VACUUM
Prev | Up | SQL Commands | Home | Next

## VACUUM

VACUUM — garbage-collect and optionally analyze a database

## Synopsis

    VACUUM [ ( option [, ...] ) ] [ table_and_columns [, ...] ]
    VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ table_and_columns [, ...] ]

    where option can be one of:

        FULL [ boolean ]
        FREEZE [ boolean ]
        VERBOSE [ boolean ]
        ANALYZE [ boolean ]
        DISABLE_PAGE_SKIPPING [ boolean ]
        SKIP_LOCKED [ boolean ]
        INDEX_CLEANUP [ boolean ]
        TRUNCATE [ boolean ]
        PARALLEL integer

    and table_and_columns is:

        table_name [ ( column_name [, ...] ) ]

## Description

`VACUUM` reclaims storage occupied by dead tuples. In normal PostgreSQL operation, tuples that are deleted or obsoleted by an update are not physically removed from their table; they remain present until a `VACUUM` is done. Therefore it's necessary to do `VACUUM` periodically, especially on frequently-updated tables.

Without a `table_and_columns` list, `VACUUM` processes every table and materialized view in the current database that the current user has permission to vacuum. With a list, `VACUUM` processes only those table(s).

`VACUUM ANALYZE` performs a `VACUUM` and then an `ANALYZE` for each selected table. This is a handy combination form for routine maintenance scripts. See ANALYZE for more details about its processing.

Plain `VACUUM` (without `FULL`) simply reclaims space and makes it available for re-use. This form of the command can operate in parallel with normal reading and writing of the table, as an exclusive lock is not obtained. However, extra space is not returned to the operating system (in most cases); it's just kept available for re-use within the same table. It also allows us to leverage multiple CPUs in order to process indexes. This feature is known as parallel vacuum. To disable this feature, one can use `PARALLEL` option and specify parallel workers as zero. `VACUUM FULL` rewrites the entire contents of the table into a new disk file with no extra space, allowing unused space to be returned to the operating system. This form is much slower and requires an `ACCESS EXCLUSIVE` lock on each table while it is being processed.

When the option list is surrounded by parentheses, the options can be written in any order. Without parentheses, options must be specified in exactly the order shown above. The parenthesized syntax was added in PostgreSQL 9.0; the unparenthesized syntax is deprecated.

## Parameters

`FULL`

  Selects “full” vacuum, which can reclaim more space, but takes much longer and exclusively locks the table. This method also requires extra disk space, since it writes a new copy of the table and doesn't release the old copy until the operation is complete. Usually this should only be used when a significant amount of space needs to be reclaimed from within the table.

`FREEZE`

  Selects aggressive “freezing” of tuples. Specifying `FREEZE` is equivalent to performing `VACUUM` with the vacuum_freeze_min_age and vacuum_freeze_table_age parameters set to zero. Aggressive freezing is always performed when the table is rewritten, so this option is redundant when `FULL` is specified.

`VERBOSE`

  Prints a detailed vacuum activity report for each table.

`ANALYZE`

  Updates statistics used by the planner to determine the most efficient way to execute a query.

`DISABLE_PAGE_SKIPPING`

  Normally, `VACUUM` will skip pages based on the visibility map. Pages where all tuples are known to be frozen can always be skipped, and those where all tuples are known to be visible to all transactions may be skipped except when performing an aggressive vacuum. Furthermore, except when performing an aggressive vacuum, some pages may be skipped in order to avoid waiting for other sessions to finish using them. This option disables all page-skipping behavior, and is intended to be used only when the contents of the visibility map are suspect, which should happen only if there is a hardware or software issue causing database corruption.

`SKIP_LOCKED`

  Specifies that `VACUUM` should not wait for any conflicting locks to be released when beginning work on a relation: if a relation cannot be locked immediately without waiting, the relation is skipped. Note that even with this option, `VACUUM` may still block when opening the relation's indexes. Additionally, `VACUUM ANALYZE` may still block when acquiring sample rows from partitions, table inheritance children, and some types of foreign tables. Also, while `VACUUM` ordinarily processes all partitions of specified partitioned tables, this option will cause `VACUUM` to skip all partitions if there is a conflicting lock on the partitioned table.

`INDEX_CLEANUP`

  Specifies that `VACUUM` should attempt to remove index entries pointing to dead tuples. This is normally the desired behavior and is the default unless the `vacuum_index_cleanup` option has been set to false for the table to be vacuumed. Setting this option to false may be useful when it is necessary to make vacuum run as quickly as possible, for example to avoid imminent transaction ID wraparound (see Section 24.1.5). However, if index cleanup is not performed regularly, performance may suffer, because as the table is modified, indexes will accumulate dead tuples and the table itself will accumulate dead line pointers that cannot be removed until index cleanup is completed. This option has no effect for tables that do not have an index and is ignored if the `FULL` option is used.

`TRUNCATE`

  Specifies that `VACUUM` should attempt to truncate off any empty pages at the end of the table and allow the disk space for the truncated pages to be returned to the operating system. This is normally the desired behavior and is the default unless the `vacuum_truncate` option has been set to false for the table to be vacuumed. Setting this option to false may be useful to avoid `ACCESS EXCLUSIVE` lock on the table that the truncation requires. This option is ignored if the `FULL` option is used.

`PARALLEL`

  Perform index vacuum and index cleanup phases of `VACUUM` in parallel using `integer` background workers (for the details of each vacuum phase, please refer to Table 27.37). The number of workers used to perform the operation is equal to the number of indexes on the relation that support parallel vacuum which is limited by the number of workers specified with `PARALLEL` option if any which is further limited by max_parallel_maintenance_workers. An index can participate in parallel vacuum if and only if the size of the index is more than min_parallel_index_scan_size. Please note that it is not guaranteed that the number of parallel workers specified in `integer` will be used during execution. It is possible for a vacuum to run with fewer workers than specified, or even with no workers at all. Only one worker can be used per index. So parallel workers are launched only when there are at least `2` indexes in the table. Workers for vacuum are launched before the start of each phase and exit at the end of the phase. These behaviors might change in a future release. This option can't be used with the `FULL` option.

`boolean`

  Specifies whether the selected option should be turned on or off. You can write `TRUE`, `ON`, or `1` to enable the option, and `FALSE`, `OFF`, or `0` to disable it. The `boolean` value can also be omitted, in which case `TRUE` is assumed.

`integer`

  Specifies a non-negative integer value passed to the selected option.

`table_name`

  The name (optionally schema-qualified) of a specific table or materialized view to vacuum. If the specified table is a partitioned table, all of its leaf partitions are vacuumed.

`column_name`

  The name of a specific column to analyze. Defaults to all columns. If a column list is specified, `ANALYZE` must also be specified.

## Outputs

When `VERBOSE` is specified, `VACUUM` emits progress messages to indicate which table is currently being processed. Various statistics about the tables are printed as well.

## Notes

To vacuum a table, one must ordinarily be the table's owner or a superuser. However, database owners are allowed to vacuum all tables in their databases, except shared catalogs. (The restriction for shared catalogs means that a true database-wide `VACUUM` can only be performed by a superuser.) `VACUUM` will skip over any tables that the calling user does not have permission to vacuum.

`VACUUM` cannot be executed inside a transaction block.

For tables with GIN indexes, `VACUUM` (in any form) also completes any pending index insertions, by moving pending index entries to the appropriate places in the main GIN index structure. See Section 66.4.1 for details.

We recommend that active production databases be vacuumed frequently (at least nightly), in order to remove dead rows. After adding or deleting a large number of rows, it might be a good idea to issue a `VACUUM ANALYZE` command for the affected table. This will update the system catalogs with the results of all recent changes, and allow the PostgreSQL query planner to make better choices in planning queries.

The `FULL` option is not recommended for routine use, but might be useful in special cases. An example is when you have deleted or updated most of the rows in a table and would like the table to physically shrink to occupy less disk space and allow faster table scans. `VACUUM FULL` will usually shrink the table more than a plain `VACUUM` would.

The `PARALLEL` option is used only for vacuum purposes. If this option is specified with the `ANALYZE` option, it does not affect `ANALYZE`.

`VACUUM` causes a substantial increase in I/O traffic, which might cause poor performance for other active sessions. Therefore, it is sometimes advisable to use the cost-based vacuum delay feature. For parallel vacuum, each worker sleeps in proportion to the work done by that worker. See Section 19.4.4 for details.

PostgreSQL includes an “autovacuum” facility which can automate routine vacuum maintenance. For more information about automatic and manual vacuuming, see Section 24.1.

## Examples

To clean a single table `onek`, analyze it for the optimizer and print a detailed vacuum activity report:

    VACUUM (VERBOSE, ANALYZE) onek;

## Compatibility

There is no `VACUUM` statement in the SQL standard.

## See Also

vacuumdb, Section 19.4.4, Section 24.1.6

Prev | Up | Next
UPDATE | Home | VALUES