MySQL 5.6 Reference Manual Including MySQL NDB Cluster 7.3-7.4 Reference Guide
The EXPLAIN
statement provides
information about how MySQL executes statements:
The EXPLAIN
statement
provides information about how MySQL executes statements.
EXPLAIN
works with
SELECT
,
DELETE
,
INSERT
,
REPLACE
, and
UPDATE
statements.
When EXPLAIN
is used with an
explainable statement, MySQL displays information from the
optimizer about the statement execution plan. That is, MySQL
explains how it would process the statement, including
information about how tables are joined and in which order.
For information about using
EXPLAIN
to obtain execution
plan information, see Section 8.8.2, “EXPLAIN Output Format”.
EXPLAIN EXTENDED
produces
additional execution plan information that can be displayed
using SHOW WARNINGS
. See
Section 8.8.3, “Extended EXPLAIN Output Format”.
EXPLAIN
PARTITIONS
is useful for examining queries
involving partitioned tables. See
Section 19.3.5, “Obtaining Information About Partitions”.
The FORMAT
option can be used to select
the output format. TRADITIONAL
presents
the output in tabular format. This is the default if no
FORMAT
option is present.
JSON
format displays the information in
JSON format. With FORMAT = JSON
, the
output includes extended and partition information.
With the help of EXPLAIN
, you can
see where you should add indexes to tables so that the statement
executes faster by using indexes to find rows. You can also use
EXPLAIN
to check whether the
optimizer joins the tables in an optimal order. To give a hint
to the optimizer to use a join order corresponding to the order
in which the tables are named in a
SELECT
statement, begin the
statement with SELECT STRAIGHT_JOIN
rather
than just SELECT
. (See
Section 13.2.9, “SELECT Statement”.) However,
STRAIGHT_JOIN
may prevent indexes from being
used because it disables semijoin transformations. See
Section 8.2.2.1, “Optimizing Subqueries with Semijoin Transformations”.
If you have a problem with indexes not being used when you
believe that they should be, run ANALYZE
TABLE
to update table statistics, such as cardinality
of keys, that can affect the choices the optimizer makes. See
Section 13.7.2.1, “ANALYZE TABLE Statement”.
EXPLAIN
can also be used to
obtain information about the columns in a table.
EXPLAIN
is synonymous
with tbl_name
DESCRIBE
and
tbl_name
SHOW COLUMNS FROM
. For more
information, see Section 13.8.1, “DESCRIBE Statement”, and
Section 13.7.5.6, “SHOW COLUMNS Statement”.
tbl_name