Movatterモバイル変換

ANALYZE

ANALYZE — collect statistics about a database

Synopsis

ANALYZE [ (option [, ...] ) ] [table_and_columns [, ...] ]whereoption can be one of:    VERBOSE [boolean ]    SKIP_LOCKED [boolean ]    BUFFER_USAGE_LIMITsizeandtable_and_columns is:table_name [ (column_name [, ...] ) ]

Description

ANALYZE collects statistics about the contents of tables in the database, and stores the results in thepg_statistic system catalog. Subsequently, the query planner uses these statistics to help determine the most efficient execution plans for queries.

Without atable_and_columns list,ANALYZE processes every table and materialized view in the current database that the current user has permission to analyze. With a list,ANALYZE processes only those table(s). It is further possible to give a list of column names for a table, in which case only the statistics for those columns are collected.

Parameters

Outputs

WhenVERBOSE 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 have theMAINTAIN privilege on the table. However, database owners are allowed to analyze all tables in their databases, except shared catalogs.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 supportANALYZE. If the table's wrapper does not supportANALYZE, the command prints a warning and does nothing.

In the defaultPostgreSQL configuration, the autovacuum daemon (seeSection 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 runANALYZE 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 runVACUUM andANALYZE once a day during a low-usage time of day. (This will not be sufficient if there is heavy update activity.)

WhileANALYZE is running, thesearch_path is temporarily changed topg_catalog, pg_temp.

ANALYZE requires only a read lock on the target table, so it can run in parallel with other non-DDL activity on the table.

The statistics collected byANALYZE 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 ifANALYZE 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 inChapter 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 timeANALYZE is run, even if the actual table contents did not change. This might result in small changes in the planner's estimated costs shown byEXPLAIN. In rare situations, this non-determinism will cause the planner's choices of query plans to change afterANALYZE is run. To avoid this, raise the amount of statistics collected byANALYZE, as described below.

The extent of analysis can be controlled by adjusting thedefault_statistics_target configuration variable, or on a column-by-column basis by setting the per-column statistics target withALTER TABLE ... ALTER COLUMN ... SET STATISTICS. 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 forANALYZE and the amount of space occupied inpg_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 theWHERE,GROUP BY, orORDER 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 doANALYZE.

One of the values estimated byANALYZE 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 withALTER TABLE ... ALTER COLUMN ... SET (n_distinct = ...).

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 runANALYZE 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 manualANALYZE 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 supportANALYZE, 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.

Each backend runningANALYZE will report its progress in thepg_stat_progress_analyze view. SeeSection 27.4.1 for details.

Compatibility

There is noANALYZE statement in the SQL standard.

The following syntax was used beforePostgreSQL version 11 and is still supported:

ANALYZE [ VERBOSE ] [table_and_columns [, ...] ]

See Also