/usr/man2/cat.l/analyze.l.Z

NAME

       ANALYZE - collect statistics about a database

SYNOPSIS

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

DESCRIPTION

       ANALYZE  collects  statistics about the contents of tables in the data-
       base, and stores the results in the system table  pg_statistic.  Subse-
       quently,  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  data-
       base. With a parameter, ANALYZE examines only that table. It is further
       possible to give a list of column names, in which case only the statis-
       tics for those columns are collected.

PARAMETERS

       VERBOSE
              Enables display of progress messages.

       table  The name (possibly schema-qualified) of a specific table to ana-
              lyze. Defaults to all tables in the current database.

       column The name of a specific column to analyze. Defaults to  all  col-
              umns.

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 VAC-
       UUM  [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 ta-
       ble, 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 informa-
       tion about the statistics 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 statis-
       tics 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_sta-
       tistics_target  configuration  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 tar-
       get value sets the maximum number of entries in  the  most-common-value
       list  and the maximum number of bins in the histogram. The default tar-
       get value is 10, but this can be adjusted up or down to trade off accu-
       racy  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  deter-
       mines  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.

SQL - Language Statements         2005-11-05                         ANALYZE()