ANALYZE(7)                                                SQL Commands                                                ANALYZE(7)



NAME
       ANALYZE - collect statistics about a database


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


DESCRIPTION
       ANALYZE collects statistics about the contents of tables in the database, and stores the results in the pg_statistic sys-
       tem catalog. Subsequently, 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 database. With a parameter, ANALYZE examines only that ta-
       ble. It is further possible to give a list of column names, in which case only the statistics for those columns are  col-
       lected.

PARAMETERS
       VERBOSE
              Enables display of progress messages.

       table  The  name  (possibly schema-qualified) of a specific table to analyze. Defaults to all tables in the current data-
              base.

       column The name of a specific column to analyze. Defaults to all columns.

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
       In  the  default PostgreSQL configuration, in the documentation takes care of automatic analyzing of tables when they are
       first loaded with data, and as they change throughout regular operation.  When autovacuum is disabled, 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 strat-
       egy is to run VACUUM [vacuum(7)] and ANALYZE once a day during a low-usage time of day.

       ANALYZE requires only a read lock on the target table, 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 his-
       togram showing the approximate data distribution in each column. One or both of these can 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 information about the statistics in 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 statistics are only approximate, and
       will change slightly each time ANALYZE is run, even if the actual table contents did not change.  This  might  result  in
       small  changes  in the planner's estimated costs shown by EXPLAIN [explain(7)].  In rare situations, this non-determinism
       will cause the planner's choices of query plans to change after ANALYZE is run.  To avoid this, raise the amount of  sta-
       tistics collected by ANALYZE, as described below.

       The  extent of analysis can be controlled by adjusting the default_statistics_target configuration variable, or on a col-
       umn-by-column basis by setting the per-column statistics target with ALTER TABLE ... ALTER COLUMN ... SET STATISTICS (see
       ALTER  TABLE [alter_table(7)]). The target value sets the maximum number of entries in the most-common-value list and the
       maximum number of bins in the histogram. The default target value is 100, but this can be adjusted up or  down  to  trade
       off accuracy 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 might  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 determines 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.

SEE ALSO
       VACUUM [vacuum(7)], vacuumdb [vacuumdb(1)], in the documentation, in the documentation



SQL - Language Statements                                  2011-09-22                                                 ANALYZE(7)