1ANALYZE(7) SQL Commands ANALYZE(7)
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6 ANALYZE - collect statistics about a database
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10 ANALYZE [ VERBOSE ] [ table [ ( column [, ...] ) ] ]
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14 ANALYZE collects statistics about the contents of tables in the data‐
15 base, and stores the results in the pg_statistic system catalog. Subse‐
16 quently, the query planner uses these statistics to help determine the
17 most efficient execution plans for queries.
18 With no parameter, ANALYZE examines every table in the current data‐
20 base. With a parameter, ANALYZE examines only that table. It is further
21 possible to give a list of column names, in which case only the statis‐
22 tics for those columns are collected.
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25 VERBOSE
26 Enables display of progress messages.
27 table The name (possibly schema-qualified) of a specific table to ana‐
29 lyze. Defaults to all tables in the current database.
30 column The name of a specific column to analyze. Defaults to all col‐
32 umns.
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35 When VERBOSE is specified, ANALYZE emits progress messages to indicate
36 which table is currently being processed. Various statistics about the
37 tables are printed as well.
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40 In the default PostgreSQL configuration, in the documentation takes
41 care of automatic analyzing of tables when they are first loaded with
42 data, and as they change throughout regular operation. When autovacuum
43 is disabled, it is a good idea to run ANALYZE periodically, or just
44 after making major changes in the contents of a table. Accurate statis‐
45 tics will help the planner to choose the most appropriate query plan,
46 and thereby improve the speed of query processing. A common strategy is
47 to run VACUUM [vacuum(7)] and ANALYZE once a day during a low-usage
48 time of day.
49 ANALYZE requires only a read lock on the target table, so it can run in
51 parallel with other activity on the table.
52 The statistics collected by ANALYZE usually include a list of some of
54 the most common values in each column and a histogram showing the
55 approximate data distribution in each column. One or both of these can
56 be omitted if ANALYZE deems them uninteresting (for example, in a
57 unique-key column, there are no common values) or if the column data
58 type does not support the appropriate operators. There is more informa‐
59 tion about the statistics in in the documentation.
60 For large tables, ANALYZE takes a random sample of the table contents,
62 rather than examining every row. This allows even very large tables to
63 be analyzed in a small amount of time. Note, however, that the statis‐
64 tics are only approximate, and will change slightly each time ANALYZE
65 is run, even if the actual table contents did not change. This might
66 result in small changes in the planner's estimated costs shown by
67 EXPLAIN [explain(7)]. In rare situations, this non-determinism will
68 cause the planner's choices of query plans to change after ANALYZE is
69 run. To avoid this, raise the amount of statistics collected by ANA‐
70 LYZE, as described below.
71 The extent of analysis can be controlled by adjusting the default_sta‐
73 tistics_target configuration variable, or on a column-by-column basis
74 by setting the per-column statistics target with ALTER TABLE ... ALTER
75 COLUMN ... SET STATISTICS (see ALTER TABLE [alter_table(7)]). The tar‐
76 get value sets the maximum number of entries in the most-common-value
77 list and the maximum number of bins in the histogram. The default tar‐
78 get value is 100, but this can be adjusted up or down to trade off
79 accuracy of planner estimates against the time taken for ANALYZE and
80 the amount of space occupied in pg_statistic. In particular, setting
81 the statistics target to zero disables collection of statistics for
82 that column. It might be useful to do that for columns that are never
83 used as part of the WHERE, GROUP BY, or ORDER BY clauses of queries,
84 since the planner will have no use for statistics on such columns.
85 The largest statistics target among the columns being analyzed deter‐
87 mines the number of table rows sampled to prepare the statistics.
88 Increasing the target causes a proportional increase in the time and
89 space needed to do ANALYZE.
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92 There is no ANALYZE statement in the SQL standard.
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95 VACUUM [vacuum(7)], vacuumdb [vacuumdb(1)], in the documentation, in
96 the documentation
97SQL - Language Statements 2011-09-22 ANALYZE(7)