Provided by: postgresql-client-9.1_9.1.3-2_i386
ANALYZE - collect statistics about a database
ANALYZE [ VERBOSE ] [ table [ ( column [, ...] ) ] ]
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.
Enables display of progress messages.
The name (possibly schema-qualified) of a specific table to
analyze. Defaults to all tables in the current database.
The name of a specific column to analyze. Defaults to all columns.
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.
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
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
There is no ANALYZE statement in the SQL standard.
VACUUM(7), vacuumdb(1), Section 18.4.3, “Cost-based Vacuum Delay”, in
the documentation, Section 23.1.5, “The Autovacuum Daemon”, in the