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POWhat indexes can be used to improve this query?
 

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    <p>This query selects all the unique visitor sessions in a certain date range:</p> <pre><code>select distinct(accessid) from accesslog where date &gt; '2009-09-01' </code></pre> <p>I have indexes on the following fields:</p> <ul> <li>accessid</li> <li>date</li> <li>some other fields </li> </ul> <p>Here's what explain looks like:</p> <pre><code>mysql&gt; explain select distinct(accessid) from accesslog where date &gt; '2009-09-01'; +----+-------------+-----------+-------+----------------------+------+---------+------+-------+------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-----------+-------+----------------------+------+---------+------+-------+------------------------------+ | 1 | SIMPLE | accesslog | range | date,dateurl,dateaff | date | 3 | NULL | 64623 | Using where; Using temporary | +----+-------------+-----------+-------+----------------------+------+---------+------+-------+------------------------------+ mysql&gt; explain select distinct(accessid) from accesslog; +----+-------------+-----------+-------+---------------+----------+---------+------+---------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-----------+-------+---------------+----------+---------+------+---------+-------------+ | 1 | SIMPLE | accesslog | index | NULL | accessid | 257 | NULL | 1460253 | Using index | +----+-------------+-----------+-------+---------------+----------+---------+------+---------+-------------+ </code></pre> <p>Why doesn't the query with the date clause use the accessid index?</p> <p>Are there any other indexes I can use to speed up queries for distinct accessid's in certain date spans?</p> <p><b>Edit - Resolution</b></p> <p>Reducing column width on <code>accessid</code> from varchar 255 to char 32 improved query time by ~75%.</p> <p>Adding a <code>date+accessid</code> index had no effect on query time.</p>
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