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 run time 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 actual startup time and time spent in each 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. Run time of the entire statement
is always measured, even when node-level timing is turned off with
this option. 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,
CREATE TABLE AS, or CREATE MATERIALIZED VIEW 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=9.54..9.54 rows=1 width=8) (actual time=0.156..0.161 rows=11 loops=1)
Group Key: foo
-> Index Scan using test_pkey on test (cost=0.29..9.29 rows=50 width=8) (actual time=0.039..0.091 rows=99 loops=1)
Index Cond: ((id > $1) AND (id < $2))
Planning time: 0.197 ms
Execution time: 0.225 ms
(6 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)
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