Provided by: postgresql-client-9.1_9.1.3-2_i386 bug


       EXPLAIN - show the execution plan of a statement


       EXPLAIN [ ( option [, ...] ) ] statement
       EXPLAIN [ ANALYZE ] [ VERBOSE ] statement

       where option can be one of:

           ANALYZE [ boolean ]
           VERBOSE [ boolean ]
           COSTS [ boolean ]
           BUFFERS [ boolean ]
           FORMAT { TEXT | XML | JSON | YAML }


       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.

           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
           without letting the command affect your data, use this approach:

               EXPLAIN ANALYZE ...;

       Only the ANALYZE and VERBOSE options can be specified, and only in that
       order, without surrounding the option list in parentheses. Prior to
       PostgreSQL 9.0, the unparenthesized syntax was the only one supported.
       It is expected that all new options will be supported only in the
       parenthesized syntax.


           Carry out the command and show the actual run times. This parameter
           defaults to FALSE.

           Display additional information regarding the plan. Specifically,
           include the output column list for each node in the plan tree,
           schema-qualify table and function names, always label variables in
           expressions with their range table alias, and always print the name
           of each trigger for which statistics are displayed. This parameter
           defaults to FALSE.

           Include information on the estimated startup and total cost of each
           plan node, as well as the estimated number of rows and the
           estimated width of each row. This parameter defaults to TRUE.

           Include information on buffer usage. Specifically, include the
           number of shared blocks hits, reads, and writes, the number of
           local blocks hits, reads, and writes, and the number of temp blocks
           reads and writes. Shared blocks, local blocks, and temp blocks
           contain tables and indexes, temporary tables and temporary indexes,
           and disk blocks used in sort and materialized plans, respectively.
           The number of blocks shown for an upper-level node includes those
           used by all its child nodes. In text format, only non-zero values
           are printed. This parameter may only be used with ANALYZE
           parameter. It defaults to FALSE.

           Specify the output format, which can be TEXT, XML, JSON, or YAML.
           Non-text output contains the same information as the text output
           format, but is easier for programs to parse. This parameter
           defaults to TEXT.

           Specifies whether the selected option should be turned on or off.
           You can write TRUE, ON, or 1 to enable the option, and FALSE, OFF,
           or 0 to disable it. The boolean value can also be omitted, in which
           case TRUE is assumed.

           CREATE TABLE AS statement, whose execution plan you wish to see.


       There is only sparse documentation on the optimizer's use of cost
       information in PostgreSQL. Refer to Section 14.1, “Using EXPLAIN”, in
       the documentation for more information.

       In order to allow the PostgreSQL query planner to make reasonably
       informed decisions when optimizing queries, the 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.

       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.


       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)

       Here is the same query, with JSON formatting:

                      QUERY PLAN
            [                             +
              {                           +
                "Plan": {                 +
                  "Node Type": "Seq Scan",+
                  "Relation Name": "foo", +
                  "Alias": "foo",         +
                  "Startup Cost": 0.00,   +
                  "Total Cost": 155.00,   +
                  "Plan Rows": 10000,     +
                  "Plan 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 the same query, but in YAML output:

                     QUERY PLAN
            - Plan:                      +
                Node Type: "Index Scan"  +
                Scan Direction: "Forward"+
                Index Name: "fi"         +
                Relation Name: "foo"     +
                Alias: "foo"             +
                Startup Cost: 0.00       +
                Total Cost: 5.98         +
                Plan Rows: 1             +
                Plan Width: 4            +
                Index Cond: "(i = 4)"
           (1 row)

       XML output is left as an exercise to the reader.

       Here is the same plan with costs suppressed:


                   QUERY PLAN
            Index Scan using fi on foo
              Index Cond: (i = 4)
           (2 rows)

       Here is an example of a query plan for a query using an aggregate

           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.


       There is no EXPLAIN statement defined in the SQL standard.