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POMySQL datetime index is not working
 

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    <p>Table structure:</p> <pre><code>+-------------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------------+----------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | total | int(11) | YES | | NULL | | | thedatetime | datetime | YES | MUL | NULL | | +-------------+----------+------+-----+---------+----------------+ </code></pre> <p>Total rows: <strong>137967</strong></p> <pre><code>mysql&gt; explain select * from out where thedatetime &lt;= NOW(); +----+-------------+-------------+------+---------------+------+---------+------+--------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------------+------+---------------+------+---------+------+--------+-------------+ | 1 | SIMPLE | out | ALL | thedatetime | NULL | NULL | NULL | 137967 | Using where | +----+-------------+-------------+------+---------------+------+---------+------+--------+-------------+ </code></pre> <p>The real query is much more longer with more table joins, the point is, I can't get the table to use the <code>datetime</code> index. This is going to be hard for me if I want to select all data until certain date. However, I noticed that I can get MySQL to use the index if I select a smaller subset of data. </p> <pre><code>mysql&gt; explain select * from out where thedatetime &lt;= '2008-01-01'; +----+-------------+-------------+-------+---------------+-------------+---------+------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------------+-------+---------------+-------------+---------+------+-------+-------------+ | 1 | SIMPLE | out | range | thedatetime | thedatetime | 9 | NULL | 15826 | Using where | +----+-------------+-------------+-------+---------------+-------------+---------+------+-------+-------------+ mysql&gt; select count(*) from out where thedatetime &lt;= '2008-01-01'; +----------+ | count(*) | +----------+ | 15990 | +----------+ </code></pre> <p>So, what can I do to make sure MySQL will use the index no matter what date that I put?</p>
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    1. COAs `EXPLAIN` tells you the index is 'used', or more correctly, it is *considered* - in both cases. My answer below explains in detail. On the other hand if you *are* experiencing bad performance then you have simplified the question too much.
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