Provided by: postgresql-client-9.1_9.1.1-1_i386 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