Index for Section 5

Alphabetical listing for A

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ANALYZE(5)
NAME
  ANALYZE - collect 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.

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

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.

  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. There is more information about the statistics in
  the chapter called ``Routine Database Maintenance'' in the documentation.

  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, as described below.

  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
  There is no ANALYZE statement in the SQL standard.

Index for Section 5

Alphabetical listing for A

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