EXPLAIN(7)							   SQL Commands 							EXPLAIN(7)

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 sequential scan, index scan, etc. -- and if  multiple  tables  are  refer-
       enced, 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 execution 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
       estimates are close to reality.

	      Important: Keep in mind that the statement is actually executed when the ANALYZE option 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, CREATE TABLE AS, 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
	      Include the output column list for each node in the plan tree.

       statement
	      Any SELECT, INSERT, UPDATE, DELETE, VALUES, EXECUTE, DECLARE, or CREATE TABLE AS statement, 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 in the documentation for more  infor-
       mation.

       In  order  to  allow  the  PostgreSQL query planner to make reasonably informed decisions when optimizing queries, the ANALYZE [analyze(7)]
       statement should be run to record statistics about the distribution of data within the table. If you have not done this (or if the  statis-
       tical  distribution  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 might be chosen.

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

       In  order  to measure the run-time cost of each node in the execution plan, the current implementation of EXPLAIN ANALYZE can add consider-
       able profiling overhead to query execution. As a result, running EXPLAIN ANALYZE on a query can sometimes take  significantly  longer  than
       executing the query normally. The amount of overhead depends on the nature of the query.

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 condition, 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)

       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 contents of the tables involved. Also note that the numbers, and even the
       selected query strategy, might vary between PostgreSQL releases due to planner improvements. In addition, the ANALYZE command  uses  random
       sampling  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					    2010-05-14								EXPLAIN(7)