# Re: RM formalism supporting partial information

Date: Tue, 27 Nov 2007 04:43:55 -0800 (PST)

Message-ID: <c691dfeb-3a5f-446b-8005-cb900e90f1fe_at_s8g2000prg.googlegroups.com>

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:
**>
**> > > On Nov 24, 3:56 am, Jan Hidders <hidd..._at_gmail.com> wrote:
**>
**> > > > On 21 nov, 04:05, David BL <davi..._at_iinet.net.au> wrote:
**>
**> > > I have wondered whether what you're after is a model that allows the
**> > > CWA to apply to some projections and not to others.
**>
**> > Not only projections, also selections, or actually any part of the
**> > database that can be identified by a query.
**>
**> So would you say that CWA most generally applies to particular views
**> and not necessarily base relvars?
*

Exactly.

> > > Well I agree, but

*> > > I still think it's interesting to investigate the simpler case of
**> > > where the CWA is never applicable.
**>
**> > Really? That case looks rather simple to me. If we consider the usual
**> > 6 algebra operators from the named perspective then, under the
**> > assumption that the operations contain the set of certain answers then
**> > all except the difference also return the certain answers, and the
**> > difference returns something that is neither the certain nor the
**> > possible answers.
**>
**> It is rather simple, but I think that's a good thing! I have doubts
**> whether an intricate model will be worthwhile. I'm put off by the way
**> (in 3vls) unknowns propagate all other the place, so there seems
**> little gain modelling all that, compared to merely propagating what's
**> certain. There seems a lot of complexity and not much to show for all
**> the trouble.
*

That may have more to do with the fact that coming up with algebra operators that deal with unknowns are simply not such a good idea. A much more promising approach for dealing with generalized CWAs and unknown values is that you take the query (which now also indicates whether you want the certain answers, the possible answers or both) and translate it to a classical algebra expression that computes the certain answers / possible answers / both. No special algebra operators or 3VL is needed.

> > > 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.

- Jan Hidders