EXPLAIN(7) PostgreSQL 9.1.9 Documentation EXPLAIN(7)NAMEEXPLAIN - show the execution plan of a statement
SYNOPSISEXPLAIN [ ( 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 }
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, 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 the actual run times. 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 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 the ANALYZE
parameter. It defaults to FALSE.
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.
NOTES
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.
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 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 output:
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 output is left as an exercise to the reader.
Here is the same plan with costs 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 ALSOANALYZE(7)PostgreSQL 9.1.9 2013-04-01 EXPLAIN(7)
