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Posted by ZeldorBlat on 06/09/06 20:51
Alexander Kuznetsov wrote:
> > > 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.
>
> Why don't you try it out in practice? You might be in for some
> surprise. If the index is smaller than the table, and contains all the
> necessary information, it is likely to be used instead of the table.
> Google up "index covering".
I did try it in practice -- as did the OP who posted his results in
this thread.
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