The EXPLAIN statement can be used
either as a way to obtain information about how MySQL executes a
statement, or as a synonym for
DESCRIBE:
When you precede a SELECT
statement with the keyword
EXPLAIN, MySQL displays
information from the optimizer about the query execution
plan. That is, MySQL explains how it would process the
statement, including information about how tables are joined
and in which order.
EXPLAIN
EXTENDED can be used to provide additional
information.
The following sections describe how to use
EXPLAIN and
EXPLAIN
EXTENDED to obtain query execution plan
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 12.2.8, “SELECT Syntax”.)
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 12.7.2.1, “ANALYZE TABLE Syntax”.
© 2012, Oracle Corporation and/or its affiliates
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