The EXPLAIN statement provides
information about how MySQL executes statements:
When you precede a SELECT
statement with the keyword
EXPLAIN, 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 can be used
to obtain additional execution plan information. See
Section 8.8.3, “EXPLAIN EXTENDED Output Format”.
EXPLAIN
PARTITIONS is useful for examining queries
involving partitioned tables. See
Section 19.3.4, “Obtaining Information About Partitions”.
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 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 13.7.2.1, “ANALYZE TABLE Syntax”.
EXPLAIN can also be used to
obtain information about the columns in a table.
EXPLAIN
is synonymous
with tbl_nameDESCRIBE
and
tbl_nameSHOW COLUMNS FROM
. For more
information, see Section 13.8.1, “DESCRIBE Syntax”, and
Section 13.7.5.6, “SHOW COLUMNS Syntax”.
tbl_name