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Re: Why does MySQL's performance decrease when queries are executed in parallel?
Posted by: Rick James
Date: June 30, 2012 09:00AM

Do
EXPLAIN EXTENDED SELECT ...;
SHOW WARNINGS;
In the "IS NULL" case, I expect it to still say LEFT.
In the "IS NOT NULL" case, I expect it to throw away "LEFT", since it is irrelevant. That is JOIN and LEFT JOIN have identical semantics when you are throwing out NULL values.

My point that is that you are doing two radically different queries, with radically different EXPLAIN plans (though the look very similar), hence the locks, timing, etc, may be radically different.

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