VACUUM() SQL Commands VACUUM()
NAME
VACUUM - garbage-collect and optionally analyze a database
SYNOPSIS
VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ table ]
VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] ANALYZE [ table [ (column [, ...] ) ] ]
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.
With no parameter, VACUUM processes every table in the current data-
base. 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 [analyze(l)] for more details about its process-
ing.
Plain VACUUM (without FULL) simply reclaims space and makes it avail-
able 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 num-
ber of disk blocks. This form is much slower and requires an exclusive
lock on each table while it is being processed.
PARAMETERS
FULL Selects ``full'' vacuum, which can reclaim more space, but takes
much longer and exclusively locks the table.
FREEZE Selects aggressive ``freezing'' of tuples. Specifying FREEZE is
equivalent to performing VACUUM with the vacuum_freeze_min_age
parameter set to zero. The FREEZE option is deprecated and will
be removed in a future release; set the parameter instead.
VERBOSE
Prints a detailed vacuum activity report for each table. Can be
used to help determine appropriate settings for max_fsm_pages,
max_fsm_relations, and default_statistics_target.
ANALYZE
Updates statistics used by the planner to determine the most
efficient way to execute a query.
table The name (optionally schema-qualified) of a specific table to
vacuum. Defaults to all tables in the current database.
column The name of a specific column to analyze. Defaults to all col-
umns.
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
VACUUM cannot be executed inside a transaction block.
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 FULL
option does not shrink indexes; a periodic REINDEX is still recom-
mended. In fact, it is often faster to drop all indexes, VACUUM FULL,
and recreate the indexes.
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. See in the docu-
mentation for details.
PostgreSQL includes an ``autovacuum'' facility which can automate rou-
tine vacuum maintenance. For more information about automatic and man-
ual vacuuming, see in the documentation.
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
vacuumdb [vacuumdb(1)], in the documentation, in the documentation
SQL - Language Statements 2008-09-19 VACUUM()
postgresql 8.3.4 - Generated Sun Oct 5 09:40:26 CDT 2008