ANALYZE — collect statistics about a database
ANALYZE [ VERBOSE ] [
column_name[, ...] ) ] ]
ANALYZE collects statistics about the contents
of tables in the database, and stores the results in the
system catalog. Subsequently, the query planner uses these
statistics to help determine the most efficient execution plans for
With no parameter,
ANALYZE examines every table in the
current database. With a parameter,
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. 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.
The name of a specific column to analyze. Defaults to all columns.
VERBOSE is specified,
progress messages to indicate which table is currently being
processed. Various statistics about the tables are printed as well.
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
ANALYZE once a day during a low-usage time of day.
(This will not be sufficient if there is heavy update activity.)
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
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
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
ALTER TABLE ... ALTER COLUMN ... SET
STATISTICS (see ALTER TABLE).
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
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
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
ALTER TABLE ... ALTER COLUMN ... SET (n_distinct = ...)
(see ALTER TABLE).
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
If any of the child tables are foreign tables whose foreign data wrappers
do not support
ANALYZE, those child tables are ignored while
gathering inheritance statistics.
If the table being analyzed is completely empty,
will not record new statistics for that table. Any existing statistics
will be retained.
There is no
ANALYZE statement in the SQL standard.