Re: RM formalism supporting partial information
Date: Wed, 28 Nov 2007 07:15:40 -0800 (PST)
On 28 nov, 01:58, David BL <davi..._at_iinet.net.au> wrote:
> On Nov 27, 9:43 pm, Jan Hidders <hidd..._at_gmail.com> wrote:
> > On 26 nov, 15:06, David BL <davi..._at_iinet.net.au> wrote:
> > > On Nov 26, 7:47 pm, Jan Hidders <hidd..._at_gmail.com> wrote:
> > > > On 26 nov, 08:52, David BL <davi..._at_iinet.net.au> wrote:
> > > > > Firstly a minor nit pick: you can't say "possible answers", because
> > > > > they don't actually represent an upper bound on the result in the
> > > > > omniscient database.
> > > > ?? They do so by definition.
> > > What I meant was that unless CWA is available on an appropriate
> > > projection there may be so much missing information (eg all
> > > information about an entity) that the query purported to return the
> > > "possible answers" does no such thing. ie it suffers a similar
> > > problem to negation (it returns neither the certain nor the possible
> > > answers).
> > I'm not sure what you mean by "the query purported to return the
> > 'possible answers'". If the user formulates a query then this will now
> > include an indication of whether he or she wants the possible/certain
> > answers. It is up to the DBMS to efficiently compute the answer, and
> > this is not necessarily done by the usual translation of calculus to
> > algebra or even one very similar to it.
> Consider a query to find all the 27 year old pilots from a census
> recorded in an RDB. If the age or occupation is missing we could
> think of the person as a possible answer. However we cannot say the
> query returns all possible answers unless we assume every person took
> part in the census.
Ok. Forget my other reply, for some reason I had missed something very simple. Whether the suggested computation gives you all possible answers or not depends on the query that is being asked. If it concerned only the persons that took part in the census and you are assuming the CWA for the value-unknown interpretation, then it does. If you really meant all persons, then it doesn't, and you need another computation if you want that answer.
- Jan Hidders