10.8.1 Optimizing Queries with EXPLAIN

The EXPLAIN statement provides information about how MySQL executes statements:

• EXPLAIN works with SELECT, DELETE, INSERT, REPLACE, and UPDATE statements.

• When 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 10.8.2, “EXPLAIN Output Format”.

• When EXPLAIN is used with FOR CONNECTION connection_id rather than an explainable statement, it displays the execution plan for the statement executing in the named connection. See Section 10.8.4, “Obtaining Execution Plan Information for a Named Connection”.

• For SELECT statements, EXPLAIN produces additional execution plan information that can be displayed using SHOW WARNINGS. See Section 10.8.3, “Extended EXPLAIN Output Format”.

• EXPLAIN is useful for examining queries involving partitioned tables. See Section 26.3.5, “Obtaining Information About Partitions”.

• 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 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 15.2.13, “SELECT Statement”.) However, STRAIGHT_JOIN may prevent indexes from being used because it disables semijoin transformations. See Optimizing IN and EXISTS Subquery Predicates 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 15.7.3.1, “ANALYZE TABLE Statement”.

Note

EXPLAIN can also be used to obtain information about the columns in a table. EXPLAIN tbl_name is synonymous with DESCRIBE tbl_name and SHOW COLUMNS FROM tbl_name. For more information, see Section 15.8.1, “DESCRIBE Statement”, and Section 15.7.7.6, “SHOW COLUMNS Statement”.