9.27. 系統管理函式
The functions described in this section are used to control and monitor a PostgreSQL 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
9.27.2. Server Signaling Functions
The functions shown in Table 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 using GRANT
, with noted exceptions.
Each of these functions returns true
if successful and false
otherwise.
Table 9.84. Server Signaling Functions
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.
9.27.3. Backup Control Functions
The functions shown in Table 9.85 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
).
For details about proper usage of these functions, see Section 25.3.
Table 9.85. Backup Control Functions
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 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 use pg_walfile_name_offset
to extract the corresponding write-ahead log file name and byte offset from a pg_lsn
value. For example:
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 in Table 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
The functions shown in Table 9.87 control the progress of recovery. These functions may be executed only during recovery.
Table 9.87. Recovery Control Functions
pg_wal_replay_pause
and pg_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
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 9.88, and imported with the SET TRANSACTION command.
Table 9.88. Snapshot Synchronization Functions
9.27.6. Replication Management Functions
The functions shown in Table 9.89 are for controlling and interacting with replication features. See Section 26.2.5, Section 26.2.6, and Chapter 49 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 having REPLICATION
privilege.
Many of these functions have equivalent commands in the replication protocol; see Section 52.4.
The functions described in Section 9.27.3, Section 9.27.4, and Section 9.27.5 are also relevant for replication.
Table 9.89. Replication Management Functions
9.27.7. Database Object Management Functions
The functions shown in Table 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
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.
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
Table 9.92 lists functions used to manage collations.
Table 9.92. Collation Management Functions
Table 9.93 lists functions that provide information about the structure of partitioned tables.
Table 9.93. Partitioning Information Functions
For example, to check the total size of the data contained in a partitioned table measurement
, one could use the following query:
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
9.27.9. Generic File Access Functions
The functions shown in Table 9.95 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 a superuser or 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 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 the pg_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 optional missing_ok
parameter, which specifies the behavior when the file or directory does not exist. If true
, the function returns NULL
or an empty result set, as appropriate. If false
, an error is raised. The default is false
.
Table 9.95. Generic File Access Functions
9.27.10. Advisory Lock Functions
The functions shown in Table 9.96 manage advisory locks. For details about proper use of these functions, see Section 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 a false
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
Last updated