Without going into specific details on queries...
Using mysql 5.1 as provided with CentOS6, I've noticed some queries
providing what I can best explain as inconsistent optimization. The
database can be quieted to just controlled queries and at times the same
query will return very quickly when at other times may take minutes.
I don't see the same behavior with mysql5.0 under CentOS5. The same
queries on the same data returns quickly consistently.
When the queries run slowly they show in a process list as either in a
"copy to temp table" or "sending data" state. At first I thought query
restructuring to avoid the copy to temp table was a path to a solution,
but now I don't think so since the same query changed so that it no
longer needs a temp table will sit in the "sending data" state for a
long time.
The queries do eventually come back with correct results, but it takes
minutes rather than milliseconds (sometimes slow; sometimes fast).
Have others seen this behavior? Any explanations?
Any reading to point to for further understanding?
Thanks.
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