VACUUM

Description

VACUUM reclaims storage occupied by deleted 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.

With no parameter, VACUUM processes every table in the current database. With a parameter, VACUUM processes only that table.

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. VACUUM FULL does more extensive processing, including moving of tuples across blocks to try to compact the table to the minimum number of disk blocks. This form is much slower and requires an exclusive lock on each table while it is being processed.

FREEZE is a special-purpose option that causes tuples to be marked "frozen" as soon as possible, rather than waiting until they are quite old. If this is done when there are no other open transactions in the same database, then it is guaranteed that all tuples in the database are "frozen" and will not be subject to transaction ID wraparound problems, no matter how long the database is left unvacuumed. FREEZE is not recommended for routine use. Its only intended usage is in connection with preparation of user-defined template databases, or other databases that are completely read-only and will not receive routine maintenance VACUUM operations. See Chapter 22 for details.

Parameters

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

We recommend that active production databases be vacuumed frequently (at least nightly), in order to remove expired rows. After adding or deleting a large number of rows, it may 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 may be useful in special cases. An example is when you have deleted most of the rows in a table and would like the table to physically shrink to occupy less disk space. VACUUM FULL will usually shrink the table more than a plain VACUUM would.

VACUUM causes a substantial increase in I/O traffic, which can cause poor performance for other active sessions. Therefore, it is sometimes advisable to use the cost-based vacuum delay feature. See Section 17.4.4 for details.

Examples

The following is an example from running VACUUM on a table in the regression database:

regression=# VACUUM VERBOSE ANALYZE onek;
INFO:  vacuuming "public.onek"
INFO:  index "onek_unique1" now contains 1000 tuples in 14 pages
DETAIL:  3000 index tuples were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.01s/0.08u sec elapsed 0.18 sec.
INFO:  index "onek_unique2" now contains 1000 tuples in 16 pages
DETAIL:  3000 index tuples were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.07u sec elapsed 0.23 sec.
INFO:  index "onek_hundred" now contains 1000 tuples in 13 pages
DETAIL:  3000 index tuples were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.01s/0.08u sec elapsed 0.17 sec.
INFO:  index "onek_stringu1" now contains 1000 tuples in 48 pages
DETAIL:  3000 index tuples were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.01s/0.09u sec elapsed 0.59 sec.
INFO:  "onek": removed 3000 tuples in 108 pages
DETAIL:  CPU 0.01s/0.06u sec elapsed 0.07 sec.
INFO:  "onek": found 3000 removable, 1000 nonremovable tuples in 143 pages
DETAIL:  0 dead tuples cannot be removed yet.
There were 0 unused item pointers.
0 pages are entirely empty.
CPU 0.07s/0.39u sec elapsed 1.56 sec.
INFO:  analyzing "public.onek"
INFO:  "onek": 36 pages, 1000 rows sampled, 1000 estimated total rows
VACUUM

Compatibility

There is no VACUUM statement in the SQL standard.

See Also