Ubuntu Manpage: ANALYZE - collect statistics about a database

Provided by: postgresql-client-9.3_9.3.24-0ubuntu0.14.04_amd64

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 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.