Re: RM formalism supporting partial information

On 26 nov, 15:06, David BL <davi...@xxxxxxxxxxxx> wrote:
On Nov 26, 7:47 pm, Jan Hidders <hidd...@xxxxxxxxx> wrote:



On 26 nov, 08:52, David BL <davi...@xxxxxxxxxxxx> wrote:

On Nov 24, 3:56 am, Jan Hidders <hidd...@xxxxxxxxx> wrote:

On 21 nov, 04:05, David BL <davi...@xxxxxxxxxxxx> 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
.

Relevant Pages

  • Re: RM formalism supporting partial information
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  • Re: RM formalism supporting partial information
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