EXPLAIN(7)		PostgreSQL 9.2.24 Documentation		    EXPLAIN(7)



NAME
       EXPLAIN - show the execution plan of a statement

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

       where option can be one of:

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

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 cost units that are arbitrary, but
       conventionally mean disk page fetches). Actually two numbers are shown:
       the start-up cost before the first row can be returned, and the total
       cost to return all the rows. For most queries the total cost is what
       matters, but in contexts such as a subquery in EXISTS, the planner will
       choose the smallest start-up cost instead of the smallest total cost
       (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. Then actual runtime statistics are added to the display,
       including the total elapsed time expended within each plan node (in
       milliseconds) and the total number of rows it actually returned. 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;

       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.

PARAMETERS
       ANALYZE
	   Carry out the command and show actual run times and other
	   statistics. This parameter defaults to FALSE.

       VERBOSE
	   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.

       COSTS
	   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.

       BUFFERS
	   Include information on buffer usage. Specifically, include the
	   number of shared blocks hit, read, dirtied, and written, the number
	   of local blocks hit, read, dirtied, and written, and the number of
	   temp blocks read and written. A hit means that a read was avoided
	   because the block was found already in cache when needed. Shared
	   blocks contain data from regular tables and indexes; local blocks
	   contain data from temporary tables and indexes; while temp blocks
	   contain short-term working data used in sorts, hashes, Materialize
	   plan nodes, and similar cases. The number of blocks dirtied
	   indicates the number of previously unmodified blocks that were
	   changed by this query; while the number of blocks written indicates
	   the number of previously-dirtied blocks evicted from cache by this
	   backend during query processing. 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 when ANALYZE is also enabled. It defaults to FALSE.

       TIMING
	   Include the actual startup time and time spent in the node in the
	   output. The overhead of repeatedly reading the system clock can
	   slow down the query significantly on some systems, so it may be
	   useful to set this parameter to FALSE when only actual row counts,
	   and not exact times, are needed. This parameter may only be used
	   when ANALYZE is also enabled. It defaults to TRUE.

       FORMAT
	   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.

       boolean
	   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.

       statement
	   Any SELECT, INSERT, UPDATE, DELETE, VALUES, EXECUTE, DECLARE, or
	   CREATE TABLE AS statement, whose execution plan you wish to see.

OUTPUTS
       The command's result is a textual description of the plan selected for
       the statement, optionally annotated with execution statistics.  Section
       14.1, "Using EXPLAIN", in the documentation describes the information
       provided.

NOTES
       In order to allow the PostgreSQL query planner to make reasonably
       informed decisions when optimizing queries, the pg_statistic data
       should be up-to-date for all tables used in the query. Normally the
       autovacuum daemon will take care of that automatically. But if a table
       has recently had substantial changes in its contents, you might need to
       do a manual ANALYZE(7) rather than wait for autovacuum to catch up with
       the changes.

       In order to measure the run-time cost of each node in the execution
       plan, the current implementation of EXPLAIN ANALYZE adds 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, as
       well as the platform being used. The worst case occurs for plan nodes
       that in themselves require very little time per execution, and on
       machines that have relatively slow operating system calls for obtaining
       the time of day.

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)

       Here is the same query, with JSON output formatting:

	   EXPLAIN (FORMAT JSON) SELECT * FROM foo;
		      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 format:

	   EXPLAIN (FORMAT YAML) SELECT * FROM foo WHERE i='4';
		     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 format is left as an exercise for the reader.

       Here is the same plan with cost estimates suppressed:

	   EXPLAIN (COSTS FALSE) SELECT * FROM foo WHERE i = 4;

		   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
       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(7)



PostgreSQL 9.2.24		  2017-11-06			    EXPLAIN(7)