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ANALYZE() SQL Commands ANALYZE()
NAME
ANALYZE - collect statistics about a database
SYNOPSIS
ANALYZE [ VERBOSE ] [ table [ ( column [, ...] ) ] ]
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 The name (possibly schema-qualified) of a specific table to analyze.
Defaults to all tables in the current database.
column 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
In the default PostgreSQL configuration, in the documentation takes care of auto-
matic 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 [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 com-
mon 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 uninter-
esting (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 informa-
tion about the statistics in 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 [explain(7)]. In rare situations, this non-determinism will cause the
query optimizer to choose a different query plan between runs of ANALYZE. 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 10, 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 occu-
pied 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 num-
ber of table rows sampled to prepare the statistics. Increasing the target causes a
proportional increase in the time and space needed to do ANALYZE.
COMPATIBILITY
There is no ANALYZE statement in the SQL standard.
SEE ALSO
VACUUM [vacuum(7)], vacuumdb [vacuumdb(1)], in the documentation, in the documenta-
tion
SQL - Language Statements 2009-03-12 ANALYZE()
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