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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 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 let-
ting 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 Post-
greSQL. 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 sta-
tistical 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 num-
ber 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 execu-
tion. 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 esti-
mates 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 2011-09-22 EXPLAIN()
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On Apache
Under GNU General Public License
2012-05-23 16:16 @38.107.179.239 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)