Provided by: postgresql-client-9.1_9.1.3-2_amd64 bug

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. 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
           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, the autovacuum daemon (see Section 23.1.5, “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 is to run 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 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. The autovacuum daemon, however, will only consider
       inserts or updates on the parent table when deciding whether to trigger an automatic
       analyze. If that table is rarely inserted into or updated, the inheritance statistics will
       not be up to date unless you run ANALYZE manually.

COMPATIBILITY

       There is no ANALYZE statement in the SQL standard.

SEE ALSO

       VACUUM(7), vacuumdb(1), Section 18.4.3, “Cost-based Vacuum Delay”, in the documentation,
       Section 23.1.5, “The Autovacuum Daemon”, in the documentation