See SET TRANSACTION for details of how to use an exported snapshot.
The functions shown in Table 9-63 calculate the disk space usage of database objects.
Table 9-63. 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_indexes_size(regclass)
| bigint | Total disk space used by indexes attached to the specified table |
pg_relation_size(relation regclass, fork text)
| bigint | Disk space used by the specified fork ('main', 'fsm' or 'vm') of the specified table or index |
pg_relation_size(relation regclass)
| bigint | Shorthand for pg_relation_size(..., 'main') |
pg_size_pretty(bigint)
| text | Converts a size in bytes expressed as a 64-bit integer into a human-readable format with size units |
pg_size_pretty(numeric)
| text | Converts a size in bytes expressed as a numeric value into a human-readable format with size units |
pg_table_size(regclass)
| bigint | Disk space used by the specified table, excluding indexes (but including TOAST, free space map, and visibility map) |
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(regclass)
| bigint | Total disk space used by the specified table, including all indexes and TOAST data |
Note: The following description applies only to Postgres-XC
The object size functions pg_database_size, pg_indexes_size, pg_relation_size, pg_table_size, and pg_total_relation_size return the cumulative size from all the Datanodes. For e.g., pg_relation_size returns the sum of disk space used up by the specified fork at all the Datanodes where the table is distributed or replicated. If the table is replicated on 3 tables, the size will be 3 times that of individual nodes. If you need to retrieve the local results from a particular Coordinator or Datanode, you should issue these function calls explicitly through EXECUTE DIRECT statement. All other system functions run locally at the Coordinator, unless explicitly specified otherwise in this document.
Note: The following description applies only to Postgres-XC
Please note that these functions works just locally. To issue these functions to another Coordinators or Datanodes, you should issue these functions through EXECUTE DIRECT statement.
pg_column_size
shows the space used to store any individual
data value.
pg_total_relation_size
accepts the OID or name of a
table or toast table, and returns the total on-disk space used for
that table, including all associated indexes. This function is
equivalent to pg_table_size
+ pg_indexes_size
.
pg_table_size
accepts the OID or name of a table and
returns the disk space needed for that table, exclusive of indexes.
(TOAST space, free space map, and visibility map are included.)
pg_indexes_size
accepts the OID or name of a table and
returns the total disk space used by all the indexes attached to that
table.
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 on-disk size in bytes. Specifying
'main' or leaving out the second argument returns the
size of the main data fork of the relation. Specifying
'fsm' returns the size of the
Free Space Map (see Section 58.3) associated with the
relation. Specifying 'vm' returns the size of the
Visibility Map (see Section 58.4) associated with the
relation. Note that this function shows the size of only one fork;
for most purposes it is more convenient to use the higher-level
functions pg_total_relation_size
or
pg_table_size
.
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 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. Just write the table name enclosed in single quotes so that it looks like a literal constant. For compatibility with the handling of ordinary SQL names, the string will be converted to lower case unless it contains double quotes around the table name.
If an OID that does not represent an existing object is passed as argument to one of the above functions, NULL is returned.
The functions shown in Table 9-64 assist in identifying the specific disk files associated with database objects.
Table 9-64. Database Object Location Functions
Name | Return Type | Description |
---|---|---|
pg_relation_filenode(relation regclass)
| oid | Filenode number of the specified relation |
pg_relation_filepath(relation regclass)
| text | File path name of the specified relation |
pg_relation_filenode
accepts the OID or name of a table,
index, sequence, or toast table, and returns the "filenode" number
currently assigned to it. The filenode is the base component of the file
name(s) used for the relation (see Section 58.1
for more information). For most tables 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
is similar to
pg_relation_filenode
, but it returns the entire file path name
(relative to the database cluster's data directory PGDATA) of
the relation.
The functions shown in Table 9-65 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. Use a relative path for files in the cluster directory, and a path matching the log_directory configuration setting for log files. Use of these functions is restricted to superusers.
Table 9-65. 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_read_binary_file(filename text [, offset bigint, length bigint])
| bytea | Return the contents of a 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.
If offset and length are omitted, the entire
file is returned. The bytes read from the file are interpreted as a string
in the server encoding; an error is thrown if they are not valid in that
encoding.
pg_read_binary_file
is similar to
pg_read_file
, except that the result is a bytea value;
accordingly, no encoding checks are performed.
In combination with the convert_from
function, this function
can be used to read a file in a specified encoding:
SELECT convert_from(pg_read_binary_file('file_in_utf8.txt'), 'UTF8');
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 9-66 manage advisory locks. For details about proper use of these functions, see Section 13.3.4.
Table 9-66. Advisory Lock Functions
Name | Return Type | Description |
---|---|---|
pg_advisory_lock(key bigint)
| void | Obtain exclusive session level advisory lock |
pg_advisory_lock(key1 int, key2 int)
| void | Obtain exclusive session level advisory lock |
pg_advisory_lock_shared(key bigint)
| void | Obtain shared session level advisory lock |
pg_advisory_lock_shared(key1 int, key2 int)
| void | Obtain shared session level advisory lock |
pg_advisory_unlock(key bigint)
| boolean | Release an exclusive session level advisory lock |
pg_advisory_unlock(key1 int, key2 int)
| boolean | Release an exclusive session level advisory lock |
pg_advisory_unlock_all()
| void | Release all session level advisory locks held by the current session |
pg_advisory_unlock_shared(key bigint)
| boolean | Release a shared session level advisory lock |
pg_advisory_unlock_shared(key1 int, key2 int)
| boolean | Release a shared session level advisory lock |
pg_advisory_xact_lock(key bigint)
| void | Obtain exclusive transaction level advisory lock |
pg_advisory_xact_lock(key1 int, key2 int)
| void | Obtain exclusive transaction level advisory lock |
pg_advisory_xact_lock_shared(key bigint)
| void | Obtain shared transaction level advisory lock |
pg_advisory_xact_lock_shared(key1 int, key2 int)
| void | Obtain shared transaction level advisory lock |
pg_try_advisory_lock(key bigint)
| boolean | Obtain exclusive session level advisory lock if available |
pg_try_advisory_lock(key1 int, key2 int)
| boolean | Obtain exclusive session level advisory lock if available |
pg_try_advisory_lock_shared(key bigint)
| boolean | Obtain shared session level advisory lock if available |
pg_try_advisory_lock_shared(key1 int, key2 int)
| boolean | Obtain shared session level advisory lock if available |
pg_try_advisory_xact_lock(key bigint)
| boolean | Obtain exclusive transaction level advisory lock if available |
pg_try_advisory_xact_lock(key1 int, key2 int)
| boolean | Obtain exclusive transaction level advisory lock if available |
pg_try_advisory_xact_lock_shared(key bigint)
| boolean | Obtain shared transaction level advisory lock if available |
pg_try_advisory_xact_lock_shared(key1 int, key2 int)
| boolean | Obtain shared transaction level advisory lock if available |
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 identifier,
this 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 then be 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 immediately.
pg_try_advisory_lock_shared
works the same as
pg_try_advisory_lock
, except it attempts to acquire
a shared rather than an exclusive lock.
pg_advisory_unlock
will release a previously-acquired
exclusive session level advisory lock. It
returns true if the lock is successfully released.
If the lock was not held, it will return false,
and in addition, an SQL warning will be reported by the server.
pg_advisory_unlock_shared
works the same as
pg_advisory_unlock
,
except it releases a shared session level advisory lock.
pg_advisory_unlock_all
will release 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_xact_lock
works the same as
pg_advisory_lock
, except the lock is automatically released
at the end of the current transaction and cannot be released explicitly.
pg_advisory_xact_lock_shared
works the same as
pg_advisory_lock_shared
, except the lock is automatically released
at the end of the current transaction and cannot be released explicitly.
pg_try_advisory_xact_lock
works the same as
pg_try_advisory_lock
, except the lock, if acquired,
is automatically released at the end of the current transaction and
cannot be released explicitly.
pg_try_advisory_xact_lock_shared
works the same as
pg_try_advisory_lock_shared
, except the lock, if acquired,
is automatically released at the end of the current transaction and
cannot be released explicitly.
Note: The following description applies only to Postgres-XC
The advisory lock functions are aware of the Postgres XC cluster. Hence, if you use a function like pg_advisory_lock() from a particular Coordinator, the resource will be locked across the complete cluster, so another application calling the same function from a different Coordinator will see this lock, and will wait on the resource until the lock is released. This applies to both transaction and session level advisory locks.
Note: The following description applies only to Postgres-XC
The functions shown in Table 9-67 manage Postgres-XC pooler. For details about Postgres-XC pooler, see Section 46.2.6.
Table 9-67. Postgres-XC pooler functions
Name | Return Type | Description |
---|---|---|
pgxc_pool_check()
| boolean | Check if connection data cached in pooler is consistent with pgxc_node. |
pgxc_pool_reload()
| boolean | Reload connection data cached in pooler and reload sessions in server |
pgxc_pool_check
verifies if connection data cached in Postgres-XC pooler
is consistent with pgxc_node
catalog. Data checked for consistency is Node Oid (pgxc_node.oid),
node port (pgxc_node.node_port)
and node host (pgxc_node.node_host).
pgxc_pool_reload
reloads connection data cached in pooler from
pgxc_node catalog
and reloads all the information info cached in pooler. All the active transactions
are aborted and all existing pooler connections are dropped. This results in having
all the temporary and prepared objects dropped on remote and local node for session.
Note: The following description applies only to Postgres-XC
The functions shown in Table 9-68 manage addition of a new node to Postgres-XC cluster.
Table 9-68. Postgres-XC functions to manage addition of a new node
Name | Return Type | Description |
---|---|---|
pgxc_lock_for_backup()
| boolean | Locks the cluster for taking backup that would be restored on the new node to be added. |
pgxc_lock_for_backup
locks the cluster for taking backup using pg_dump/pg_dumpall.
Locking means that we disallow the statemets that change the portions of the catalog which are
backed up by pg_dump/pg_dumpall. When locked only the following utility statemets are allowed.
Please note that this function does not impact SELECTs or DMLs.
Table 9-69. Utility statements allowed while the cluster is locked for backup
Statement |
---|
EXECUTE |
CREATE NODE |
START TRANSACTION |
BEGIN |
COMMIT |
ROLLBACK |
PREPARE TRANSACTION |
COMMIT PREPARED |
ROLLBACK PREPARED |
DECLARE CURSOR |
CLOSE CURSOR |
FETCH |
TRUNCATE |
COPY |
PREPARE |
DEALLOCATE |
DO |
NOTIFY |
LISTEN |
UNLISTEN |
LOAD |
CLUSTER |
VACUUM |
EXPLAIN |
SET |
SHOW |
DISCARD |
LOCK |
SET CONSTRAINTS |
CHECKPOINT |
CREATE BARRIER |
REINDEX |
CLEAN CONNECTION |
To lock the cluster for backup while adding a new node Postgres-XC uses advisory locks.
Every time a disallowed statement is issued the system tries to acquire a transaction
level advisory lock in shared mode and the lock is released when
the DDL or the transaction issuing the DDL ends. The function pgxc_lock_for_backup
tries to acquire the same advisory lock in exclusive mode at session level. It is therefore necessary
to keep the session issuing pgxc_lock_for_backup
alive as long as the issuer wants
the system to keep the lock.
Postgres-XC uses the key pair (0xFFFF, 0xFFFF) as object ID while using
advisory lock for backup.
The function fails to acquire the lock if any one of the following is true:
The caller of the function is not a superuser.
There are some uncommitted prepared transactions, because they might contain any utility statement belonging to the disallowed group.
The lock is already held by an uncommitted transaction that has issued a utility statement belonging to the disallowed group.
The lock is already held by a previous call to the same function and current request is being issued from a different session.