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, partitioned tables, and materialized views in the current database are
           analyzed (but not foreign tables). If the specified table is a partitioned table, both
           the inheritance statistics of the partitioned table as a whole and statistics of the
           individual partitions are updated.

       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

       To analyze a table, one must ordinarily be the table's owner or a superuser. However,
       database owners are allowed to analyze all tables in their databases, except shared
       catalogs. (The restriction for shared catalogs means that a true database-wide ANALYZE can
       only be performed by a superuser.)  ANALYZE will skip over any tables that the calling
       user does not have permission to analyze.

       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 24.1.6) 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 24.

       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 inheritance children, ANALYZE gathers two sets of
       statistics: one on the rows of the parent table only, and a second including rows of both
       the parent table and all of its children. This second set of statistics is needed when
       planning queries that process the inheritance tree as a whole. The child tables themselves
       are not individually analyzed in this case. 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.

       For partitioned tables, ANALYZE gathers statistics by sampling rows from all partitions;
       in addition, it will recurse into each partition and update its statistics. Each leaf
       partition is analyzed only once, even with multi-level partitioning. No statistics are
       collected for only the parent table (without data from its partitions), because with
       partitioning it's guaranteed to be empty.

       The autovacuum daemon does not process partitioned tables, nor does it process inheritance
       parents if only the children are ever modified. It is usually necessary to periodically
       run a manual ANALYZE to keep the statistics of the table hierarchy up to date.

       If any child tables or partitions are foreign tables whose foreign data wrappers do not
       support ANALYZE, those 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.

SEE ALSO

       VACUUM(7), vacuumdb(1), Section 19.4.4, Section 24.1.6