The functions described in this section are used to control and monitor a PostgreSQL installation.
Table 9.77 shows the functions available to query and alter run-time configuration parameters.
Table 9.77. Configuration Settings Functions
    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)
    If there is no setting named setting_name,
    current_setting throws an error
    unless missing_ok is supplied and is
    true.
   
    set_config sets the parameter
    setting_name to
    new_value.  If
    is_local is true, the
    new value will only apply to the current transaction. If you want
    the new value to apply for the current session, use
    false instead. The function corresponds to the
    SQL command SET. An example:
SELECT set_config('log_statement_stats', 'off', false);
 set_config
------------
 off
(1 row)
    The functions shown in Table 9.78 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.
   
Table 9.78. Server Signaling Functions
| Name | Return Type | Description | 
|---|---|---|
|  | boolean | Cancel a backend's current query.  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. | 
|  | boolean | Cause server processes to reload their configuration files | 
|  | boolean | Rotate server's log file | 
|  | boolean | Terminate a backend.  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. | 
    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.
   
    The functions shown in Table 9.79 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).
   
Table 9.79. Backup Control Functions
| Name | Return Type | Description | 
|---|---|---|
|  | pg_lsn | Create a named point for performing restore (restricted to superusers by default, but other users can be granted EXECUTE to run the function) | 
|  | pg_lsn | Get current write-ahead log flush location | 
|  | pg_lsn | Get current write-ahead log insert location | 
|  | pg_lsn | Get current write-ahead log write location | 
|  | pg_lsn | Prepare for performing on-line backup (restricted to superusers by default, but other users can be granted EXECUTE to run the function) | 
|  | pg_lsn | Finish performing exclusive on-line backup (restricted to superusers by default, but other users can be granted EXECUTE to run the function) | 
|  | setof record | Finish performing exclusive or non-exclusive on-line backup (restricted to superusers by default, but other users can be granted EXECUTE to run the function) | 
|  | bool | True if an on-line exclusive backup is still in progress. | 
|  | timestamp with time zone | Get start time of an on-line exclusive backup in progress. | 
|  | pg_lsn | Force switch to a new write-ahead log file (restricted to superusers by default, but other users can be granted EXECUTE to run the function) | 
|  | text | Convert write-ahead log location to file name | 
|  | text,integer | Convert write-ahead log location to file name and decimal byte offset within file | 
|  | numeric | Calculate the difference between two write-ahead 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.) When used in exclusive mode, the 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,
    performs a checkpoint, and then returns the backup's starting write-ahead
    log location as text.  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 written to the backup
    by the caller.
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.
   
    In an exclusive backup, pg_stop_backup removes the label file
    and, if it exists, the tablespace_map file created by
    pg_start_backup. In a non-exclusive backup, the contents of
    the backup_label and tablespace_map are returned
    in the result of the function, and should be written to files in the
    backup (and not in the data directory).  There is an optional second
    parameter of type boolean.  If false, the pg_stop_backup
    will return immediately after the backup is completed without waiting for
    WAL to be archived.  This behavior is only useful for backup
    software which independently monitors WAL archiving. Otherwise, WAL
    required to make the backup consistent might be missing and make the backup
    useless.  When this parameter is set to true, pg_stop_backup
    will wait for WAL to be archived when archiving is enabled; on the 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, the 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.  The return
    value is the backup's ending write-ahead log location (which again
    can be ignored).  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.
   
    pg_switch_wal moves to the next write-ahead log file, allowing the
    current file to be archived (assuming you are using continuous archiving).
    The return value is the ending write-ahead log location + 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.
   
    pg_create_restore_point creates a named write-ahead log
    record that can be used as 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.
   
    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 flush
    location is the location guaranteed 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 both 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 the results of any of the
    above functions.  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.
   
    pg_wal_lsn_diff calculates the difference in bytes
    between two write-ahead log locations. It can be used with
    pg_stat_replication or some functions shown in
    Table 9.79 to get the replication lag.
   
For details about proper usage of these functions, see Section 25.3.
The functions shown in Table 9.80 provide information about the current status of the standby. These functions may be executed both during recovery and in normal running.
Table 9.80. Recovery Information Functions
| Name | Return Type | Description | 
|---|---|---|
|  | bool | True if recovery is still in progress. | 
|  | pg_lsn | Get last write-ahead 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_lsn | Get last write-ahead 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. | 
|  | 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 9.81 control the progress of recovery. These functions may be executed only during recovery.
Table 9.81. Recovery Control Functions
| Name | Return Type | Description | 
|---|---|---|
|  | bool | True if recovery is paused. | 
|  | void | Pauses recovery immediately (restricted to superusers by default, but other users can be granted EXECUTE to run the function). | 
|  | void | Restarts recovery if it was paused (restricted to superusers by default, but other users can be granted EXECUTE to run the function). | 
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 9.82, and
    imported with the SET TRANSACTION command.
   
Table 9.82. Snapshot Synchronization Functions
| Name | Return Type | Description | 
|---|---|---|
|  | 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 9.83 are for controlling and interacting with replication features. See Section 26.2.5, Section 26.2.6, and Chapter 50 for information about the underlying features. Use of these functions is restricted to superusers.
Many of these functions have equivalent commands in the replication protocol; see Section 53.4.
The functions described in Section 9.26.3, Section 9.26.4, and Section 9.26.5 are also relevant for replication.
Table 9.83. Replication SQL Functions
| Function | Return Type | Description | 
|---|---|---|
|  | ( slot_namename,lsnpg_lsn) | Creates a new physical replication slot named slot_name. The optional second parameter,
        whentrue, 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 current session. Temporary slots are also
        released upon any error. This function corresponds
        to the replication protocol commandCREATE_REPLICATION_SLOT
        ... PHYSICAL. | 
|  | void | Drops the physical or logical replication slot
        named slot_name. Same as replication protocol
        commandDROP_REPLICATION_SLOT. For logical slots, this must
        be called when connected to the same database the slot was created on. | 
|  | ( slot_namename,lsnpg_lsn) | Creates a new logical (decoding) replication slot named slot_nameusing the output pluginplugin. 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 current session. Temporary slots are also
        released upon any error. A call to this function has the same
        effect as the replication protocol commandCREATE_REPLICATION_SLOT ... LOGICAL. | 
|  | ( lsnpg_lsn,xidxid,datatext) | Returns changes in the slot slot_name, starting
        from the point at which since changes have been consumed last.  Ifupto_lsnandupto_nchangesare NULL,
        logical decoding will continue until end of WAL.  Ifupto_lsnis non-NULL, decoding will include only
        those transactions which commit prior to the specified LSN.  Ifupto_nchangesis 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. | 
|  | ( lsnpg_lsn,xidxid,datatext) | 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. | 
|  | ( lsnpg_lsn,xidxid,databytea) | Behaves just like
        the pg_logical_slot_get_changes()function,
        except that changes are returned asbytea. | 
|  | ( lsnpg_lsn,xidxid,databytea) | Behaves just like
        the pg_logical_slot_get_changes()function,
        except that changes are returned asbyteaand that
        changes are not consumed; that is, they will be returned again
        on future calls. | 
|  | ( slot_namename,end_lsnpg_lsn)bool | 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
        name of the slot and real position to which it was advanced to. | 
|  | oid | Create a replication origin with the given external name, and return the internal id assigned to it. | 
|  | void | Delete a previously created replication origin, including any associated replay progress. | 
|  | oid | Lookup a replication origin by name and return the internal id. If no corresponding replication origin is found an error is thrown. | 
|  | void | Mark the current session as replaying from the given
        origin, allowing replay progress to be tracked.  Use pg_replication_origin_session_resetto revert.
        Can only be used if no previous origin is configured. | 
|  | void | Cancel the effects
        of pg_replication_origin_session_setup(). | 
|  | bool | Has a replication origin been configured in the current session? | 
|  | pg_lsn | Return the replay location for the replication origin configured in
        the current session. The parameter flushdetermines whether the corresponding local transaction will be
        guaranteed to have been flushed to disk or not. | 
|  | void | Mark 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 previously been
        configured using pg_replication_origin_session_setup(). | 
|  | void | Cancel the effects of pg_replication_origin_xact_setup(). | 
| pg_replication_origin_advance | void | Set replication progress for the given node to the given location. This primarily is useful for setting up the initial location or a new location after configuration changes and similar. Be aware that careless use of this function can lead to inconsistently replicated data. | 
|  | pg_lsn | Return the replay location for the given replication origin. The
        parameter flushdetermines whether the
        corresponding local transaction will be guaranteed to have been
        flushed to disk or not. | 
|  | pg_lsn | Emit text logical decoding message. This can be used to pass generic
        messages to logical decoding plugins through WAL. The parameter transactionalspecifies if the message should
        be part of current transaction or if it should be written immediately
        and decoded as soon as the logical decoding reads the record. Theprefixis textual prefix used by the logical
        decoding plugins to easily recognize interesting messages for them.
        Thecontentis the text of the message. | 
|  | pg_lsn | Emit binary logical decoding message. This can be used to pass generic
        messages to logical decoding plugins through WAL. The parameter transactionalspecifies if the message should
        be part of current transaction or if it should be written immediately
        and decoded as soon as the logical decoding reads the record. Theprefixis textual prefix used by the logical
        decoding plugins to easily recognize interesting messages for them.
        Thecontentis the binary content of the
        message. | 
The functions shown in Table 9.84 calculate the disk space usage of database objects.
Table 9.84. Database Object Size Functions
| Name | Return Type | Description | 
|---|---|---|
|  | int | Number of bytes used to store a particular value (possibly compressed) | 
|  | bigint | Disk space used by the database with the specified OID | 
|  | bigint | Disk space used by the database with the specified name | 
|  | bigint | Total disk space used by indexes attached to the specified table | 
|  | bigint | Disk space used by the specified fork ( 'main','fsm','vm', or'init')
        of the specified table or index | 
|  | bigint | Shorthand for pg_relation_size(..., 'main') | 
|  | bigint | Converts a size in human-readable format with size units into bytes | 
|  | text | Converts a size in bytes expressed as a 64-bit integer into a human-readable format with size units | 
|  | text | Converts a size in bytes expressed as a numeric value into a human-readable format with size units | 
|  | bigint | Disk space used by the specified table, excluding indexes (but including TOAST, free space map, and visibility map) | 
|  | bigint | Disk space used by the tablespace with the specified OID | 
|  | bigint | Disk space used by the tablespace with the specified name | 
|  | bigint | Total disk space used by the specified table, including all indexes and TOAST data | 
    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.  To use pg_database_size,
    you must have CONNECT permission on the specified database
    (which is granted by default), or be a member of the pg_read_all_stats
    role. To use pg_tablespace_size, you must have
    CREATE permission 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_relation_size accepts the OID or name of a table, index
    or toast table, and returns the on-disk size in bytes of one fork of
    that 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, it 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 68.3) associated with the relation.
      
       'vm' returns the size of the Visibility Map
       (see Section 68.4) associated with the relation.
      
       'init' returns the size of the initialization
       fork, if any, associated with the relation.
      
    pg_size_pretty can be used to format the result of one of
    the other functions in a human-readable way, using bytes, kB, MB, GB or TB
    as appropriate.
   
    pg_size_bytes can be used to get the size in bytes from a
    string in human-readable format. The input may have units of bytes, kB,
    MB, GB or TB, and is parsed case-insensitively. If no units are specified,
    bytes are assumed.
   
     The units kB, MB, GB and TB used by the functions
     pg_size_pretty and pg_size_bytes are defined
     using powers of 2 rather than powers of 10, so 1kB is 1024 bytes, 1MB is
     10242 = 1048576 bytes, and so on.
    
    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 9.85 assist in identifying the specific disk files associated with database objects.
Table 9.85. Database Object Location Functions
| Name | Return Type | Description | 
|---|---|---|
|  | oid | Filenode number of the specified relation | 
|  | text | File path name of the specified relation | 
|  | regclass | Find the relation associated with a given tablespace and filenode | 
    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 68.1
    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.
   
    pg_filenode_relation is the reverse of
    pg_relation_filenode. Given a “tablespace” OID and
    a “filenode”, it returns the associated relation's OID. For a table
    in the database's default tablespace, the tablespace can be specified as 0.
   
Table 9.86 lists functions used to manage collations.
Table 9.86. Collation Management Functions
    pg_collation_actual_version 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 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 23.2.2 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.
   
Table 9.87 shows the functions available for index maintenance tasks. These functions cannot be executed during recovery. Use of these functions is restricted to superusers and the owner of the given index.
Table 9.87. Index Maintenance Functions
| Name | Return Type | Description | 
|---|---|---|
|  | integer | summarize page ranges not already summarized | 
|  | integer | summarize the page range covering the given block, if not already summarized | 
|  | integer | de-summarize the page range covering the given block, if summarized | 
|  | bigint | move GIN pending list entries into main index structure | 
    brin_summarize_new_values accepts the OID or name of a
    BRIN index and inspects the 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 the table pages.
    It returns the number of new page range summaries that were inserted
    into the index.  brin_summarize_range does the same, except
    it only summarizes the range that covers the given block number.
   
    gin_clean_pending_list accepts the OID or name of
    a GIN index and cleans up the pending list of the specified index
    by moving entries in it to the main GIN data structure in bulk.
    It returns the number of pages removed from the pending list.
    Note that if the argument is a GIN index built with
    the fastupdate option disabled, no cleanup happens and the
    return value is 0, because the index doesn't have a pending list.
    Please see Section 66.4.1 and Section 66.5
    for details of the pending list and fastupdate option.
   
    The functions shown in Table 9.88 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 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 which the database can read and
    that those reads bypass all in-database privilege checks.  This means that,
    among other things, a user with this access is able to read the contents of the
    pg_authid table where authentication information is contained,
    as well as read any file in the database.  Therefore, granting access to these
    functions should be carefully considered.
   
Table 9.88. Generic File Access Functions
| Name | Return Type | Description | 
|---|---|---|
|  | setof text | List the contents of a directory. Restricted to superusers by default, but other users can be granted EXECUTE to run the function. | 
|  | setof record | List the name, size, and last modification time of files in the log
        directory. Access is granted to members of the pg_monitorrole and may be granted to other non-superuser roles. | 
|  | setof record | List the name, size, and last modification time of files in the WAL
        directory. Access is granted to members of the pg_monitorrole and may be granted to other non-superuser roles. | 
|  | text | Return the contents of a text file. Restricted to superusers by default, but other users can be granted EXECUTE to run the function. | 
|  | bytea | Return the contents of a file. Restricted to superusers by default, but other users can be granted EXECUTE to run the function. | 
|  | record | Return information about a file. Restricted to superusers by default, but other users can be granted EXECUTE to run the function. | 
    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 (except
    pg_ls_dir, which returns an empty result set). If
    false, an error is raised. The default is false.
   
    pg_ls_dir returns the names of all files (and directories
    and other special files) in the specified directory. The 
    include_dot_dirs indicates whether “.” and “..” are
    included in the result set. The default is to exclude them
    (false), but including them can be useful when
    missing_ok is true, to distinguish an
    empty directory from an non-existent directory.
   
    pg_ls_logdir returns the name, size, and last modified time
    (mtime) of each file in the log directory. By default, only superusers
    and members of the pg_monitor role can use this function.
    Access may be granted to others using GRANT.
   
    pg_ls_waldir returns the name, size, and last modified time
    (mtime) of each file in the write ahead log (WAL) directory. By
    default only superusers and members of the pg_monitor role
    can use this function. Access may be granted to others using
    GRANT.
   
    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 9.89 manage advisory locks. For details about proper use of these functions, see Section 13.3.5.
Table 9.89. Advisory Lock Functions
| Name | Return Type | Description | 
|---|---|---|
|  | void | Obtain exclusive session level advisory lock | 
|  | void | Obtain exclusive session level advisory lock | 
|  | void | Obtain shared session level advisory lock | 
|  | void | Obtain shared session level advisory lock | 
|  | boolean | Release an exclusive session level advisory lock | 
|  | boolean | Release an exclusive session level advisory lock | 
|  | void | Release all session level advisory locks held by the current session | 
|  | boolean | Release a shared session level advisory lock | 
|  | boolean | Release a shared session level advisory lock | 
|  | void | Obtain exclusive transaction level advisory lock | 
|  | void | Obtain exclusive transaction level advisory lock | 
|  | void | Obtain shared transaction level advisory lock | 
|  | void | Obtain shared transaction level advisory lock | 
|  | boolean | Obtain exclusive session level advisory lock if available | 
|  | boolean | Obtain exclusive session level advisory lock if available | 
|  | boolean | Obtain shared session level advisory lock if available | 
|  | boolean | Obtain shared session level advisory lock if available | 
|  | boolean | Obtain exclusive transaction level advisory lock if available | 
|  | boolean | Obtain exclusive transaction level advisory lock if available | 
|  | boolean | Obtain shared transaction level advisory lock if available | 
|  | 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.