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. As of MySQL 4.1, EXPLAIN EXTENDED can be used to provide additional information.

For information on how to use EXPLAIN and EXPLAIN EXTENDED to obtain query execution plan information, see Section 7.2.1, “Optimizing Queries with EXPLAIN”.

EXPLAIN tbl_name is synonymous with DESCRIBE tbl_name or SHOW COLUMNS FROM tbl_name.

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”.