Posted by ZeldorBlat on 06/09/06 04:57

Alexander Kuznetsov wrote:
> Baihao Yuan wrote:
> > Hi,
> >
> > I created a composite index (lastname, firstname). I know the following
> > queries will use this index:
> >
> > WHERE lastname = ...
> > WHERE lastname = ... AND firstname = ...
> >
> > Also this won't use the index:
> > WHERE firstname = ...
> >
>
> Not necessarily. Consider the following query:
>
> select lastname, firstname from some_table where firstname = ...
>
> It will use the index, and, more to the point, it will not touch the
> table at all - the index already has all the information the query
> needs. It is called "index covering".

No, it won't. If you had a list of people on a piece of paper, sorted
by last name and then by first name, explain how you would use that
list to find everyone with a first name of "Joe" without looking
through the entire list.

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