explain
EXPLAIN() SQL Commands EXPLAIN()
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 row from each input table.
The most critical part of the display is the estimated statement exe-
cution 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 cheap-
est.
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 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
debugging PostgreSQL. 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 [XRef to USING-EXPLAIN] for more
information.
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
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 infe-
rior 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
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.
SQL - Language Statements 2008-01-03 EXPLAIN()
