ANALYZE

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ANALYZE

Collects 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 system table pg_statistic. 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.

ANALYZE does not collect statistics on external tables.

Parameters

VERBOSE
Enables display of progress messages. When specified, ANALYZE emits progress messages to indicate which table is currently being processed. Various statistics about the tables are printed as well.
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.

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 query 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.

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

You can run ANALYZE on a partitioned table or a child table that is used by the partitioned table. When you create a partitioned table with the CREATE TABLE command, Greenplum Database creates the table that you specify (the root or parent table), and also creates a hierarchy of tables based on the partition hierarchy that you specified (the child tables).

  • When you run ANALYZE on the root table, statistics are collected for all the leaf child tables (the lowest-level tables in the hierarchy of child tables created by Greenplum Database for use by the partitioned table).
  • When you run ANALYZE on a leaf child table, statistics are collected only for that leaf child table. When you run ANALYZE on a child table that is not a leaf child table, statistics are not collected.

    For example, you can create a partitioned table with partitions for the years 2000 to 2010 and subpartitions for each month in each year. If you run ANALYZE on the child table for the year 2005 no statistics are collected. If you run ANALYZE on the leaf child table for March of 2005, statistics are collected only for that leaf child table.

    Note: Partitioned tables, child tables and their inheritance level relationships are tracked in the system view pg_partitions.

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 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.

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. 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 by adjusting the default_statistics_target configuration parameter, 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.

Examples

Collect statistics for the table mytable:

ANALYZE mytable;

Compatibility

There is no ANALYZE statement in the SQL standard.