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    <p>It looks like MySQL is making a poor decision about how to use the index.</p> <p>From that query plan, it is showing it could have used either the PRIMARY or is_active index, and it has chosen is_active in order to narrow by track.is_active first. However, it is only using the first column of the index (track.is_active). That gets it 3747543 results which then have to be filtered and sorted.</p> <p>If it had chosen the PRIMARY index, it would be able to narrow down to 1597518 rows using the index, and they would be retrieved in order of track.id already, which should require no further sorting. That would be faster.</p> <p>New information:</p> <p>In the third case where you are using <a href="http://dev.mysql.com/doc/refman/5.5/en/index-hints.html" rel="noreferrer">FORCE INDEX</a>, MySQL is using the is_active index but now instead of only using the first column, it is using both columns (see key_len). It is therefore now able to narrow by is_active and sort and filter by id using the same index, and since is_active is a single constant, the ORDER BY is satisfied by the second column (ie the rows from a single branch of the index are already in sorted order). This seems to be an even better outcome than using PRIMARY - and probably what you intended in the first place, right?</p> <p>I don't know why it wasn't using both columns of this index without FORCE INDEX, unless the query has changed in a subtle way in between. If not I'd put it down to MySQL making bad decisions.</p>
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    1. POWhy does removing this index in MySQL speed up my query 100x?
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    1. COOf course, if you know better than MySQL you can always use [USE INDEX ()](http://dev.mysql.com/doc/refman/5.5/en/index-hints.html) to suggest which index it should prefer. You can also try [ANALYZE TABLE](http://dev.mysql.com/doc/refman/5.5/en/analyze-table.html) to give MySQL a chance to figure it out on its own, which may work sometimes.
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    2. COIf I use FORCE INDEX(is_active) the query happens instantly (see recent edits). Any ideas?
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    3. COI'm not sure - possibly a cache of some sort? Maybe add the EXPLAIN output for that? Do you get the same output, in the same order?
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