Optimizing Queries with EXPLAIN

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

• When you precede a statement with the keyword EXPLAIN, MariaDB displays information from the optimizer about the query execution plan. That is, MariaDB explains how it would process the statement, including information about how tables are joined and in which order. EXPLAIN EXTENDED can be used to provide additional information.

  As of MariaDB 5.6.3, EXPLAIN provides information about SELECT, DELETE, INSERT, REPLACE, and UPDATE statements. Before MariaDB 5.6.3, EXPLAIN provides information only about SELECT statements.

  The following sections describe how to use EXPLAIN and EXPLAIN EXTENDED to obtain query execution plan information.

• EXPLAIN PARTITIONS is useful only when examining queries involving partitioned tables. For details, see , "Obtaining Information About Partitions".
• EXPLAIN tbl_name is synonymous with DESCRIBE tbl_name or SHOW COLUMNS FROM tbl_name. For information about DESCRIBE and SHOW COLUMNS, see , "DESCRIBE Syntax", and , "SHOW COLUMNS Syntax".

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 , "SELECT Syntax".)

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: Optimizer tracing.

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 , "ANALYZE TABLE Syntax".

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