8.8.1 Optimizing Queries with EXPLAIN

The EXPLAIN statement can be used to obtain information about how MySQL executes a statement:

• As of MySQL 5.6.3, permitted explainable statements for EXPLAIN are SELECT, DELETE, INSERT, REPLACE, and UPDATE. Before MySQL 5.6.3, SELECT is the only explainable statement.

• 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 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 19.3.5, “Obtaining Information About Partitions”.

• As of MySQL 5.6.3, 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 FORMAT = JSON, the output includes extended and partition 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 13.2.9, “SELECT Syntax”.) However, STRAIGHT_JOIN may prevent indexes from being used because it disables semi-join transformations. See Section 8.2.1.18.1, “Optimizing Subqueries with Semi-Join Transformations”.

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 tbl_name is synonymous with DESCRIBE tbl_name and SHOW COLUMNS FROM tbl_name. For more information, see Section 13.8.1, “DESCRIBE Syntax”, and Section 13.7.5.6, “SHOW COLUMNS Syntax”.