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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 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.

       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(l)]

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

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