Use the EXPLAIN statement

Precede any valid SELECT statement with the word "EXPLAIN" and MySQL will explain to you how it would process that SELECT - how many rows it would have returned, etc, which is a great way to spot problems. The faster of the two queries from above, when used with EXPLAIN, produces the following output:

| table  | type | possible_keys | key  | key_len | re f | rows   | Extra      |
| mydata | ALL  | NULL          | NULL | NULL    | NULL | 199999 | where used |

The important parts are "possible_keys", "rows", and "Extra". Possible keys shows "NULL", which means MySQL cannot run the query using indexes - we will be solving that later. Rows shows how many rows MySQL would have had to search through, and "Extra" shows what extra functionality it would have used to perform the search - here we can see it would have filtered the search using the where clause.


Want to learn PHP 7?

Hacking with PHP has been fully updated for PHP 7, and is now available as a downloadable PDF. Get over 1200 pages of hands-on PHP learning today!

If this was helpful, please take a moment to tell others about Hacking with PHP by tweeting about it!

Next chapter: Change your hardware >>

Previous chapter: Use shorter queries where possible

Jump to:


Home: Table of Contents

Copyright ©2015 Paul Hudson. Follow me: @twostraws.