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EXPLAIN()			 SQL Commands			     EXPLAIN()



NAME
       EXPLAIN - show the execution plan of a statement


SYNOPSIS
       EXPLAIN [ ANALYZE ] [ VERBOSE ] statement


DESCRIPTION
       This  command  displays	the execution plan that the PostgreSQL planner
       generates for the supplied statement. The execution plan shows how  the
       table(s) referenced by the statement will be scanned -- by plain sequen-
       tial scan, index scan, etc. -- and if multiple  tables  are  referenced,
       what  join  algorithms will be used to bring together the required rows
       from each input table.

       The most critical part of the display is the estimated statement execu-
       tion cost, which is the planner's guess at how long it will take to run
       the statement (measured in units of disk page  fetches).	 Actually  two
       numbers	are  shown:  the  start-up  time  before  the first row can be
       returned, and the total time to return all the rows. For	 most  queries
       the  total  time is what matters, but in contexts such as a subquery in
       EXISTS, the planner will choose the smallest start-up time  instead  of
       the smallest total time (since the executor will stop after getting one
       row, anyway).  Also, if you limit the number of rows to return  with  a
       LIMIT  clause,  the  planner makes an appropriate interpolation between
       the endpoint costs to estimate which plan is really the cheapest.

       The ANALYZE option causes the statement to be  actually	executed,  not
       only planned. The total elapsed time expended within each plan node (in
       milliseconds) and total number of rows it actually returned  are	 added
       to  the	display. This is useful for seeing whether the planner's esti-
       mates are close to reality.

	      Important: Keep in mind that the statement is actually  executed
	      when  ANALYZE  is used. Although EXPLAIN will discard any output
	      that a SELECT would return, other side effects of the  statement
	      will  happen  as usual. If you wish to use EXPLAIN ANALYZE on an
	      INSERT, UPDATE, DELETE, or EXECUTE statement without letting the
	      command affect your data, use this approach:

	      BEGIN;
	      EXPLAIN ANALYZE ...;
	      ROLLBACK;



PARAMETERS
       ANALYZE
	      Carry out the command and show the actual run times.

       VERBOSE
	      Show  the	 full internal representation of the plan tree, rather
	      than just a summary. Usually this option is only useful for spe-
	      cialized	debugging  purposes.  The  VERBOSE  output  is	either
	      pretty-printed  or  not,	depending  on  the  setting   of   the
	      explain_pretty_print configuration parameter.

       statement
	      Any  SELECT,  INSERT, UPDATE, DELETE, EXECUTE, or DECLARE state-
	      ment, whose execution plan you wish to see.

NOTES
       There is only sparse documentation  on  the  optimizer's	 use  of  cost
       information in PostgreSQL. Refer to the documentation for more informa-
       tion.

       In order to allow the  PostgreSQL  query	 planner  to  make  reasonably
       informed	 decisions  when  optimizing  queries,	the  ANALYZE statement
       should be run to record	statistics  about  the	distribution  of  data
       within the table. If you have not done this (or if the statistical dis-
       tribution of the data in the table has changed significantly since  the
       last time ANALYZE was run), the estimated costs are unlikely to conform
       to the real properties of the query, and consequently an inferior query
       plan may be chosen.

       Genetic	query  optimization  (GEQO)  randomly  tests  execution plans.
       Therefore, when the number of  tables  exceeds  geqo_threshold  causing
       genetic	query optimization to be used, the execution plan is likely to
       change each time the statement is executed.

       Prior to PostgreSQL 7.3, the plan was emitted in the form of  a	NOTICE
       message.	 Now it appears as a query result (formatted like a table with
       a single text column).

EXAMPLES
       To show the plan for a simple query on a table with  a  single  integer
       column and 10000 rows:

       EXPLAIN SELECT * FROM foo;

			      QUERY PLAN
       ---------------------------------------------------------
	Seq Scan on foo	 (cost=0.00..155.00 rows=10000 width=4)
       (1 row)


       If  there is an index and we use a query with an indexable WHERE condi-
       tion, EXPLAIN might show a different plan:

       EXPLAIN SELECT * FROM foo WHERE i = 4;

				QUERY PLAN
       --------------------------------------------------------------
	Index Scan using fi on foo  (cost=0.00..5.98 rows=1 width=4)
	  Index Cond: (i = 4)
       (2 rows)


       And here is an example of a query plan for a query using	 an  aggregate
       function:

       EXPLAIN SELECT sum(i) FROM foo WHERE i < 10;

				    QUERY PLAN
       ---------------------------------------------------------------------
	Aggregate  (cost=23.93..23.93 rows=1 width=4)
	  ->  Index Scan using fi on foo  (cost=0.00..23.92 rows=6 width=4)
		Index Cond: (i < 10)
       (3 rows)


       Here  is	 an  example of using EXPLAIN EXECUTE to display the execution
       plan for a prepared query:

       PREPARE query(int, int) AS SELECT sum(bar) FROM test
	   WHERE id > $1 AND id < $2
	   GROUP BY foo;

       EXPLAIN ANALYZE EXECUTE query(100, 200);

							      QUERY PLAN
       -------------------------------------------------------------------------------------------------------------------------
	HashAggregate  (cost=39.53..39.53 rows=1 width=8) (actual time=0.661..0.672 rows=7 loops=1)
	  ->  Index Scan using test_pkey on test  (cost=0.00..32.97 rows=1311 width=8) (actual time=0.050..0.395 rows=99 loops=1)
		Index Cond: ((id > $1) AND (id < $2))
	Total runtime: 0.851 ms
       (4 rows)


       Of course, the specific numbers shown here depend on  the  actual  con-
       tents  of the tables involved. Also note that the numbers, and even the
       selected query strategy, may vary between PostgreSQL  releases  due  to
       planner improvements. In addition, the ANALYZE command uses random sam-
       pling to estimate data statistics; therefore, it is possible  for  cost
       estimates  to  change  after a fresh run of ANALYZE, even if the actual
       distribution of data in the table has not changed.

COMPATIBILITY
       There is no EXPLAIN statement defined in the SQL standard.

SEE ALSO
       ANALYZE [analyze(7)]



SQL - Language Statements	  2010-12-14			     EXPLAIN()
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