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7.2.2 EXPLAIN Output Format »
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7.2.1. Optimizing Queries with EXPLAIN

The EXPLAIN statement can be used either as a synonym for DESCRIBE or as a way to obtain information about how MySQL executes a SELECT statement:

  • EXPLAIN tbl_name is synonymous with DESCRIBE tbl_name or SHOW COLUMNS FROM tbl_name: 

    EXPLAIN tbl_name

  • When you precede a SELECT statement with the keyword EXPLAIN, MySQL displays information from the optimizer about the query execution plan. That is, MySQL explains how it would process the SELECT, including information about how tables are joined and in which order: 

    EXPLAIN [EXTENDED] SELECT select_options

This section describes the second use of EXPLAIN for obtaining query execution plan information. See also Section 12.7.2, “EXPLAIN Syntax”. For a description of the DESCRIBE and SHOW COLUMNS statements, see Section 12.7.1, “DESCRIBE Syntax”, and Section 12.4.5.5, “SHOW COLUMNS Syntax”.

With the help of EXPLAIN, you can see where you should add indexes to tables to get a faster SELECT that uses 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 the SELECT statement, begin the statement with SELECT STRAIGHT_JOIN rather than just SELECT. (See Section 12.2.7, “SELECT Syntax”.)

If you have a problem with indexes not being used when you believe that they should be, you should run ANALYZE TABLE to update table statistics such as cardinality of keys, that can affect the choices the optimizer makes. See Section 12.4.2.1, “ANALYZE TABLE Syntax”.