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 data-
base, and stores the results in the pg_statistic system catalog. Subse-
quently, 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 data-
base. With a parameter, ANALYZE examines only that table. It is further
possible to give a list of column names, in which case only the statis-
tics for those columns are collected.
PARAMETERS
VERBOSE
Enables display of progress messages.
table The name (possibly schema-qualified) of a specific table to ana-
lyze. Defaults to all tables in the current database.
column The name of a specific column to analyze. Defaults to all col-
umns.
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 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 statis-
tics 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(l)] 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 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 statis-
tics 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(l)]. 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_sta-
tistics_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(l)]). The tar-
get value sets the maximum number of entries in the most-common-value
list and the maximum number of bins in the histogram. The default tar-
get value is 10, but this can be adjusted up or down to trade off accu-
racy 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 deter-
mines 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.
COMPATIBILITY
There is no ANALYZE statement in the SQL standard.
SEE ALSO
VACUUM [vacuum(l)], vacuumdb [vacuumdb(1)], in the documentation, in
the documentation
SQL - Language Statements 2008-09-19 ANALYZE()
postgresql 8.3.4 - Generated Thu Oct 2 08:35:23 CDT 2008