Hi Andre,

> If you do a plain left outer join without a WHERE-clause on the left
> side an index should not help you too much.
>
> > How is selectivity and statistic related. From the book I learn that
> > selectivity that closes to 1 is the better(do we have a treshold here
> > as to what is acceptable is a selectivity of .5 still good?)
> I think that this _is_ explained in the book, and at least so that _I_
> could understand it.
>
> selectivity of 0.5 means that in average you do have 2 records per
> index-key. And that should not be a problem. There are more functions
> to find out if this average is outweighted by extreme high
> duplications of only a few fields.
> Check your documentation and your management-tool for
> functions/statistics about "duplications" or "dupes" ... something
> like that.
>
> ciao,
> André

Thanks for your answer. But I am still quite confuse with selectivity
and statistics. I thinks I was use interchangeble in the Book. I very
clear with what is selectivity where 1 is the perfect and as long as
it is close to 1 it is good. and as you said a selectivity of .5 can
be good or bad depending on the duplicate values. Where as about
statistics I think it wasn't mention cleary what is it how it was
computed.

Apinot