ANALYZE()			 SQL Commands			     ANALYZE()



NAME
       ANALYZE - collect statistics about a database


SYNOPSIS
       ANALYZE [ VERBOSE ] [ table [ (column [, ...] ) ] ]


DESCRIPTION
       ANALYZE	collects  statistics about the contents of tables in the data-
       base, and stores the results in the system table	 pg_statistic.	Subse-
       quently,	 the query planner uses these statistics to help determine the
       most efficient execution plans for queries.

       With no parameter, ANALYZE examines every table in  the	current	 data-
       base. With a parameter, ANALYZE examines only that table. It is further
       possible to give a list of column names, in which case only the statis-
       tics for those columns are collected.

PARAMETERS
       VERBOSE
	      Enables display of progress messages.

       table  The name (possibly schema-qualified) of a specific table to ana-
	      lyze. 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, ANALYZE emits progress messages to indicate
       which table is currently being processed. Various statistics about  the
       tables are printed as well.

NOTES
       It  is  a  good	idea to run ANALYZE periodically, or just after making
       major changes in the contents of a table. Accurate statistics will help
       the  planner  to	 choose	 the  most appropriate query plan, and thereby
       improve the speed of query processing. A common strategy is to run VAC-
       UUM [vacuum(7)] and ANALYZE once a day during a low-usage time of day.

       Unlike VACUUM FULL, ANALYZE requires only a read lock on the target ta-
       ble, so it can run in parallel with other activity on the table.

       The statistics collected by ANALYZE usually include a list of  some  of
       the  most  common  values  in  each  column and a histogram showing the
       approximate data distribution in each column. One or both of these  may
       be  omitted  if	ANALYZE	 deems	them  uninteresting (for example, in a
       unique-key column, there are no common values) or if  the  column  data
       type does not support the appropriate operators. There is more informa-
       tion about the statistics in the documentation.

       For large tables, ANALYZE takes a random sample of the table  contents,
       rather  than examining every row. This allows even very large tables to
       be analyzed in a small amount of time. Note, however, that the  statis-
       tics  are  only approximate, and will change slightly each time ANALYZE
       is run, even if the actual table contents  did  not  change.  This  may
       result  in  small  changes  in  the  planner's estimated costs shown by
       EXPLAIN. In rare situations, this non-determinism will cause the	 query
       optimizer  to choose a different query plan between runs of ANALYZE. To
       avoid this, raise the amount of statistics  collected  by  ANALYZE,  as
       described below.

       The  extent of analysis can be controlled by adjusting the default_sta-
       tistics_target configuration variable, or on a  column-by-column	 basis
       by  setting the per-column statistics target with ALTER TABLE ... ALTER
       COLUMN ... SET STATISTICS (see ALTER TABLE [alter_table(7)]). The  tar-
       get  value  sets the maximum number of entries in the most-common-value
       list and the maximum number of bins in the histogram. The default  tar-
       get value is 10, but this can be adjusted up or down to trade off accu-
       racy of planner estimates against the time taken for  ANALYZE  and  the
       amount  of  space  occupied in pg_statistic. In particular, setting the
       statistics target to zero disables collection of	 statistics  for  that
       column.	It may be useful to do that for columns that are never used as
       part of the WHERE, GROUP BY, or ORDER BY clauses of queries, since  the
       planner will have no use for statistics on such columns.

       The  largest  statistics target among the columns being analyzed deter-
       mines the number of table  rows	sampled	 to  prepare  the  statistics.
       Increasing  the	target	causes a proportional increase in the time and
       space needed to do ANALYZE.

COMPATIBILITY
       There is no ANALYZE statement in the SQL standard.



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