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     ANALYZE(l)	  SQL - Language Statements (2002-11-22)    ANALYZE(l)

     NAME
	  ANALYZE - collect statistics about a database

     SYNOPSIS
	  ANALYZE [ VERBOSE ] [ table [ (column [, ...] ) ] ]

	INPUTS
	  VERBOSE
	       Enables display of progress messages.

	  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.

	OUTPUTS
	  ANALYZE
	       The command is complete.

     DESCRIPTION
	  ANALYZE collects statistics about the contents of PostgreSQL
	  tables, and stores the results in the system table
	  pg_statistic. Subsequently, the query planner uses the
	  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 updated.

	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
	  [vacuum(l)] 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

     Page 1					     (printed 3/24/03)

     ANALYZE(l)	  SQL - Language Statements (2002-11-22)    ANALYZE(l)

	  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 [alter_table(l)]). 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.

     COMPATIBILITY
	SQL92
	  There is no ANALYZE statement in SQL92.

     Page 2					     (printed 3/24/03)

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