Re: Problems with query not using expeced indexes
Posted by:
Rick James
Date: March 18, 2010 07:41AM
A column can be in multiple indexes.
About the only constraint is that there can be only one PRIMARY KEY.
Here is a wasteful (but not invalid) situation:
INDEX(a),
INDEX(a)
one is redundant.
UNIQUE(a),
INDEX(a)
remove the INDEX; UNIQUE is an index with an extra constraint (no dups of `a`).
INDEX(a,b)
INDEX(a)
Usually the second one is unnecessary; the first one can be used whenever the second one is desired.
INDEX(a,b)
INDEX(b,a)
These are significantly different; sometimes it is advisable to have both.
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Re: Problems with query not using expeced indexes
1911
March 18, 2010 07:41AM
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