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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(7)]  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 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  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(7)]). 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.