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    <p>Index order matters when your query conditions only apply to <strong>PART</strong> of the index. Consider:</p> <ol> <li><p><code>SELECT * FROM table WHERE first_name="john" AND last_name="doe"</code></p></li> <li><p><code>SELECT * FROM table WHERE first_name="john"</code></p></li> <li><p><code>SELECT * FROM table WHERE last_name="doe"</code></p></li> </ol> <p>If your index is (<code>first_name</code>, <code>last_name</code>) queries 1 and 2 will use it, query #3 won't. If your index is (<code>last_name</code>, <code>first_name</code>) queries 1 and 3 will use it, query #2 won't. Changing the condition order within WHERE clause has no effect in either case.</p> <p>Details are <a href="http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html" rel="noreferrer">here</a></p> <p><strong>Update</strong>:<br> In case the above is not clear - MySQL can only use an index if the columns in query conditions form a leftmost prefix of the index. Query #2 above can not use (<code>last_name</code>, <code>first_name</code>) index because it's only based on <code>first_name</code> and <code>first_name</code> is NOT the leftmost prefix of the (<code>last_name</code>, <code>first_name</code>) index.</p> <p>The order of conditions WITHIN the query does not matter; query #1 above will be able to use (<code>last_name</code>, <code>first_name</code>) index just fine because its conditions are <code>first_name</code> and <code>last_name</code> and, taken together, they DO form a leftmost prefix of (<code>last_name</code>, <code>first_name</code>) index.</p>
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    1. COSo I guess SELECT * FROM table WHERE last_name="doe" AND first_name="john" won't? If that's the case, then since all the query building is hidden by my ORM, I probably have to define the 2 indices... which really sucks, because I have an index on 3 columns to define...
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    2. CONo, you've misunderstood. WHERE `last_name`="doe" AND `first_name`="john" query would use either (`first_name`, `last_name`) or (`last_name`, `first_name`) indexes just fine.
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