The case of the mysterious MySQL caching across restarts

Posted by shanusmagnus on Stack Overflow See other posts from Stack Overflow or by shanusmagnus
Published on 2011-01-14T18:47:52Z Indexed on 2011/01/14 18:54 UTC
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I found a very slow MySQL query in my web app. The weird thing is that the query is only slow the first time it's executed, despite the fact that the query_cache is set to its default (query_cache_size 0) like so:

mysql> show variables like 'query%';
+------------------------------+---------+
| Variable_name                | Value   |
+------------------------------+---------+
| query_alloc_block_size       | 8192    |
| query_cache_limit            | 1048576 |
| query_cache_min_res_unit     | 4096    |
| query_cache_size             | 0       |
| query_cache_type             | ON      |
| query_cache_wlock_invalidate | OFF     |
| query_prealloc_size          | 8192    |
+------------------------------+---------+

The even weirder thing is that this speedup persists even after the MySQL server has been stopped and restarted (I'm using OSX, and perform this restart using the system preferences pane.) The only way I can re-create the poor performance of the initial query is by rebooting the system.

So my question is: how is this happening? Obviously some sort of caching at work, but where? And how does it persist across database restarts? This query is mediated through our web app, which comes via PHP/Apache, but there are no extra bells and whistles, and the curious caching also persists across Apache restarts.

Help?

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