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9.27. System Administration Functions
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9.27. System Administration Functions

9.27.1. Configuration Settings Functions
9.27.2. Server Signaling Functions
9.27.3. Backup Control Functions
9.27.4. Recovery Control Functions
9.27.5. Snapshot Synchronization Functions
9.27.6. Replication Management Functions
9.27.7. Database Object Management Functions
9.27.8. Index Maintenance Functions
9.27.9. Generic File Access Functions
9.27.10. Advisory Lock Functions
9.27.11. Compression Control Functions
9.27.12. Operation Log Support Functions
9.27.13. Debugging Functions

The functions described in this section are used to control and monitor aPostgres Pro installation.

9.27.1. Configuration Settings Functions

Table 9.83 shows the functions available to query and alter run-time configuration parameters.

Table 9.83. Configuration Settings Functions

Function Description Example(s)
`current_setting` (`setting_name``text` [,`missing_ok``boolean`] ) →`text` Returns the current value of the setting`setting_name`. If there is no such setting,`current_setting` throws an error unless`missing_ok` is supplied and is`true` (in which case NULL is returned). This function corresponds to theSQL command SHOW. `current_setting('datestyle')` →`ISO, MDY`
`set_config` (`setting_name``text`,`new_value``text`,`is_local``boolean` ) →`text` Sets the parameter`setting_name` to`new_value`, and returns that value. If`is_local` is`true`, the new value will only apply during the current transaction. If you want the new value to apply for the rest of the current session, use`false` instead. This function corresponds to the SQL commandSET. `set_config('log_statement_stats', 'off', false)` →`off`
`pg_backend_set_config` (`pid``int`,`config``text`,`wait``int` default 0 ) →`boolean` Sets one or more parameters provided in the`config` string for the backend with the specified PID. Following the`postgresql.conf` convention, each configuration parameter in the`config` string must be written on a separate line, in the`name`=`value` format. The`pg_backend_set_config` function only affects run-time parameters. When called, it modifies the main configuration until the end of the session or the next`pg_backend_set_config` call. The changes take effect starting from the next transaction.
`pg_backend_load_library` (`pid``int`,`name``text`,`wait``int` default 0 ) →`boolean` Loads the library with the provided name for the backend with the specified PID. You can load only one library at a time. When run without superuser privileges,`pg_backend_load_library` only allows to load libraries located in`$libdir/plugins`. If you need to load a library from a different location, run this function on behalf of a superuser.

Function

Description

Example(s)

current_setting (setting_nametext [,missing_okboolean] ) →text

Returns the current value of the settingsetting_name. If there is no such setting,current_setting throws an error unlessmissing_ok is supplied and istrue (in which case NULL is returned). This function corresponds to theSQL command SHOW.

current_setting('datestyle') →ISO, MDY

set_config (setting_nametext,new_valuetext,is_localboolean ) →text

Sets the parametersetting_name tonew_value, and returns that value. Ifis_local istrue, the new value will only apply during the current transaction. If you want the new value to apply for the rest of the current session, usefalse instead. This function corresponds to the SQL commandSET.

set_config('log_statement_stats', 'off', false) →off

pg_backend_set_config (pidint,configtext,waitint default 0 ) →boolean

Sets one or more parameters provided in theconfig string for the backend with the specified PID. Following thepostgresql.conf convention, each configuration parameter in theconfig string must be written on a separate line, in thename=value format. Thepg_backend_set_config function only affects run-time parameters. When called, it modifies the main configuration until the end of the session or the nextpg_backend_set_config call. The changes take effect starting from the next transaction.

pg_backend_load_library (pidint,nametext,waitint default 0 ) →boolean

Loads the library with the provided name for the backend with the specified PID. You can load only one library at a time. When run without superuser privileges,pg_backend_load_library only allows to load libraries located in$libdir/plugins. If you need to load a library from a different location, run this function on behalf of a superuser.

Functionspg_backend_set_config andpg_backend_load_library are designed to set configuration parameters and load shared libraries in other sessions, which may be useful for tracing sessions with unexpected behavior. To avoid potential security issues, these functions can only be called by a superuser.

Note

This functionality cannot be used together with the built-in connection pooler described inChapter 33.

By default,pg_backend_set_config andpg_backend_load_library returntrue, without waiting for response from the other backend. You can use the optionalwait parameter to specify the time interval, in seconds, for which to wait for the confirmation that the target backend has received the command. If the confirmation is received, the functions returntrue. Otherwise,false is returned. Note that the returned value does not reflect the actual result of the operation. If a configuration update fails, an error occurs on the target backend, and the current command on this backend is aborted.

Consider the following examples:

SELECT pg_backend_set_config(pg_backend_pid(), 'statement_timeout=10000'); pg_backend_set_config----------------------- t(1 row)SELECT pg_backend_set_config(pg_backend_pid(),       'statement_timeout=20000        lock_timeout=10000'); pg_backend_set_config----------------------- t(1 row)SELECT pg_backend_set_config(pg_backend_pid(), 'fsync=on');ERROR:  parameter "fsync" cannot be changed nowSELECT pg_backend_set_config(pg_backend_pid(), 'log_min_messages=INFO');ERROR:  permission denied to set parameter "log_min_messages"

SELECT pg_backend_load_library(pg_backend_pid(), 'pgoutput'); pg_backend_load_library------------------------- t(1 row)

9.27.2. Server Signaling Functions

The functions shown inTable 9.84 send control signals to other server processes. Use of these functions is restricted to superusers by default but access may be granted to others usingGRANT, with noted exceptions.

Each of these functions returnstrue if successful andfalse otherwise.

Table 9.84. Server Signaling Functions

Function Description
`pg_cancel_backend` (`pid``integer` ) →`boolean` Cancels the current query of the session whose backend process has the specified process ID. This is also allowed if the calling role is a member of the role whose backend is being canceled or the calling role has been granted`pg_signal_backend`, however only superusers can cancel superuser backends.
`pg_reload_conf` () →`boolean` Causes all processes of thePostgres Pro server to reload their configuration files. (This is initiated by sending aSIGHUP signal to the postmaster process, which in turn sendsSIGHUP to each of its children.)
`pg_rotate_logfile` () →`boolean` Signals the log-file manager to switch to a new output file immediately. This works only when the built-in log collector is running, since otherwise there is no log-file manager subprocess.
`pg_terminate_backend` (`pid``integer` ) →`boolean` Terminates the session whose backend process has the specified process ID. This is also allowed if the calling role is a member of the role whose backend is being terminated or the calling role has been granted`pg_signal_backend`, however only superusers can terminate superuser backends.

Function

Description

pg_cancel_backend (pidinteger ) →boolean

Cancels the current query of the session whose backend process has the specified process ID. This is also allowed if the calling role is a member of the role whose backend is being canceled or the calling role has been grantedpg_signal_backend, however only superusers can cancel superuser backends.

pg_reload_conf () →boolean

Causes all processes of thePostgres Pro server to reload their configuration files. (This is initiated by sending aSIGHUP signal to the postmaster process, which in turn sendsSIGHUP to each of its children.)

pg_rotate_logfile () →boolean

Signals the log-file manager to switch to a new output file immediately. This works only when the built-in log collector is running, since otherwise there is no log-file manager subprocess.

pg_terminate_backend (pidinteger ) →boolean

Terminates the session whose backend process has the specified process ID. This is also allowed if the calling role is a member of the role whose backend is being terminated or the calling role has been grantedpg_signal_backend, however only superusers can terminate superuser backends.

pg_cancel_backend andpg_terminate_backend send signals (SIGINT orSIGTERM respectively) to backend processes identified by process ID. The process ID of an active backend can be found from thepid column of thepg_stat_activity view, or by listing thepostgres processes on the server (usingps on Unix or theTask Manager onWindows). The role of an active backend can be found from theusename column of thepg_stat_activity view.

9.27.3. Backup Control Functions

The functions shown inTable 9.85 assist in making on-line backups. These functions cannot be executed during recovery (except non-exclusivepg_start_backup, non-exclusivepg_stop_backup,pg_is_in_backup,pg_backup_start_time andpg_wal_lsn_diff).

For details about proper usage of these functions, seeSection 25.3.

Table 9.85. Backup Control Functions

Function Description
`pg_create_restore_point` (`name``text` ) →`pg_lsn` Creates a named marker record in the write-ahead log that can later be used as a recovery target, and returns the corresponding write-ahead log location. The given name can then be used withrecovery_target_name to specify the point up to which recovery will proceed. Avoid creating multiple restore points with the same name, since recovery will stop at the first one whose name matches the recovery target. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
`pg_current_wal_flush_lsn` () →`pg_lsn` Returns the current write-ahead log flush location (see notes below).
`pg_current_wal_insert_lsn` () →`pg_lsn` Returns the current write-ahead log insert location (see notes below).
`pg_current_wal_lsn` () →`pg_lsn` Returns the current write-ahead log write location (see notes below).
`pg_start_backup` (`label``text` [,`fast``boolean` [,`exclusive``boolean`]] ) →`pg_lsn` Prepares the server to begin an on-line backup. The only required parameter is an arbitrary user-defined label for the backup. (Typically this would be the name under which the backup dump file will be stored.) If the optional second parameter is given as`true`, it specifies executing`pg_start_backup` as quickly as possible. This forces an immediate checkpoint which will cause a spike in I/O operations, slowing any concurrently executing queries. The optional third parameter specifies whether to perform an exclusive or non-exclusive backup (default is exclusive). When used in exclusive mode, this function writes a backup label file (`backup_label`) and, if there are any links in the`pg_tblspc/` directory, a tablespace map file (`tablespace_map`) into the database cluster's data directory, then performs a checkpoint, and then returns the backup's starting write-ahead log location. (The user can ignore this result value, but it is provided in case it is useful.) When used in non-exclusive mode, the contents of these files are instead returned by the`pg_stop_backup` function, and should be copied to the backup area by the user. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
`pg_stop_backup` (`exclusive``boolean` [,`wait_for_archive``boolean`] ) →`setof record` (`lsn``pg_lsn`,`labelfile``text`,`spcmapfile``text` ) Finishes performing an exclusive or non-exclusive on-line backup. The`exclusive` parameter must match the previous`pg_start_backup` call. In an exclusive backup,`pg_stop_backup` removes the backup label file and, if it exists, the tablespace map file created by`pg_start_backup`. In a non-exclusive backup, the desired contents of these files are returned as part of the result of the function, and should be written to files in the backup area (not in the data directory). There is an optional second parameter of type`boolean`. If false, the function will return immediately after the backup is completed, without waiting for WAL to be archived. This behavior is only useful with backup software that independently monitors WAL archiving. Otherwise, WAL required to make the backup consistent might be missing and make the backup useless. By default or when this parameter is true,`pg_stop_backup` will wait for WAL to be archived when archiving is enabled. (On a standby, this means that it will wait only when`archive_mode` =`always`. If write activity on the primary is low, it may be useful to run`pg_switch_wal` on the primary in order to trigger an immediate segment switch.) When executed on a primary, this function also creates a backup history file in the write-ahead log archive area. The history file includes the label given to`pg_start_backup`, the starting and ending write-ahead log locations for the backup, and the starting and ending times of the backup. After recording the ending location, the current write-ahead log insertion point is automatically advanced to the next write-ahead log file, so that the ending write-ahead log file can be archived immediately to complete the backup. The result of the function is a single record. The`lsn` column holds the backup's ending write-ahead log location (which again can be ignored). The second and third columns are`NULL` when ending an exclusive backup; after a non-exclusive backup they hold the desired contents of the label and tablespace map files. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
`pg_stop_backup` () →`pg_lsn` Finishes performing an exclusive on-line backup. This simplified version is equivalent to`pg_stop_backup(true, true)`, except that it only returns the`pg_lsn` result. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
`pg_is_in_backup` () →`boolean` Returns true if an on-line exclusive backup is in progress.
`pg_backup_start_time` () →`timestamp with time zone` Returns the start time of the current on-line exclusive backup if one is in progress, otherwise`NULL`.
`pg_switch_wal` () →`pg_lsn` Forces the server to switch to a new write-ahead log file, which allows the current file to be archived (assuming you are using continuous archiving). The result is the ending write-ahead log location plus 1 within the just-completed write-ahead log file. If there has been no write-ahead log activity since the last write-ahead log switch,`pg_switch_wal` does nothing and returns the start location of the write-ahead log file currently in use. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
`pg_walfile_name` (`lsn``pg_lsn` ) →`text` Converts a write-ahead log location to the name of the WAL file holding that location.
`pg_walfile_name_offset` (`lsn``pg_lsn` ) →`record` (`file_name``text`,`file_offset``integer` ) Converts a write-ahead log location to a WAL file name and byte offset within that file.
`pg_wal_lsn_diff` (`lsn1``pg_lsn`,`lsn2``pg_lsn` ) →`numeric` Calculates the difference in bytes (`lsn1` -`lsn2`) between two write-ahead log locations. This can be used with`pg_stat_replication` or some of the functions shown inTable 9.85 to get the replication lag.

Function

Description

pg_create_restore_point (nametext ) →pg_lsn

Creates a named marker record in the write-ahead log that can later be used as a recovery target, and returns the corresponding write-ahead log location. The given name can then be used withrecovery_target_name to specify the point up to which recovery will proceed. Avoid creating multiple restore points with the same name, since recovery will stop at the first one whose name matches the recovery target.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

pg_current_wal_flush_lsn () →pg_lsn

Returns the current write-ahead log flush location (see notes below).

pg_current_wal_insert_lsn () →pg_lsn

Returns the current write-ahead log insert location (see notes below).

pg_current_wal_lsn () →pg_lsn

Returns the current write-ahead log write location (see notes below).

pg_start_backup (labeltext [,fastboolean [,exclusiveboolean]] ) →pg_lsn

Prepares the server to begin an on-line backup. The only required parameter is an arbitrary user-defined label for the backup. (Typically this would be the name under which the backup dump file will be stored.) If the optional second parameter is given astrue, it specifies executingpg_start_backup as quickly as possible. This forces an immediate checkpoint which will cause a spike in I/O operations, slowing any concurrently executing queries. The optional third parameter specifies whether to perform an exclusive or non-exclusive backup (default is exclusive).

When used in exclusive mode, this function writes a backup label file (backup_label) and, if there are any links in thepg_tblspc/ directory, a tablespace map file (tablespace_map) into the database cluster's data directory, then performs a checkpoint, and then returns the backup's starting write-ahead log location. (The user can ignore this result value, but it is provided in case it is useful.) When used in non-exclusive mode, the contents of these files are instead returned by thepg_stop_backup function, and should be copied to the backup area by the user.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

pg_stop_backup (exclusiveboolean [,wait_for_archiveboolean] ) →setof record (lsnpg_lsn,labelfiletext,spcmapfiletext )

Finishes performing an exclusive or non-exclusive on-line backup. Theexclusive parameter must match the previouspg_start_backup call. In an exclusive backup,pg_stop_backup removes the backup label file and, if it exists, the tablespace map file created bypg_start_backup. In a non-exclusive backup, the desired contents of these files are returned as part of the result of the function, and should be written to files in the backup area (not in the data directory).

There is an optional second parameter of typeboolean. If false, the function will return immediately after the backup is completed, without waiting for WAL to be archived. This behavior is only useful with backup software that independently monitors WAL archiving. Otherwise, WAL required to make the backup consistent might be missing and make the backup useless. By default or when this parameter is true,pg_stop_backup will wait for WAL to be archived when archiving is enabled. (On a standby, this means that it will wait only whenarchive_mode =always. If write activity on the primary is low, it may be useful to runpg_switch_wal on the primary in order to trigger an immediate segment switch.)

When executed on a primary, this function also creates a backup history file in the write-ahead log archive area. The history file includes the label given topg_start_backup, the starting and ending write-ahead log locations for the backup, and the starting and ending times of the backup. After recording the ending location, the current write-ahead log insertion point is automatically advanced to the next write-ahead log file, so that the ending write-ahead log file can be archived immediately to complete the backup.

The result of the function is a single record. Thelsn column holds the backup's ending write-ahead log location (which again can be ignored). The second and third columns areNULL when ending an exclusive backup; after a non-exclusive backup they hold the desired contents of the label and tablespace map files.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

pg_stop_backup () →pg_lsn

Finishes performing an exclusive on-line backup. This simplified version is equivalent topg_stop_backup(true, true), except that it only returns thepg_lsn result.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

pg_is_in_backup () →boolean

Returns true if an on-line exclusive backup is in progress.

pg_backup_start_time () →timestamp with time zone

Returns the start time of the current on-line exclusive backup if one is in progress, otherwiseNULL.

pg_switch_wal () →pg_lsn

Forces the server to switch to a new write-ahead log file, which allows the current file to be archived (assuming you are using continuous archiving). The result is the ending write-ahead log location plus 1 within the just-completed write-ahead log file. If there has been no write-ahead log activity since the last write-ahead log switch,pg_switch_wal does nothing and returns the start location of the write-ahead log file currently in use.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

pg_walfile_name (lsnpg_lsn ) →text

Converts a write-ahead log location to the name of the WAL file holding that location.

pg_walfile_name_offset (lsnpg_lsn ) →record (file_nametext,file_offsetinteger )

Converts a write-ahead log location to a WAL file name and byte offset within that file.

pg_wal_lsn_diff (lsn1pg_lsn,lsn2pg_lsn ) →numeric

Calculates the difference in bytes (lsn1 -lsn2) between two write-ahead log locations. This can be used withpg_stat_replication or some of the functions shown inTable 9.85 to get the replication lag.

pg_current_wal_lsn displays the current write-ahead log write location in the same format used by the above functions. Similarly,pg_current_wal_insert_lsn displays the current write-ahead log insertion location andpg_current_wal_flush_lsn displays the current write-ahead log flush location. The insertion location is the“logical” end of the write-ahead log at any instant, while the write location is the end of what has actually been written out from the server's internal buffers, and the flush location is the last location known to be written to durable storage. The write location is the end of what can be examined from outside the server, and is usually what you want if you are interested in archiving partially-complete write-ahead log files. The insertion and flush locations are made available primarily for server debugging purposes. These are all read-only operations and do not require superuser permissions.

You can usepg_walfile_name_offset to extract the corresponding write-ahead log file name and byte offset from apg_lsn value. For example:

postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());        file_name         | file_offset--------------------------+------------- 00000001000000000000000D |     4039624(1 row)

Similarly,pg_walfile_name extracts just the write-ahead log file name. When the given write-ahead log location is exactly at a write-ahead log file boundary, both these functions return the name of the preceding write-ahead log file. This is usually the desired behavior for managing write-ahead log archiving behavior, since the preceding file is the last one that currently needs to be archived.

9.27.4. Recovery Control Functions

The functions shown inTable 9.86 provide information about the current status of a standby server. These functions may be executed both during recovery and in normal running.

Table 9.86. Recovery Information Functions

Function Description
`pg_is_in_recovery` () →`boolean` Returns true if recovery is still in progress.
`pg_last_wal_receive_lsn` () →`pg_lsn` Returns the last write-ahead log location that has been received and synced to disk by streaming replication. While streaming replication is in progress this will increase monotonically. If recovery has completed then this will remain static at the location of the last WAL record received and synced to disk during recovery. If streaming replication is disabled, or if it has not yet started, the function returns`NULL`.
`pg_last_wal_replay_lsn` () →`pg_lsn` Returns the last write-ahead log location that has been replayed during recovery. If recovery is still in progress this will increase monotonically. If recovery has completed then this will remain static at the location of the last WAL record applied during recovery. When the server has been started normally without recovery, the function returns`NULL`.
`pg_last_xact_replay_timestamp` () →`timestamp with time zone` Returns the time stamp of the last transaction replayed during recovery. This is the time at which the commit or abort WAL record for that transaction was generated on the primary. If no transactions have been replayed during recovery, the function returns`NULL`. Otherwise, if recovery is still in progress this will increase monotonically. If recovery has completed then this will remain static at the time of the last transaction applied during recovery. When the server has been started normally without recovery, the function returns`NULL`.

Function

Description

pg_is_in_recovery () →boolean

Returns true if recovery is still in progress.

pg_last_wal_receive_lsn () →pg_lsn

Returns the last write-ahead log location that has been received and synced to disk by streaming replication. While streaming replication is in progress this will increase monotonically. If recovery has completed then this will remain static at the location of the last WAL record received and synced to disk during recovery. If streaming replication is disabled, or if it has not yet started, the function returnsNULL.

pg_last_wal_replay_lsn () →pg_lsn

Returns the last write-ahead log location that has been replayed during recovery. If recovery is still in progress this will increase monotonically. If recovery has completed then this will remain static at the location of the last WAL record applied during recovery. When the server has been started normally without recovery, the function returnsNULL.

pg_last_xact_replay_timestamp () →timestamp with time zone

Returns the time stamp of the last transaction replayed during recovery. This is the time at which the commit or abort WAL record for that transaction was generated on the primary. If no transactions have been replayed during recovery, the function returnsNULL. Otherwise, if recovery is still in progress this will increase monotonically. If recovery has completed then this will remain static at the time of the last transaction applied during recovery. When the server has been started normally without recovery, the function returnsNULL.

The functions shown inTable 9.87 control the progress of recovery. These functions may be executed only during recovery.

Table 9.87. Recovery Control Functions

Function Description
`pg_is_wal_replay_paused` () →`boolean` Returns true if recovery is paused.
`pg_promote` (`wait``booleanDEFAULTtrue`,`wait_seconds``integerDEFAULT60` ) →`boolean` Promotes a standby server to primary status. With`wait` set to`true` (the default), the function waits until promotion is completed or`wait_seconds` seconds have passed, and returns`true` if promotion is successful and`false` otherwise. If`wait` is set to`false`, the function returns`true` immediately after sending a`SIGUSR1` signal to the postmaster to trigger promotion. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
`pg_wal_replay_pause` () →`void` Pauses recovery. While recovery is paused, no further database changes are applied. If hot standby is active, all new queries will see the same consistent snapshot of the database, and no further query conflicts will be generated until recovery is resumed. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
`pg_wal_replay_resume` () →`void` Restarts recovery if it was paused. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

Function

Description

pg_is_wal_replay_paused () →boolean

Returns true if recovery is paused.

pg_promote (waitbooleanDEFAULTtrue,wait_secondsintegerDEFAULT60 ) →boolean

Promotes a standby server to primary status. Withwait set totrue (the default), the function waits until promotion is completed orwait_seconds seconds have passed, and returnstrue if promotion is successful andfalse otherwise. Ifwait is set tofalse, the function returnstrue immediately after sending aSIGUSR1 signal to the postmaster to trigger promotion.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

pg_wal_replay_pause () →void

Pauses recovery. While recovery is paused, no further database changes are applied. If hot standby is active, all new queries will see the same consistent snapshot of the database, and no further query conflicts will be generated until recovery is resumed.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

pg_wal_replay_resume () →void

Restarts recovery if it was paused.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

pg_wal_replay_pause andpg_wal_replay_resume cannot be executed while a promotion is ongoing. If a promotion is triggered while recovery is paused, the paused state ends and promotion continues.

If streaming replication is disabled, the paused state may continue indefinitely without a problem. If streaming replication is in progress then WAL records will continue to be received, which will eventually fill available disk space, depending upon the duration of the pause, the rate of WAL generation and available disk space.

9.27.5. Snapshot Synchronization Functions

Postgres Pro allows database sessions to synchronize their snapshots. Asnapshot 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 twoSTART TRANSACTION commands, so that one session sees the effects of that transaction and the other does not.

To solve this problem,Postgres Pro allows a transaction toexport the snapshot it is using. As long as the exporting transaction remains open, other transactions canimport 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 thepg_export_snapshot function, shown inTable 9.88, and imported with theSET TRANSACTION command.

Table 9.88. Snapshot Synchronization Functions

Function Description
`pg_export_snapshot` () →`text` Saves the transaction's current snapshot and returns a`text` string identifying the snapshot. This string must be passed (outside the database) to clients that want to import the snapshot. The snapshot is available for import only until the end of the transaction that exported it. A transaction can export more than one snapshot, if needed. Note that doing so is only useful in`READ COMMITTED` transactions, since in`REPEATABLE READ` and higher isolation levels, transactions use the same snapshot throughout their lifetime. Once a transaction has exported any snapshots, it cannot be prepared withPREPARE TRANSACTION.

Function

Description

pg_export_snapshot () →text

Saves the transaction's current snapshot and returns atext 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 inREAD COMMITTED transactions, since inREPEATABLE READ and higher isolation levels, transactions use the same snapshot throughout their lifetime. Once a transaction has exported any snapshots, it cannot be prepared withPREPARE TRANSACTION.

9.27.6. Replication Management Functions

The functions shown inTable 9.89 are for controlling and interacting with replication features. SeeSection 26.2.5,Section 26.2.6, andChapter 52 for information about the underlying features. Use of functions for replication origin is restricted to superusers. Use of functions for replication slots is restricted to superusers and users havingREPLICATION privilege.

Many of these functions have equivalent commands in the replication protocol; seeSection 55.4.

The functions described inSection 9.27.3,Section 9.27.4, andSection 9.27.5 are also relevant for replication.

Table 9.89. Replication Management Functions

Function Description
`pg_create_physical_replication_slot` (`slot_name``name` [,`immediately_reserve``boolean`,`temporary``boolean`] ) →`record` (`slot_name``name`,`lsn``pg_lsn` ) Creates a new physical replication slot named`slot_name`. The optional second parameter, when`true`, specifies that theLSN 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 55.4. The optional third parameter,`temporary`, when set to true, specifies that the slot should not be permanently stored to disk and is only meant for use by the current session. Temporary slots are also released upon any error. This function corresponds to the replication protocol command`CREATE_REPLICATION_SLOT ... PHYSICAL`.
`pg_drop_replication_slot` (`slot_name``name` ) →`void` Drops the physical or logical replication slot named`slot_name`. Same as replication protocol command`DROP_REPLICATION_SLOT`. For logical slots, this must be called while connected to the same database the slot was created on.
`pg_create_logical_replication_slot` (`slot_name``name`,`plugin``name` [,`temporary``boolean`] ) →`record` (`slot_name``name`,`lsn``pg_lsn` ) Creates a new logical (decoding) replication slot named`slot_name` using the output plugin`plugin`. The optional third parameter,`temporary`, when set to true, specifies that the slot should not be permanently stored to disk and is only meant for use by the current session. Temporary slots are also released upon any error. A call to this function has the same effect as the replication protocol command`CREATE_REPLICATION_SLOT ... LOGICAL`.
`pg_copy_physical_replication_slot` (`src_slot_name``name`,`dst_slot_name``name` [,`temporary``boolean`] ) →`record` (`slot_name``name`,`lsn``pg_lsn` ) Copies an existing physical replication slot named`src_slot_name` to a physical replication slot named`dst_slot_name`. The copied physical slot starts to reserve WAL from the sameLSN as the source slot.`temporary` is optional. If`temporary` is omitted, the same value as the source slot is used.
`pg_copy_logical_replication_slot` (`src_slot_name``name`,`dst_slot_name``name` [,`temporary``boolean` [,`plugin``name`]] ) →`record` (`slot_name``name`,`lsn``pg_lsn` ) Copies an existing logical replication slot named`src_slot_name` to a logical replication slot named`dst_slot_name`, optionally changing the output plugin and persistence. The copied logical slot starts from the sameLSN as the source logical slot. Both`temporary` and`plugin` are optional; if they are omitted, the values of the source slot are used.
`pg_logical_slot_get_changes` (`slot_name``name`,`upto_lsn``pg_lsn`,`upto_nchanges``integer`,`VARIADIC``options``text[]` ) →`setof record` (`lsn``pg_lsn`,`xid``xid`,`data``text` ) Returns changes in the slot`slot_name`, starting from the point from which changes have been consumed last. If`upto_lsn` and`upto_nchanges` are NULL, logical decoding will continue until end of WAL. If`upto_lsn` is non-NULL, decoding will include only those transactions which commit prior to the specified LSN. If`upto_nchanges` is non-NULL, decoding will stop when the number of rows produced by decoding exceeds the specified value. Note, however, that the actual number of rows returned may be larger, since this limit is only checked after adding the rows produced when decoding each new transaction commit.
`pg_logical_slot_peek_changes` (`slot_name``name`,`upto_lsn``pg_lsn`,`upto_nchanges``integer`,`VARIADIC``options``text[]` ) →`setof record` (`lsn``pg_lsn`,`xid``xid`,`data``text` ) Behaves just like the`pg_logical_slot_get_changes()` function, except that changes are not consumed; that is, they will be returned again on future calls.
`pg_logical_slot_get_binary_changes` (`slot_name``name`,`upto_lsn``pg_lsn`,`upto_nchanges``integer`,`VARIADIC``options``text[]` ) →`setof record` (`lsn``pg_lsn`,`xid``xid`,`data``bytea` ) Behaves just like the`pg_logical_slot_get_changes()` function, except that changes are returned as`bytea`.
`pg_logical_slot_peek_binary_changes` (`slot_name``name`,`upto_lsn``pg_lsn`,`upto_nchanges``integer`,`VARIADIC``options``text[]` ) →`setof record` (`lsn``pg_lsn`,`xid``xid`,`data``bytea` ) Behaves just like the`pg_logical_slot_peek_changes()` function, except that changes are returned as`bytea`.
`pg_replication_slot_advance` (`slot_name``name`,`upto_lsn``pg_lsn` ) →`record` (`slot_name``name`,`end_lsn``pg_lsn` ) Advances the current confirmed position of a replication slot named`slot_name`. The slot will not be moved backwards, and it will not be moved beyond the current insert location. Returns the name of the slot and the actual position that it was advanced to. The updated slot position information is written out at the next checkpoint if any advancing is done. So in the event of a crash, the slot may return to an earlier position.
`pg_replication_origin_create` (`node_name``text` ) →`oid` Creates a replication origin with the given external name, and returns the internal ID assigned to it.
`pg_replication_origin_drop` (`node_name``text` ) →`void` Deletes a previously-created replication origin, including any associated replay progress.
`pg_replication_origin_oid` (`node_name``text` ) →`oid` Looks up a replication origin by name and returns the internal ID. If no such replication origin is found,`NULL` is returned.
`pg_replication_origin_session_setup` (`node_name``text` ) →`void` Marks the current session as replaying from the given origin, allowing replay progress to be tracked. Can only be used if no origin is currently selected. Use`pg_replication_origin_session_reset` to undo.
`pg_replication_origin_session_reset` () →`void` Cancels the effects of`pg_replication_origin_session_setup()`.
`pg_replication_origin_session_is_setup` () →`boolean` Returns true if a replication origin has been selected in the current session.
`pg_replication_origin_session_progress` (`flush``boolean` ) →`pg_lsn` Returns the replay location for the replication origin selected in the current session. The parameter`flush` determines whether the corresponding local transaction will be guaranteed to have been flushed to disk or not.
`pg_replication_origin_xact_setup` (`origin_lsn``pg_lsn`,`origin_timestamp``timestamp with time zone` ) →`void` Marks the current transaction as replaying a transaction that has committed at the givenLSN and timestamp. Can only be called when a replication origin has been selected using`pg_replication_origin_session_setup`.
`pg_replication_origin_xact_reset` () →`void` Cancels the effects of`pg_replication_origin_xact_setup()`.
`pg_replication_origin_advance` (`node_name``text`,`lsn``pg_lsn` ) →`void` Sets replication progress for the given node to the given location. This is primarily useful for setting up the initial location, or setting a new location after configuration changes and similar. Be aware that careless use of this function can lead to inconsistently replicated data.
`pg_replication_origin_progress` (`node_name``text`,`flush``boolean` ) →`pg_lsn` Returns the replay location for the given replication origin. The parameter`flush` determines whether the corresponding local transaction will be guaranteed to have been flushed to disk or not.
`pg_logical_emit_message` (`transactional``boolean`,`prefix``text`,`content``text` ) →`pg_lsn` `pg_logical_emit_message` (`transactional``boolean`,`prefix``text`,`content``bytea` ) →`pg_lsn` Emits a logical decoding message. This can be used to pass generic messages to logical decoding plugins through WAL. The`transactional` parameter specifies if the message should be part of the current transaction, or if it should be written immediately and decoded as soon as the logical decoder reads the record. The`prefix` parameter is a textual prefix that can be used by logical decoding plugins to easily recognize messages that are interesting for them. The`content` parameter is the content of the message, given either in text or binary form.

Function

Description

pg_create_physical_replication_slot (slot_namename [,immediately_reserveboolean,temporaryboolean] ) →record (slot_namename,lsnpg_lsn )

Creates a new physical replication slot namedslot_name. The optional second parameter, whentrue, specifies that theLSN 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 55.4. The optional third parameter,temporary, when set to true, specifies that the slot should not be permanently stored to disk and is only meant for use by the current session. Temporary slots are also released upon any error. This function corresponds to the replication protocol commandCREATE_REPLICATION_SLOT ... PHYSICAL.

pg_drop_replication_slot (slot_namename ) →void

Drops the physical or logical replication slot namedslot_name. Same as replication protocol commandDROP_REPLICATION_SLOT. For logical slots, this must be called while connected to the same database the slot was created on.

pg_create_logical_replication_slot (slot_namename,pluginname [,temporaryboolean] ) →record (slot_namename,lsnpg_lsn )

Creates a new logical (decoding) replication slot namedslot_name using 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 the 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.

pg_copy_physical_replication_slot (src_slot_namename,dst_slot_namename [,temporaryboolean] ) →record (slot_namename,lsnpg_lsn )

Copies an existing physical replication slot namedsrc_slot_name to a physical replication slot nameddst_slot_name. The copied physical slot starts to reserve WAL from the sameLSN as the source slot.temporary is optional. Iftemporary is omitted, the same value as the source slot is used.

pg_copy_logical_replication_slot (src_slot_namename,dst_slot_namename [,temporaryboolean [,pluginname]] ) →record (slot_namename,lsnpg_lsn )

Copies an existing logical replication slot namedsrc_slot_name to a logical replication slot nameddst_slot_name, optionally changing the output plugin and persistence. The copied logical slot starts from the sameLSN as the source logical slot. Bothtemporary andplugin are optional; if they are omitted, the values of the source slot are used.

pg_logical_slot_get_changes (slot_namename,upto_lsnpg_lsn,upto_nchangesinteger,VARIADICoptionstext[] ) →setof record (lsnpg_lsn,xidxid,datatext )

Returns changes in the slotslot_name, starting from the point from which changes have been consumed last. Ifupto_lsn andupto_nchanges are NULL, logical decoding will continue until end of WAL. Ifupto_lsn is non-NULL, decoding will include only those transactions which commit prior to the specified LSN. Ifupto_nchanges is non-NULL, decoding will stop when the number of rows produced by decoding exceeds the specified value. Note, however, that the actual number of rows returned may be larger, since this limit is only checked after adding the rows produced when decoding each new transaction commit.

pg_logical_slot_peek_changes (slot_namename,upto_lsnpg_lsn,upto_nchangesinteger,VARIADICoptionstext[] ) →setof record (lsnpg_lsn,xidxid,datatext )

Behaves just like thepg_logical_slot_get_changes() function, except that changes are not consumed; that is, they will be returned again on future calls.

pg_logical_slot_get_binary_changes (slot_namename,upto_lsnpg_lsn,upto_nchangesinteger,VARIADICoptionstext[] ) →setof record (lsnpg_lsn,xidxid,databytea )

Behaves just like thepg_logical_slot_get_changes() function, except that changes are returned asbytea.

pg_logical_slot_peek_binary_changes (slot_namename,upto_lsnpg_lsn,upto_nchangesinteger,VARIADICoptionstext[] ) →setof record (lsnpg_lsn,xidxid,databytea )

Behaves just like thepg_logical_slot_peek_changes() function, except that changes are returned asbytea.

pg_replication_slot_advance (slot_namename,upto_lsnpg_lsn ) →record (slot_namename,end_lsnpg_lsn )

Advances the current confirmed position of a replication slot namedslot_name. The slot will not be moved backwards, and it will not be moved beyond the current insert location. Returns the name of the slot and the actual position that it was advanced to. The updated slot position information is written out at the next checkpoint if any advancing is done. So in the event of a crash, the slot may return to an earlier position.

pg_replication_origin_create (node_nametext ) →oid

Creates a replication origin with the given external name, and returns the internal ID assigned to it.

pg_replication_origin_drop (node_nametext ) →void

Deletes a previously-created replication origin, including any associated replay progress.

pg_replication_origin_oid (node_nametext ) →oid

Looks up a replication origin by name and returns the internal ID. If no such replication origin is found,NULL is returned.

pg_replication_origin_session_setup (node_nametext ) →void

Marks the current session as replaying from the given origin, allowing replay progress to be tracked. Can only be used if no origin is currently selected. Usepg_replication_origin_session_reset to undo.

pg_replication_origin_session_reset () →void

Cancels the effects ofpg_replication_origin_session_setup().

pg_replication_origin_session_is_setup () →boolean

Returns true if a replication origin has been selected in the current session.

pg_replication_origin_session_progress (flushboolean ) →pg_lsn

Returns the replay location for the replication origin selected in the current session. The parameterflush determines whether the corresponding local transaction will be guaranteed to have been flushed to disk or not.

pg_replication_origin_xact_setup (origin_lsnpg_lsn,origin_timestamptimestamp with time zone ) →void

Marks the current transaction as replaying a transaction that has committed at the givenLSN and timestamp. Can only be called when a replication origin has been selected usingpg_replication_origin_session_setup.

pg_replication_origin_xact_reset () →void

Cancels the effects ofpg_replication_origin_xact_setup().

pg_replication_origin_advance (node_nametext,lsnpg_lsn ) →void

Sets replication progress for the given node to the given location. This is primarily useful for setting up the initial location, or setting a new location after configuration changes and similar. Be aware that careless use of this function can lead to inconsistently replicated data.

pg_replication_origin_progress (node_nametext,flushboolean ) →pg_lsn

Returns the replay location for the given replication origin. The parameterflush determines whether the corresponding local transaction will be guaranteed to have been flushed to disk or not.

pg_logical_emit_message (transactionalboolean,prefixtext,contenttext ) →pg_lsn

pg_logical_emit_message (transactionalboolean,prefixtext,contentbytea ) →pg_lsn

Emits a logical decoding message. This can be used to pass generic messages to logical decoding plugins through WAL. Thetransactional parameter specifies if the message should be part of the current transaction, or if it should be written immediately and decoded as soon as the logical decoder reads the record. Theprefix parameter is a textual prefix that can be used by logical decoding plugins to easily recognize messages that are interesting for them. Thecontent parameter is the content of the message, given either in text or binary form.

9.27.7. Database Object Management Functions

The functions shown inTable 9.90 calculate the disk space usage of database objects, or assist in presentation of usage results. All these functions return sizes measured in bytes. If an OID that does not represent an existing object is passed to one of these functions,NULL is returned.

Table 9.90. Database Object Size Functions

Function Description
`pg_column_size` (`"any"` ) →`integer` Shows the number of bytes used to store any individual data value. If applied directly to a table column value, this reflects any compression that was done.
`pg_database_size` (`name` ) →`bigint` `pg_database_size` (`oid` ) →`bigint` Computes the total disk space used by the database with the specified name or OID. To use this function, you must have`CONNECT` privilege on the specified database (which is granted by default) or be a member of the`pg_read_all_stats` role.
`pg_indexes_size` (`regclass` ) →`bigint` Computes the total disk space used by indexes attached to the specified table.
`pg_relation_size` (`relation``regclass` [,`fork``text`] ) →`bigint` Computes the disk space used by one“fork” of the specified relation. (Note that for most purposes it is more convenient to use the higher-level functions`pg_total_relation_size` or`pg_table_size`, which sum the sizes of all forks.) With one argument, this returns the size of the main data fork of the relation. The second argument can be provided to specify which fork to examine: `main` returns the size of the main data fork of the relation. `fsm` returns the size of the Free Space Map (seeSection 70.3) associated with the relation. `vm` returns the size of the Visibility Map (seeSection 70.4) associated with the relation. `init` returns the size of the initialization fork, if any, associated with the relation.
`pg_temp_relation_size` (`relation``regclass` ) →`bigint` Accepts the OID or name of a temporary table and returns the total size in bytes of`main` fork of that relation. We extend physical file for temp table when table doesn't fit into temp_buffers cache anymore. So total and on-disk size of the temporary table can differ. For other kinds of relations it will not throw any error, just return NULL.
`pg_size_bytes` (`text` ) →`bigint` Converts a size in human-readable format (as returned by`pg_size_pretty`) into bytes.
`pg_size_pretty` (`bigint` ) →`text` `pg_size_pretty` (`numeric` ) →`text` Converts a size in bytes into a more easily human-readable format with size units (bytes, kB, MB, GB or TB as appropriate). Note that the units are powers of 2 rather than powers of 10, so 1kB is 1024 bytes, 1MB is 1024² = 1048576 bytes, and so on.
`pg_table_size` (`regclass` ) →`bigint` Computes the disk space used by the specified table, excluding indexes (but including its TOAST table if any, free space map, and visibility map).
`pg_tablespace_size` (`name` ) →`bigint` `pg_tablespace_size` (`oid` ) →`bigint` Computes the total disk space used in the tablespace with the specified name or OID. To use this function, you must have`CREATE` privilege on the specified tablespace or be a member of the`pg_read_all_stats` role, unless it is the default tablespace for the current database.
`pg_total_relation_size` (`regclass` ) →`bigint` Computes the total disk space used by the specified table, including all indexes andTOAST data. The result is equivalent to`pg_table_size+pg_indexes_size`.

Function

Description

pg_column_size ("any" ) →integer

Shows the number of bytes used to store any individual data value. If applied directly to a table column value, this reflects any compression that was done.

pg_database_size (name ) →bigint

pg_database_size (oid ) →bigint

Computes the total disk space used by the database with the specified name or OID. To use this function, you must haveCONNECT privilege on the specified database (which is granted by default) or be a member of thepg_read_all_stats role.

pg_indexes_size (regclass ) →bigint

Computes the total disk space used by indexes attached to the specified table.

pg_relation_size (relationregclass [,forktext] ) →bigint

Computes the disk space used by one“fork” of the specified relation. (Note that for most purposes it is more convenient to use the higher-level functionspg_total_relation_size orpg_table_size, which sum the sizes of all forks.) With one argument, this returns the size of the main data fork of the relation. The second argument can be provided to specify which fork to examine:

main returns the size of the main data fork of the relation.
fsm returns the size of the Free Space Map (seeSection 70.3) associated with the relation.
vm returns the size of the Visibility Map (seeSection 70.4) associated with the relation.
init returns the size of the initialization fork, if any, associated with the relation.

pg_temp_relation_size (relationregclass ) →bigint

Accepts the OID or name of a temporary table and returns the total size in bytes ofmain fork of that relation. We extend physical file for temp table when table doesn't fit into temp_buffers cache anymore. So total and on-disk size of the temporary table can differ. For other kinds of relations it will not throw any error, just return NULL.

pg_size_bytes (text ) →bigint

Converts a size in human-readable format (as returned bypg_size_pretty) into bytes.

pg_size_pretty (bigint ) →text

pg_size_pretty (numeric ) →text

Converts a size in bytes into a more easily human-readable format with size units (bytes, kB, MB, GB or TB as appropriate). Note that the units are powers of 2 rather than powers of 10, so 1kB is 1024 bytes, 1MB is 1024² = 1048576 bytes, and so on.

pg_table_size (regclass ) →bigint

Computes the disk space used by the specified table, excluding indexes (but including its TOAST table if any, free space map, and visibility map).

pg_tablespace_size (name ) →bigint

pg_tablespace_size (oid ) →bigint

Computes the total disk space used in the tablespace with the specified name or OID. To use this function, you must haveCREATE privilege on the specified tablespace or be a member of thepg_read_all_stats role, unless it is the default tablespace for the current database.

pg_total_relation_size (regclass ) →bigint

Computes the total disk space used by the specified table, including all indexes andTOAST data. The result is equivalent topg_table_size+pg_indexes_size.

The functions above that operate on tables or indexes accept aregclass argument, which is simply the OID of the table or index in thepg_class system catalog. You do not have to look up the OID by hand, however, since theregclass 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 ordinarySQL names, the string will be converted to lower case unless it contains double quotes around the table name.

The functions shown in Table 9.91 assist in identifying the specific disk files associated with database objects.

Table 9.91. Database Object Location Functions

Function Description
`pg_relation_filenode` (`relation``regclass` ) →`oid` Returns the“filenode” number currently assigned to the specified relation. The filenode is the base component of the file name(s) used for the relation (seeSection 70.1 for more information). For most relations the result is the same as`pg_class`.`relfilenode`, but for certain system catalogs`relfilenode` is zero and this function must be used to get the correct value. The function returns NULL if passed a relation that does not have storage, such as a view.
`pg_relation_filepath` (`relation``regclass` ) →`text` Returns the entire file path name (relative to the database cluster's data directory,`PGDATA`) of the relation.
`pg_filenode_relation` (`tablespace``oid`,`filenode``oid` ) →`regclass` Returns a relation's OID given the tablespace OID and filenode it is stored under. This is essentially the inverse mapping of`pg_relation_filepath`. For a relation in the database's default tablespace, the tablespace can be specified as zero. Returns`NULL` if no relation in the current database is associated with the given values.

Function

Description

pg_relation_filenode (relationregclass ) →oid

Returns the“filenode” number currently assigned to the specified relation. The filenode is the base component of the file name(s) used for the relation (seeSection 70.1 for more information). For most relations the result is the same aspg_class.relfilenode, but for certain system catalogsrelfilenode 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 (relationregclass ) →text

Returns the entire file path name (relative to the database cluster's data directory,PGDATA) of the relation.

pg_filenode_relation (tablespaceoid,filenodeoid ) →regclass

Returns a relation's OID given the tablespace OID and filenode it is stored under. This is essentially the inverse mapping ofpg_relation_filepath. For a relation in the database's default tablespace, the tablespace can be specified as zero. ReturnsNULL if no relation in the current database is associated with the given values.

Table 9.92 lists functions used to manage collations.

Table 9.92. Collation Management Functions

Function Description
`pg_collation_actual_version` (`oid` ) →`text` Returns the actual version of the collation object as it is currently installed in the operating system. If this is different from the value in`pg_collation`.`collversion`, then objects depending on the collation might need to be rebuilt. See alsoALTER COLLATION.
`pg_import_system_collations` (`schema``regnamespace` ) →`integer` Adds collations to the system catalog`pg_collation` based on all the locales it finds in the operating system. This is what`initdb` uses; seeSection 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. Use of this function is restricted to superusers.

Function

Description

pg_collation_actual_version (oid ) →text

Returns the actual version of the collation object as it is currently installed in the operating system. If this is different from the value inpg_collation.collversion, then objects depending on the collation might need to be rebuilt. See alsoALTER COLLATION.

pg_import_system_collations (schemaregnamespace ) →integer

Adds collations to the system catalogpg_collation based on all the locales it finds in the operating system. This is whatinitdb uses; seeSection 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 inpg_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.) Theschema parameter would typically bepg_catalog, but that is not a requirement; the collations could be installed into some other schema as well. The function returns the number of new collation objects it created. Use of this function is restricted to superusers.

Table 9.93 lists functions that provide information about the structure of partitioned tables.

Table 9.93. Partitioning Information Functions

Function Description
`pg_partition_tree` (`regclass` ) →`setof record` (`relid``regclass`,`parentrelid``regclass`,`isleaf``boolean`,`level``integer` ) Lists the tables or indexes in the partition tree of the given partitioned table or partitioned index, with one row for each partition. Information provided includes the OID of the partition, the OID of its immediate parent, a boolean value telling if the partition is a leaf, and an integer telling its level in the hierarchy. The level value is 0 for the input table or index, 1 for its immediate child partitions, 2 for their partitions, and so on. Returns no rows if the relation does not exist or is not a partition or partitioned table.
`pg_partition_ancestors` (`regclass` ) →`setof regclass` Lists the ancestor relations of the given partition, including the relation itself. Returns no rows if the relation does not exist or is not a partition or partitioned table.
`pg_partition_root` (`regclass` ) →`regclass` Returns the top-most parent of the partition tree to which the given relation belongs. Returns`NULL` if the relation does not exist or is not a partition or partitioned table.

Function

Description

pg_partition_tree (regclass ) →setof record (relidregclass,parentrelidregclass,isleafboolean,levelinteger )

Lists the tables or indexes in the partition tree of the given partitioned table or partitioned index, with one row for each partition. Information provided includes the OID of the partition, the OID of its immediate parent, a boolean value telling if the partition is a leaf, and an integer telling its level in the hierarchy. The level value is 0 for the input table or index, 1 for its immediate child partitions, 2 for their partitions, and so on. Returns no rows if the relation does not exist or is not a partition or partitioned table.

pg_partition_ancestors (regclass ) →setof regclass

Lists the ancestor relations of the given partition, including the relation itself. Returns no rows if the relation does not exist or is not a partition or partitioned table.

pg_partition_root (regclass ) →regclass

Returns the top-most parent of the partition tree to which the given relation belongs. ReturnsNULL if the relation does not exist or is not a partition or partitioned table.

For example, to check the total size of the data contained in a partitioned tablemeasurement, one could use the following query:

SELECT pg_size_pretty(sum(pg_relation_size(relid))) AS total_size  FROM pg_partition_tree('measurement');

9.27.8. Index Maintenance Functions

Table 9.94 shows the functions available for index maintenance tasks. (Note that these maintenance tasks are normally done automatically by autovacuum; use of these functions is only required in special cases.) These functions cannot be executed during recovery. Use of these functions is restricted to superusers and the owner of the given index.

Table 9.94. Index Maintenance Functions

Function Description
`brin_summarize_new_values` (`index``regclass` ) →`integer` Scans the specified BRIN index to find page ranges in the base table that are not currently summarized by the index; for any such range it creates a new summary index tuple by scanning those table pages. Returns the number of new page range summaries that were inserted into the index.
`brin_summarize_range` (`index``regclass`,`blockNumber``bigint` ) →`integer` Summarizes the page range covering the given block, if not already summarized. This is like`brin_summarize_new_values` except that it only processes the page range that covers the given table block number.
`brin_desummarize_range` (`index``regclass`,`blockNumber``bigint` ) →`void` Removes the BRIN index tuple that summarizes the page range covering the given table block, if there is one.
`gin_clean_pending_list` (`index``regclass` ) →`bigint` Cleans up the“pending” list of the specified GIN index by moving entries in it, in bulk, to the main GIN data structure. Returns the number of pages removed from the pending list. If the argument is a GIN index built with the`fastupdate` option disabled, no cleanup happens and the result is zero, because the index doesn't have a pending list. SeeSection 67.4.1 andSection 67.5 for details about the pending list and`fastupdate` option.

Function

Description

brin_summarize_new_values (indexregclass ) →integer

Scans the specified BRIN index to find page ranges in the base table that are not currently summarized by the index; for any such range it creates a new summary index tuple by scanning those table pages. Returns the number of new page range summaries that were inserted into the index.

brin_summarize_range (indexregclass,blockNumberbigint ) →integer

Summarizes the page range covering the given block, if not already summarized. This is likebrin_summarize_new_values except that it only processes the page range that covers the given table block number.

brin_desummarize_range (indexregclass,blockNumberbigint ) →void

Removes the BRIN index tuple that summarizes the page range covering the given table block, if there is one.

gin_clean_pending_list (indexregclass ) →bigint

Cleans up the“pending” list of the specified GIN index by moving entries in it, in bulk, to the main GIN data structure. Returns the number of pages removed from the pending list. If the argument is a GIN index built with thefastupdate option disabled, no cleanup happens and the result is zero, because the index doesn't have a pending list. SeeSection 67.4.1 andSection 67.5 for details about the pending list andfastupdate option.

9.27.9. Generic File Access Functions

The functions shown inTable 9.95 provide native access to files on the machine hosting the server. Only files within the database cluster directory and thelog_directory can be accessed, unless the user is a superuser or is granted the rolepg_read_server_files. Use a relative path for files in the cluster directory, and a path matching thelog_directory configuration setting for log files.

Note that granting users the EXECUTE privilege onpg_read_file(), or related functions, allows them the ability to read any file on the server that the database server process can read; these functions bypass all in-database privilege checks. This means that, for example, a user with such access is able to read the contents of thepg_authid table where authentication information is stored, as well as read any table data in the database. Therefore, granting access to these functions should be carefully considered.

Some of these functions take an optionalmissing_ok parameter, which specifies the behavior when the file or directory does not exist. Iftrue, the function returnsNULL or an empty result set, as appropriate. Iffalse, an error is raised. The default isfalse.

Table 9.95. Generic File Access Functions

Function Description
`pg_ls_dir` (`dirname``text` [,`missing_ok``boolean`,`include_dot_dirs``boolean`] ) →`setof text` Returns the names of all files (and directories and other special files) in the specified directory. The`include_dot_dirs` parameter indicates whether“.” and“..” are to be included in the result set; the default is to exclude them. Including them can be useful when`missing_ok` is`true`, to distinguish an empty directory from a non-existent directory. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
`pg_ls_logdir` () →`setof record` (`name``text`,`size``bigint`,`modification``timestamp with time zone` ) Returns the name, size, and last modification time (mtime) of each ordinary file in the server's log directory. Filenames beginning with a dot, directories, and other special files are excluded. This function is restricted to superusers and members of the`pg_monitor` role by default, but other users can be granted EXECUTE to run the function.
`pg_ls_waldir` () →`setof record` (`name``text`,`size``bigint`,`modification``timestamp with time zone` ) Returns the name, size, and last modification time (mtime) of each ordinary file in the server's write-ahead log (WAL) directory. Filenames beginning with a dot, directories, and other special files are excluded. This function is restricted to superusers and members of the`pg_monitor` role by default, but other users can be granted EXECUTE to run the function.
`pg_ls_archive_statusdir` () →`setof record` (`name``text`,`size``bigint`,`modification``timestamp with time zone` ) Returns the name, size, and last modification time (mtime) of each ordinary file in the server's WAL archive status directory (`pg_wal/archive_status`). Filenames beginning with a dot, directories, and other special files are excluded. This function is restricted to superusers and members of the`pg_monitor` role by default, but other users can be granted EXECUTE to run the function.
`pg_ls_tmpdir` ( [`tablespace``oid`] ) →`setof record` (`name``text`,`size``bigint`,`modification``timestamp with time zone` ) Returns the name, size, and last modification time (mtime) of each ordinary file in the temporary file directory for the specified`tablespace`. If`tablespace` is not provided, the`pg_default` tablespace is examined. Filenames beginning with a dot, directories, and other special files are excluded. This function is restricted to superusers and members of the`pg_monitor` role by default, but other users can be granted EXECUTE to run the function.
`pg_read_file` (`filename``text` [,`offset``bigint`,`length``bigint` [,`missing_ok``boolean`]] ) →`text` Returns all or part of a text file, starting at the given byte`offset`, returning at most`length` bytes (less if the end of file is reached first). If`offset` is negative, it is relative to the end of the file. If`offset` and`length` are omitted, the entire file is returned. The bytes read from the file are interpreted as a string in the database's encoding; an error is thrown if they are not valid in that encoding. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
`pg_read_binary_file` (`filename``text` [,`offset``bigint`,`length``bigint` [,`missing_ok``boolean`]] ) →`bytea` Returns all or part of a file. This function is identical to`pg_read_file` except that it can read arbitrary binary data, returning the result as`bytea` not`text`; accordingly, no encoding checks are performed. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. In combination with the`convert_from` function, this function can be used to read a text file in a specified encoding and convert to the database's encoding: SELECT convert_from(pg_read_binary_file('file_in_utf8.txt'), 'UTF8');
`pg_stat_file` (`filename``text` [,`missing_ok``boolean`] ) →`record` (`size``bigint`,`access``timestamp with time zone`,`modification``timestamp with time zone`,`change``timestamp with time zone`,`creation``timestamp with time zone`,`isdir``boolean` ) Returns a record containing the file's size, last access time stamp, last modification time stamp, last file status change time stamp (Unix platforms only), file creation time stamp (Windows only), and a flag indicating if it is a directory. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

Function

Description

pg_ls_dir (dirnametext [,missing_okboolean,include_dot_dirsboolean] ) →setof text

Returns the names of all files (and directories and other special files) in the specified directory. Theinclude_dot_dirs parameter indicates whether“.” and“..” are to be included in the result set; the default is to exclude them. Including them can be useful whenmissing_ok istrue, to distinguish an empty directory from a non-existent directory.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

pg_ls_logdir () →setof record (nametext,sizebigint,modificationtimestamp with time zone )

Returns the name, size, and last modification time (mtime) of each ordinary file in the server's log directory. Filenames beginning with a dot, directories, and other special files are excluded.

This function is restricted to superusers and members of thepg_monitor role by default, but other users can be granted EXECUTE to run the function.

pg_ls_waldir () →setof record (nametext,sizebigint,modificationtimestamp with time zone )

Returns the name, size, and last modification time (mtime) of each ordinary file in the server's write-ahead log (WAL) directory. Filenames beginning with a dot, directories, and other special files are excluded.

This function is restricted to superusers and members of thepg_monitor role by default, but other users can be granted EXECUTE to run the function.

pg_ls_archive_statusdir () →setof record (nametext,sizebigint,modificationtimestamp with time zone )

Returns the name, size, and last modification time (mtime) of each ordinary file in the server's WAL archive status directory (pg_wal/archive_status). Filenames beginning with a dot, directories, and other special files are excluded.

This function is restricted to superusers and members of thepg_monitor role by default, but other users can be granted EXECUTE to run the function.

pg_ls_tmpdir ( [tablespaceoid] ) →setof record (nametext,sizebigint,modificationtimestamp with time zone )

Returns the name, size, and last modification time (mtime) of each ordinary file in the temporary file directory for the specifiedtablespace. Iftablespace is not provided, thepg_default tablespace is examined. Filenames beginning with a dot, directories, and other special files are excluded.

This function is restricted to superusers and members of thepg_monitor role by default, but other users can be granted EXECUTE to run the function.

pg_read_file (filenametext [,offsetbigint,lengthbigint [,missing_okboolean]] ) →text

Returns all or part of a text file, starting at the given byteoffset, returning at mostlength bytes (less if the end of file is reached first). Ifoffset is negative, it is relative to the end of the file. Ifoffset andlength are omitted, the entire file is returned. The bytes read from the file are interpreted as a string in the database's encoding; an error is thrown if they are not valid in that encoding.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

pg_read_binary_file (filenametext [,offsetbigint,lengthbigint [,missing_okboolean]] ) →bytea

Returns all or part of a file. This function is identical topg_read_file except that it can read arbitrary binary data, returning the result asbytea nottext; accordingly, no encoding checks are performed.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

In combination with theconvert_from function, this function can be used to read a text file in a specified encoding and convert to the database's encoding:

SELECT convert_from(pg_read_binary_file('file_in_utf8.txt'), 'UTF8');

pg_stat_file (filenametext [,missing_okboolean] ) →record (sizebigint,accesstimestamp with time zone,modificationtimestamp with time zone,changetimestamp with time zone,creationtimestamp with time zone,isdirboolean )

Returns a record containing the file's size, last access time stamp, last modification time stamp, last file status change time stamp (Unix platforms only), file creation time stamp (Windows only), and a flag indicating if it is a directory.

This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

9.27.10. Advisory Lock Functions

The functions shown inTable 9.96 manage advisory locks. For details about proper use of these functions, seeSection 13.3.5.

All these functions are intended to be used to lock application-defined resources, which can be identified either by a single 64-bit key value or two 32-bit key values (note that these two key spaces do not overlap). If another session already holds a conflicting lock on the same resource identifier, the functions will either wait until the resource becomes available, or return afalse result, as appropriate for the function. Locks can be either shared or exclusive: a shared lock does not conflict with other shared locks on the same resource, only with exclusive locks. Locks can be taken at session level (so that they are held until released or the session ends) or at transaction level (so that they are held until the current transaction ends; there is no provision for manual release). Multiple session-level lock requests stack, so that if the same resource identifier is locked three times there must then be three unlock requests to release the resource in advance of session end.

Table 9.96. Advisory Lock Functions

Function Description
`pg_advisory_lock` (`key``bigint` ) →`void` `pg_advisory_lock` (`key1``integer`,`key2``integer` ) →`void` Obtains an exclusive session-level advisory lock, waiting if necessary.
`pg_advisory_lock_shared` (`key``bigint` ) →`void` `pg_advisory_lock_shared` (`key1``integer`,`key2``integer` ) →`void` Obtains a shared session-level advisory lock, waiting if necessary.
`pg_advisory_unlock` (`key``bigint` ) →`boolean` `pg_advisory_unlock` (`key1``integer`,`key2``integer` ) →`boolean` Releases a previously-acquired exclusive session-level advisory lock. Returns`true` if the lock is successfully released. If the lock was not held,`false` is returned, and in addition, an SQL warning will be reported by the server.
`pg_advisory_unlock_all` () →`void` Releases all session-level advisory locks held by the current session. (This function is implicitly invoked at session end, even if the client disconnects ungracefully.)
`pg_advisory_unlock_shared` (`key``bigint` ) →`boolean` `pg_advisory_unlock_shared` (`key1``integer`,`key2``integer` ) →`boolean` Releases a previously-acquired shared session-level advisory lock. Returns`true` if the lock is successfully released. If the lock was not held,`false` is returned, and in addition, an SQL warning will be reported by the server.
`pg_advisory_xact_lock` (`key``bigint` ) →`void` `pg_advisory_xact_lock` (`key1``integer`,`key2``integer` ) →`void` Obtains an exclusive transaction-level advisory lock, waiting if necessary.
`pg_advisory_xact_lock_shared` (`key``bigint` ) →`void` `pg_advisory_xact_lock_shared` (`key1``integer`,`key2``integer` ) →`void` Obtains a shared transaction-level advisory lock, waiting if necessary.
`pg_try_advisory_lock` (`key``bigint` ) →`boolean` `pg_try_advisory_lock` (`key1``integer`,`key2``integer` ) →`boolean` Obtains an exclusive session-level advisory lock if available. This will either obtain the lock immediately and return`true`, or return`false` without waiting if the lock cannot be acquired immediately.
`pg_try_advisory_lock_shared` (`key``bigint` ) →`boolean` `pg_try_advisory_lock_shared` (`key1``integer`,`key2``integer` ) →`boolean` Obtains a shared session-level advisory lock if available. This will either obtain the lock immediately and return`true`, or return`false` without waiting if the lock cannot be acquired immediately.
`pg_try_advisory_xact_lock` (`key``bigint` ) →`boolean` `pg_try_advisory_xact_lock` (`key1``integer`,`key2``integer` ) →`boolean` Obtains an exclusive transaction-level advisory lock if available. This will either obtain the lock immediately and return`true`, or return`false` without waiting if the lock cannot be acquired immediately.
`pg_try_advisory_xact_lock_shared` (`key``bigint` ) →`boolean` `pg_try_advisory_xact_lock_shared` (`key1``integer`,`key2``integer` ) →`boolean` Obtains a shared transaction-level advisory lock if available. This will either obtain the lock immediately and return`true`, or return`false` without waiting if the lock cannot be acquired immediately.

Function

Description

pg_advisory_lock (keybigint ) →void

pg_advisory_lock (key1integer,key2integer ) →void

Obtains an exclusive session-level advisory lock, waiting if necessary.

pg_advisory_lock_shared (keybigint ) →void

pg_advisory_lock_shared (key1integer,key2integer ) →void

Obtains a shared session-level advisory lock, waiting if necessary.

pg_advisory_unlock (keybigint ) →boolean

pg_advisory_unlock (key1integer,key2integer ) →boolean

Releases a previously-acquired exclusive session-level advisory lock. Returnstrue if the lock is successfully released. If the lock was not held,false is returned, and in addition, an SQL warning will be reported by the server.

pg_advisory_unlock_all () →void

Releases all session-level advisory locks held by the current session. (This function is implicitly invoked at session end, even if the client disconnects ungracefully.)

pg_advisory_unlock_shared (keybigint ) →boolean

pg_advisory_unlock_shared (key1integer,key2integer ) →boolean

Releases a previously-acquired shared session-level advisory lock. Returnstrue if the lock is successfully released. If the lock was not held,false is returned, and in addition, an SQL warning will be reported by the server.

pg_advisory_xact_lock (keybigint ) →void

pg_advisory_xact_lock (key1integer,key2integer ) →void

Obtains an exclusive transaction-level advisory lock, waiting if necessary.

pg_advisory_xact_lock_shared (keybigint ) →void

pg_advisory_xact_lock_shared (key1integer,key2integer ) →void

Obtains a shared transaction-level advisory lock, waiting if necessary.

pg_try_advisory_lock (keybigint ) →boolean

pg_try_advisory_lock (key1integer,key2integer ) →boolean

Obtains an exclusive session-level advisory lock if available. This will either obtain the lock immediately and returntrue, or returnfalse without waiting if the lock cannot be acquired immediately.

pg_try_advisory_lock_shared (keybigint ) →boolean

pg_try_advisory_lock_shared (key1integer,key2integer ) →boolean

Obtains a shared session-level advisory lock if available. This will either obtain the lock immediately and returntrue, or returnfalse without waiting if the lock cannot be acquired immediately.

pg_try_advisory_xact_lock (keybigint ) →boolean

pg_try_advisory_xact_lock (key1integer,key2integer ) →boolean

Obtains an exclusive transaction-level advisory lock if available. This will either obtain the lock immediately and returntrue, or returnfalse without waiting if the lock cannot be acquired immediately.

pg_try_advisory_xact_lock_shared (keybigint ) →boolean

pg_try_advisory_xact_lock_shared (key1integer,key2integer ) →boolean

Obtains a shared transaction-level advisory lock if available. This will either obtain the lock immediately and returntrue, or returnfalse without waiting if the lock cannot be acquired immediately.

9.27.11. Compression Control Functions

The functions shown inTable 9.97 provide information about CFS state and activity and control CFS garbage collection.

Table 9.97. Compression Control Functions

Name	Return Type	Description
`cfs_start_gc(n_workers integer)`	`integer`	Starts garbage collection using the specified number of workers. You can only run this function to start the garbage collection manually if background garbage collection is disabled.
`cfs_enable_gc(enabled boolean)`	`boolean`	Enables/disables background garbage collection. Alternatively, you can use thecfs_gc configuration variable.
`cfs_gc_relation(rel regclass)`	`integer`	Performs garbage collection for a particular table. This function returns the number of processed segments.
`cfs_version()`	`text`	Displays the CFS version and the specific compression algorithm used.
`cfs_estimate(rel regclass)`	`float8`	Estimates the effect of table compression. Returns the average compression ratio for the first ten blocks of the relation.
`cfs_compression_ratio(rel regclass)`	`float8`	Returns the actual compression ratio for all segments of the compressed relation.
`cfs_fragmentation(rel regclass)`	`float8`	Returns the average fragmentation ratio of the relation files.
`cfs_gc_activity_processed_bytes()`	`int64`	Returns the total size of pages processed by CFS during garbage collection.
`cfs_gc_activity_processed_pages()`	`int64`	Returns the number of pages processed by CFS during garbage collection.
`cfs_gc_activity_processed_files()`	`int64`	Returns the number of files compacted by CFS during garbage collection.
`cfs_gc_activity_scanned_files()`	`int64`	Returns the number of files scanned by CFS during garbage collection.

9.27.12. Operation Log Support Functions

An operation log stores information about system events of critical importance, such as an upgrade, execution ofpg_resetwal and so on. This information is not interesting to regular users, but is highly useful for vendor's technical support. Recording to the operation log is only done at the system level (without any actions on the part of the user), and SQL functions are used to read the operation log. The function shown inTable 9.98 reads the operation log.

Table 9.98. Operation Log Support Functions

Function Description
`pg_operation_log` () →`setof record` (`event``text`,`edition``text`,`version``text`,`lsn``pg_lsn`,`last``timestamptz`,`count``int4` ) Where: `event` — event (operation) type. Can be`bootstrap`,`startup`,`pg_resetwal`,`pg_rewind`,`pg_upgrade` or`promoted`. `edition` —Postgres Pro edition. Can be`vanilla`,`1c`,`std`,`ent` or`unknown`. `version` —Postgres Pro version. `lsn` — latest checkpoint location, returned bypg_controldata as of the moment when the event occurs. `last` — date/time of the last occurrence of this event type if events are accumulated or date/time of the event occurrence otherwise. `count` — number of events of this type if events are accumulated or 1 otherwise. This system function is created automatically for new databases and needs to be created for existing databases, as follows: CREATE OR REPLACE FUNCTION pg_operation_log( OUT event text, OUT edition text, OUT version text, OUT lsn pg_lsn, OUT last timestamptz, OUT count int4) RETURNS SETOF record LANGUAGE INTERNAL STABLE STRICT PARALLEL SAFEAS 'pg_operation_log';

Function

Description

pg_operation_log () →setof record (eventtext,editiontext,versiontext,lsnpg_lsn,lasttimestamptz,countint4 )

Where:

event — event (operation) type. Can bebootstrap,startup,pg_resetwal,pg_rewind,pg_upgrade orpromoted.
edition —Postgres Pro edition. Can bevanilla,1c,std,ent orunknown.
version —Postgres Pro version.
lsn — latest checkpoint location, returned bypg_controldata as of the moment when the event occurs.
last — date/time of the last occurrence of this event type if events are accumulated or date/time of the event occurrence otherwise.
count — number of events of this type if events are accumulated or 1 otherwise.

This system function is created automatically for new databases and needs to be created for existing databases, as follows:

CREATE OR REPLACE FUNCTION  pg_operation_log(    OUT event text,    OUT edition text,    OUT version text,    OUT lsn pg_lsn,    OUT last timestamptz,    OUT count int4) RETURNS SETOF record LANGUAGE INTERNAL STABLE STRICT PARALLEL SAFEAS 'pg_operation_log';

This is an example of what thepg_operation_log function returns:

select * from pg_operation_log();   event    | edition | version |    lsn    |          last          | count------------+---------+---------+-----------+------------------------+------- startup    | vanilla | 10.22.0 | 0/8000028 | 2022-10-27 23:06:27+03 |     1 pg_upgrade | 1c      | 15.0.0  | 0/8000028 | 2022-10-27 23:06:27+03 |     1 startup    | 1c      | 15.0.0  | 0/80001F8 | 2022-10-27 23:06:53+03 |     2(3 rows)

Note

If an operation log is empty, before a record forpg_upgrade, a record is added forstartup with the previousversion having the following specifics: as there is no information inpg_upgrade on the patch version number of the previous version for each edition (1c,std,ent), the patch version number (third number in the version number) is set to zero. For an upgrade from1c orvanilla, it cannot be distinguished whether the edition of the database before upgrade isvanilla or1c, andvanilla edition is used for thestartup record.

9.27.13. Debugging Functions

The function shown inTable 9.99 can assist you in low-level activities, such as debugging or exploring corruptedPostgres Pro databases.

Table 9.99. Snapshot Synchronization Functions

Name	Return Type	Description
`pg_snapshot_any()`	`void`	Sets the current transaction to ignore MVCC rules and see all versions of data.

Usepg_snapshot_any with care. Run it in a transaction with isolation levelREPEATABLE READ or higher, otherwise the specific snapshot will be replaced by a new one by the next query. Only superusers can run this function.

Note

If you created the database cluster using the server version that did not provide this function, execute the command:

CREATE FUNCTION pg_snapshot_any() RETURNS void AS 'pg_snapshot_any' LANGUAGE internal;

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