8.8.1 Optimizing Queries with EXPLAIN
EXPLAIN statement provides information about how MySQL executes statements:
EXPLAINis 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
EXPLAINto obtain execution plan information, see Section 8.8.2, “EXPLAIN Output Format”.
EXPLAINis used with
FOR CONNECTIONrather than an explainable statement, it displays the execution plan for the statement executing in the named connection. See Section 8.8.4, “Obtaining Execution Plan Information for a Named Connection”.
EXPLAINis useful for examining queries involving partitioned tables. See Section 22.3.5, “Obtaining Information About Partitions”.
FORMAToption can be used to select the output format.
TRADITIONALpresents the output in tabular format. This is the default if no
FORMAToption is present.
JSONformat displays the information in JSON format.
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 220.127.116.11, “Optimizing Subqueries, Derived Tables, and View References with Semijoin Transformations”.
The optimizer trace may sometimes provide information complementary to that of
EXPLAIN. However, the optimizer trace format and content are subject to change between versions. For details, see MySQL Internals: Tracing the Optimizer .
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 18.104.22.168, “ANALYZE TABLE Statement”.