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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
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March 18, 2010 07:41AM


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