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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