EXPLAIN statement can be used
to obtain information about how MySQL executes a statement:
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”.
PARTITIONS is useful for examining queries
involving partitioned tables. See
Section 19.3.5, “Obtaining Information About Partitions”.
As of MySQL 5.6.3, 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
SELECT STRAIGHT_JOIN rather
Section 13.2.9, “SELECT Syntax”.)
If you have a problem with indexes not being used when you
believe that they should be, run
TABLE to update table statistics, such as cardinality
of keys, that can affect the choices the optimizer makes. See
Section 18.104.22.168, “ANALYZE TABLE Syntax”.