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

Date: Wed, 18 Jul 2007 14:59:14 GMT

Message-ID: <6Vpni.9755$s25.3380_at_trndny04>

"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