Documentation Home
MySQL 5.7 Reference Manual
Related Documentation Download this Manual
PDF (US Ltr) - 35.6Mb
PDF (A4) - 35.6Mb
PDF (RPM) - 34.7Mb
EPUB - 8.7Mb
HTML Download (TGZ) - 8.5Mb
HTML Download (Zip) - 8.5Mb
HTML Download (RPM) - 7.3Mb
Eclipse Doc Plugin (TGZ) - 9.3Mb
Eclipse Doc Plugin (Zip) - 11.5Mb
Man Pages (TGZ) - 203.5Kb
Man Pages (Zip) - 310.2Kb
Info (Gzip) - 3.3Mb
Info (Zip) - 3.3Mb
Excerpts from this Manual

MySQL 5.7 Reference Manual  /  ...  /  Optimizing Queries with EXPLAIN

9.8.1 Optimizing Queries with EXPLAIN

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

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 14.2.9, “SELECT Syntax”.) However, STRAIGHT_JOIN may prevent indexes from being used because it disables semi-join transformations. See Section 9.2.1.18.1, “Optimizing Subqueries with Semi-Join 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 14.7.2.1, “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 14.8.1, “DESCRIBE Syntax”, and Section 14.7.5.5, “SHOW COLUMNS Syntax”.


User Comments
Sign Up Login You must be logged in to post a comment.