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POWhy does removing this index in MySQL speed up my query 100x?
 

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  1. POWhy does removing this index in MySQL speed up my query 100x?
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    <p>I have the following MySQL table (simplified):</p> <pre><code>CREATE TABLE `track` ( `id` int(11) NOT NULL AUTO_INCREMENT, `title` varchar(256) NOT NULL, `is_active` tinyint(1) NOT NULL, PRIMARY KEY (`id`), KEY `is_active` (`is_active`, `id`) ) ENGINE=MyISAM AUTO_INCREMENT=7495088 DEFAULT CHARSET=utf8 </code></pre> <p>The 'is_active' column marks rows that I want to ignore in most, but not all, of my queries. I have some queries that read chunks out of this table periodically. One of them looks like this:</p> <pre><code>SELECT id,title from track where (track.is_active=1 and track.id &gt; 5580702) ORDER BY id ASC LIMIT 10; </code></pre> <p>This query takes over a minute to execute. Here's the execution plan:</p> <pre><code>&gt; EXPLAIN SELECT id,title from track where (track.is_active=1 and track.id &gt; 5580702) ORDER BY id ASC LIMIT 10; +----+-------------+-------+------+----------------+--------+---------+-------+---------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+----------------+--------+---------+-------+---------+-------------+ | 1 | SIMPLE | t | ref | PRIMARY,is_active | is_active | 1 | const | 3747543 | Using where | +----+-------------+-------+------+----------------+--------+---------+-------+---------+-------------+ </code></pre> <p>Now, if I tell MySQL to ignore the 'is_active' index, the query happens instantaneously.</p> <pre><code>&gt; EXPLAIN SELECT id,title from track IGNORE INDEX(is_active) WHERE (track.is_active=1 AND track.id &gt; 5580702) ORDER BY id ASC LIMIT 10; +----+-------------+-------+-------+---------------+---------+---------+------+---------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+---------+-------------+ | 1 | SIMPLE | t | range | PRIMARY | PRIMARY | 4 | NULL | 1597518 | Using where | +----+-------------+-------+-------+---------------+---------+---------+------+---------+-------------+ </code></pre> <p>Now, what's really strange is that if I FORCE MySQL to use the 'is_active' index, the query once again happens instantaneously!</p> <pre><code>+----+-------------+-------+-------+---------------+---------+---------+------+---------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+---------+-------------+ | 1 | SIMPLE | t | range | is_active |is_active| 5 | NULL | 1866730 | Using where | +----+-------------+-------+-------+---------------+---------+---------+------+---------+-------------+ </code></pre> <p>I just don't understand this behavior. In the 'is_active' index, rows should be sorted by is_active, followed by id. I use both the 'is_active' and 'id' columns in my query, so it seems like it should only need to do a few hops around the tree to find the IDs, then use those IDs to retrieve the titles from the table. </p> <p>What's going on?</p> <p>EDIT: More info on what I'm doing:</p> <ul> <li>Query cache is disabled</li> <li>Running OPTIMIZE TABLE and ANALYZE TABLE had no effect</li> <li>6,620,372 rows have 'is_active' set to True. 874,714 rows have 'is_active' set to False.</li> <li>Using FORCE INDEX(is_active) once again speeds up the query.</li> <li>MySQL version 5.1.54</li> </ul>
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    1. COYou're clearing the cache before benchmarking,right ?
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    2. COAlso make sure table statistics are current and the indices are rebuilt. (However this is done in MySQL ;-)
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    3. COWhat happens if you reverse the WHERE conditions? `where (track.id > 5580702 and track.is_active=1)`
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