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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
       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
       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,  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
              specialized 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, VALUES, EXECUTE, or  DECLARE
              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
       information.

       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 statistical
       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
       causing 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
       considerable  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)]