Re: A pk is *both* a physical and a logical object.

"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message news:469e067b$0$8844$9a566e8b_at_news.aliant.net...
> Roy Hann wrote:
>
> > "Brian Selzer" <brian_at_selzer-software.com> wrote in message
> > news:_Nfni.26756$2v1.2010_at_newssvr14.news.prodigy.net...
> >
> >>>Under the closed world hypothesis, the only sensible reason to do the
> >>>update you describe to a row in a relation in which the entire header
is
> >>>the key would be to retract a falsehood. I have no interest in
> >>>falsehoods and I don't see how they are related to "individuals"
> >>>(whatever they are). What am I not getting?
> >>
> >>What does the closed world assumption have to do with it? The closed
> >>world assumption simply requires that if a tuple that does not violate
the
> >>predicate of a relation is not contained within the particular relation
> >>value at a particular world, then the atomic formula represented by the
> >>tuple is false at that particular world.
> >
> > Yes, exactly so. But a database exists only to represent what is said
to be
> > true about the real world. The real world is the only "particular"
world of
> > interest. If any particular database represents a falsehood about the
real
> > world or is updated to represent a falsehood about the real world, it is
of
> > no use and no interest.
> >
> >
> >>An update selects which possible world is actual;
> >
> > In practice, the only useful update is one that selects a world we are
> > provisionally asserting is currently like the real world.
> >
> >
> >>therefore, it operates independent of the closed world assumption.
> >
> > Notice I used the phrase "the only sensible reason" above. We are not
> > interested in random updates.
> >
> >
> >>Any possible world can become the actual world, so it follows that each
> >>possible world should be closed with respect to itself.
> >
> > Get a grip. We are talking about databases. Databases are not abstract
> > collections of symbols that can be manipulated willy-nilly. They are
> > collections of symbols that we seek to manipulate in the safe knowledge
that
> > we always end up with a representation of what we think is true in the
real
> > world.
> >
> > Roy
>
> Unless we are doing a simulation. In which case, we seek to manipulate
> in the safe knowledge that we end up with a representation of what we
> think the real world would have been had it started with the same
> boundary conditions.
>
> The truth of the matter is worlds are meaningless to mathematical
> abstractions.

It's symmetric. Mathematical abstractions are meaningless to (real) worlds. It's the semantics that establish a bridge between the concrete and the abstract. That bridge, in turn, allows what might merely be elegant to be utilitarian as well. Received on Wed Jul 18 2007 - 16:59:14 CEST