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ANALYZE(7)

NAME

   ANALYZE - collect statistics about a database

SYNOPSIS

   ANALYZE [ VERBOSE ] [ table_name [ ( column_name [, ...] ) ] ]

DESCRIPTION

   ANALYZE collects statistics about the contents of tables in the
   database, and stores the results in the pg_statistic system 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 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_name
       The name (possibly schema-qualified) of a specific table to
       analyze. If omitted, all regular tables (but not foreign tables) in
       the current database are analyzed.

   column_name
       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

   Foreign tables are analyzed only when explicitly selected. Not all
   foreign data wrappers support ANALYZE. If the table's wrapper does not
   support ANALYZE, the command prints a warning and does nothing.

   In the default PostgreSQL configuration, the autovacuum daemon (see
   Section 23.1.6, "The Autovacuum Daemon", 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 for read-mostly databases is to run VACUUM(7) and ANALYZE once
   a day during a low-usage time of day. (This will not be sufficient if
   there is heavy update activity.)

   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
   information about the statistics in Chapter 23, Routine Database
   Maintenance Tasks, 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(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.

   One of the values estimated by ANALYZE is the number of distinct values
   that appear in each column. Because only a subset of the rows are
   examined, this estimate can sometimes be quite inaccurate, even with
   the largest possible statistics target. If this inaccuracy leads to bad
   query plans, a more accurate value can be determined manually and then
   installed with ALTER TABLE ... ALTER COLUMN ... SET (n_distinct = ...)
   (see ALTER TABLE (ALTER_TABLE(7))).

   If the table being analyzed has one or more children, ANALYZE will
   gather statistics twice: once on the rows of the parent table only, and
   a second time on the rows of the parent table with all of its children.
   This second set of statistics is needed when planning queries that
   traverse the entire inheritance tree. The autovacuum daemon, however,
   will only consider inserts or updates on the parent table itself when
   deciding whether to trigger an automatic analyze for that table. If
   that table is rarely inserted into or updated, the inheritance
   statistics will not be up to date unless you run ANALYZE manually.

   If any of the child tables are foreign tables whose foreign data
   wrappers do not support ANALYZE, those child tables are ignored while
   gathering inheritance statistics.

   If the table being analyzed is completely empty, ANALYZE will not
   record new statistics for that table. Any existing statistics will be
   retained.

COMPATIBILITY

   There is no ANALYZE statement in the SQL standard.

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