Notes
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
and ANALYZE once a day during a low-usage time of day.
Unlike VACUUM FULL,
ANALYZE requires
only a read lock on the target table, so it can run in parallel with
other activity on the table.
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 may result in small changes in the planner's
estimated costs shown by EXPLAIN.
The collected statistics 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 may 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 the User's
Guide.
The extent of analysis can be controlled by adjusting the
default_statistics_target parameter 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). 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 occupied
in pg_statistic.
In particular, setting the statistics target to zero disables collection of
statistics for that column. It may 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.
