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Re: Queries slow down daily, optimizing helps -> why?
Posted by: Rick James
Date: February 19, 2009 10:53PM

key_buffer is for MyISAM indexes only.

Query_cache is for statements. And you have it "ON" with 500MB of space (a lot). There are several caveats:
* When a table is modified (INSERT/...) all entries in the QC for that table are purged.
* When the _identical_ SELECT comes in (not even so much as a space different), the engine will first look in the QC to see if it is there. If so, it will return the result set from the QC.
* Result sets that are too big won't go into the QC.

So, another explanation for the slowdown...
1. Do the SELECT -- it gets into QC.
2. The table is modified -- QC flushed (for that table)
3. Do the SELECT again -- not found in QC, so slow.
4. Run SELECT again -- now blazingly fast because it is QC.

will give some numbers that (with some dividing) can give you the hit/miss rate, etc.

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Re: Queries slow down daily, optimizing helps -> why?
February 19, 2009 10:53PM
March 18, 2009 03:58AM

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