MySQL 5.5 Release Notes
The EXPLAIN statement provides
information about how MySQL executes statements:
When you precede a
SELECTstatement with the keywordEXPLAIN, 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 usingEXPLAINto obtain execution plan information, see Section 8.8.2, “EXPLAIN Output Format”.EXPLAIN EXTENDEDcan be used to obtain additional execution plan information. See Section 8.8.3, “EXPLAIN EXTENDED Output Format”.EXPLAIN PARTITIONSis 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”.
Note
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