Re: Selectivity

"GoranG" <no_at_spam.net> wrote in message news:fpjvavcttiai53q71fh9t29chvs6kblfdc_at_4ax.com...
> On Wed, 30 Apr 2003 10:33:07 +0000, andrewst
> <member14183_at_dbforums.com> wrote:
>
> >Originally posted by Gorang
> >> Could someone, please, shed some light on why for selectivity (s)
> >> defined as the ratio of the number of tuples that satisfy the
> >> condition to the total number of tuples in the relation, and hence a
> >> number between 0 and 1, it is often (allways?) said that high
> >> selectivity corresponds to low value of this ratio and low selectivity
> >> corresponds to the high value of such ratio ?!
> >>
> >> ( GoranG79 AT hotmail.com )
> >A query has "highly selectivity"
> Would that rather be one of the following:
> A query is highly selective
> .. or ..
> A query has high selectivity
> ?
>
> >if it returns a small proportion of the
> >rows, and "low selectivity" if it returns a large proportion of the
> >rows. There is no contradiction.
>
> Yes there is! :)
>
> Selectivity is defined as a number.

I don't know where you got the definition for s. Perhaps a better definition would be one minus the ratio you gave earlier or even the reciprocal. However, if the definition has wide acceptance in some body of work, you may just have to put up with the illogic of it.

Idiomatic english use does not always match more rigorous terminology. After all, people park in driveways and drive on parkways. One challenge for english speakers related to databases is changing "I want an apple and a banana" into "Give me the fruits that are apples or bananas".

> P.S. This whole issue is actually of minor importance and might be
> caused by the fact that english is not my primary language. All I
> wanted to know is does this fall into blunder of a definition category
> or should I try to improve my english (or logic)?

It sounds like a definition blunder to me. Your comprehension of english seems fine. Received on Wed Apr 30 2003 - 21:46:19 CEST