Postgres-XC 1.1 Documentation
Prev | Up | Chapter 9. Functions and Operators | Next

# 9.26. System Administration Functions

The functions described in this section are used to control and monitor a PostgreSQL installation.

## 9.26.1. Configuration Settings Functions

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.

## 9.26.2. Server Signalling Functions

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.

## 9.26.3. Backup Control Functions

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.

## 9.26.4. Recovery Control Functions

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.

## 9.26.5. 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 9-62, 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.

## 9.26.6. Database Object Management Functions

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.

## 9.26.7. Generic File Access Functions

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;

## 9.26.8. Advisory Lock Functions

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.

Prev | Home | Next
System Information Functions | Up | Trigger Functions