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Backup and Restoration Control Functions

Updated at: Jul 15, 2020 GMT+08:00

Backup Control Functions

Backup control functions help online backup.

  • pg_create_restore_point(name text)

    Description: Creates a named point for performing the restore operation (restricted to system administrators).

    Return type: text

    Note: pg_create_restore_point creates a named transaction log record that can be used as a restoration 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 restoration will proceed. Avoid creating multiple restoration points with the same name, since restoration will stop at the first one whose name matches the restoration target.

  • pg_current_xlog_location()

    Description: Obtains the write position of the current transaction log.

    Return type: text

    Note: pg_current_xlog_location displays the write position of the current transaction log in the same format as those of the previous functions. Read-only operations do not require rights of the system administrator.

  • pg_current_xlog_insert_location()

    Description: Obtains the insert position of the current transaction log.

    Return type: text

    Note: pg_current_xlog_insert_location displays the insert position of the current transaction log. The insertion point is the logical end of the transaction log at any instant, while the write location is the end of what has been written out from the server's internal buffers. The write position is the end that can be detected externally from the server. This operation can be performed to archive only some of completed transaction log files. The insert position is mainly used for commissioning the server. Read-only operations do not require rights of the system administrator.

  • pg_start_backup(label text [, fast boolean ])

    Description: Starts executing online backup (restricted to system administrators or replication roles).

    Return type: text

    Note: pg_start_backup receives a user-defined backup label (usually the name of the position where the backup dump file is stored). This function writes a backup label file to the data directory of the database cluster and then returns the starting position of backed up transaction logs in text mode.

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    SELECT pg_start_backup('label_goes_here');
     pg_start_backup
    -----------------
     0/3000020
    (1 row)
    
  • pg_stop_backup()

    Description: Completes online backup (restricted to system administrators or replication roles).

    Return type: text

    Note: pg_stop_backup deletes 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. After the ending position is calculated, the insert position of the current transaction log automatically goes ahead to the next transaction log file. This way, the ended transaction log file can be immediately archived so that backup is complete.

  • pg_switch_xlog()

    Description: Switches to a new transaction log file (restricted to system administrators).

    Return type: text

    Note: pg_switch_xlog moves to the next transaction log file so that the current log file can be archived (if continuous archive is used). 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 switchover, pg_switch_xlog will do nothing but return the start location of the transaction log file currently in use.

  • pg_xlogfile_name(location text)

    Description: Converts the position string in a transaction log to a file name.

    Return type: text

    Note: pg_xlogfile_name extracts only the transaction log file name. If the given transaction log position is the transaction log file border, a transaction log file name will be returned for both the two functions. This is usually the desired behavior for managing transaction log archiving, since the preceding file is the last one that currently needs to be archived.

  • pg_xlogfile_name_offset(location text)

    Description: Converts the position string in a transaction log to a file name and returns the byte offset in the file.

    Return type: text, integer

    Note: pg_xlogfile_name_offset can extract transaction log file names and byte offsets from the returned results of the preceding functions. For example:

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    SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
    NOTICE:  pg_stop_backup cleanup done, waiting for required WAL segments to be archived
    NOTICE:  pg_stop_backup complete, all required WAL segments have been archived
            file_name         | file_offset 
    --------------------------+-------------
    000000010000000000000003  |         272
    (1 row)
    
  • pg_xlog_location_diff(location text, location text)

    Description: pg_xlog_location_diff calculates the difference in bytes between two transaction log locations.

    Return type: numeric

  • pgxc_get_senders_catchup_time()

    Description: Displays the catchup information of the currently active primary/standby instance sending thread on all DNs.

    Return type: record

    The following information is returned:

    Table 1 pgxc_get_senders_catchup_time() columns

    Name

    Type

    Description

    node_name

    text

    Node name

    lwpid

    integer

    Current sender lwpid

    local_role

    text

    Local role

    peer_role

    text

    Peer role

    state

    text

    Current sender's replication status

    sender

    text

    Current sender type

    catchup_start

    timestamp with time zone

    Startup time of a catchup task

    catchup_end

    timestamp with time zone

    End time of a catchup task

Restoration Control Functions

Restoration control functions provide information about the status of standby nodes. These functions may be executed both during restoration and in normal running.

  • pg_is_in_recovery()

    Description: Returns true if restoration is still in progress.

    Return type: bool

  • pg_last_xlog_receive_location()

    Description: Gets the last transaction log location received and synchronized to disk by streaming replication. While streaming replication is in progress, this will increase monotonically. If restoration has completed, then this value will remain static at the value of the last WAL record received and synchronized to disk during restoration. If streaming replication is disabled or if not yet started, the function return will return NULL.

    Return type: text

  • pg_last_xlog_replay_location()

    Description: Gets last transaction log location replayed during restoration. If restoration is still in progress, this will increase monotonically. If restoration has completed, then this value will remain static at the value of the last WAL record received during that restoration. When the server has been started normally without restoration, the function returns NULL.

    Return type: text

  • pg_last_xact_replay_timestamp()

    Description: Gets the timestamp of last transaction replayed during restoration. This is the time to commit a transaction or abort a WAL record on the primary node. If no transactions have been replayed during restoration, this function will return NULL. Otherwise, if restoration is still in progress, this will increase monotonically. If restoration has completed, then this value will remain static at the value of the last WAL record received during that restoration. If the server normally starts without manual intervention, this function will return NULL.

    Return type: timestamp with time zone

Restoration control functions control restoration processes. These functions may be executed only during restoration.

  • pg_is_xlog_replay_paused()

    Description: Returns true if restoration is paused.

    Return type: bool

  • pg_xlog_replay_pause()

    Description: Pauses restoration immediately.

    Return type: void

  • pg_xlog_replay_resume()

    Description: Restarts restoration if it was paused.

    Return type: void

While restoration is paused, no further database changes are applied. In hot standby mode, all new queries will see the same consistent snapshot of the database, and no further query conflicts will be generated until restoration 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. This progress depends on the duration of the pause, the rate of WAL generation, and available disk space.

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