vacuum
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 deleted tuples. In normal Post-
greSQL 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 [analyze(7)] 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
number of disk blocks. This form is much slower and requires an exclu-
sive 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 unvacu-
umed. 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 [XRef to MAIN-
TENANCE] for details.
PARAMETERS
FULL Selects ‘‘full’’ vacuum, which may reclaim more space, but
takes much longer and exclusively locks the table.
FREEZE Selects aggressive ‘‘freezing’’ of tuples.
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.
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
columns.
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 VAC-
UUM ANALYZE command for the affected table. This will update the sys-
tem 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.
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)]
SQL - Language Statements 2008-01-03 VACUUM()
