Index Index for
Section 5
Index Alphabetical
listing for A
Bottom of page Bottom of
page		 

ANALYZE(5)


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(5)]
  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 [alter_table(5)]). 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.

Index Index for
Section 5
Index Alphabetical
listing for A
Top of page Top of
page