Provided by: postgresql-client-9.3_9.3.24-0ubuntu0.14.04_amd64 
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, 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)