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 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(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 num-
       ber 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	 data-
       base  are  ``frozen''  and  will not be subject to transaction ID wrap-
       around 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 rou-
       tine maintenance VACUUM operations.  See the documentation 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 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
       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 system
       catalogs with the results of all recent changes, and  allow  the	 Post-
       greSQL 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 the documen-
       tation 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
       vacuumdb [vacuumdb(1)], the documentation



SQL - Language Statements	  2010-12-14			      VACUUM()