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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 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  get-
       ting  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 execu-
              tion 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 sig-
       nificantly since the last time ANALYZE was run), the estimated costs are unlikely to conform to the real  prop-
       erties 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)]



SQL - Language Statements         2014-02-17                        EXPLAIN(7)