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 system catalog. 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 database. With a parameter, ANALYZE examines only that table. It is  further
       possible to give a list of column names, in which case only the statistics for those columns are collected.

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 database.

       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 strategy 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  histogram  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  infor-
       mation 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  ANA-
       LYZE 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 statistics collected by ANALYZE, as described below.

       The  extent of analysis can be controlled by adjusting the default_statistics_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
       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                                           2010-05-14                                                          ANALYZE(7)