Posted by Alexander Kuznetsov on 06/09/06 03:14

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".

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