Re: A pk is *both* a physical and a logical object.
Date: Thu, 19 Jul 2007 11:28:07 GMT
Message-ID: <bVHni.12870$s25.12266_at_trndny04>
"Brian Selzer" <brian_at_selzer-software.com> wrote in message
news:jrHni.23333$Rw1.18184_at_newssvr25.news.prodigy.net...
>
> "Roy Hann" <specially_at_processed.almost.meat> wrote in message
> news:T-ednd6X0L5qsAPbnZ2dnUVZ8rOdnZ2d_at_pipex.net...
> > "Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
> > news:469e067b$0$8844$9a566e8b_at_news.aliant.net...
> >> Roy Hann wrote:
> >> 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.
> >
> > Fair enough, and the practical example would be when we are running
> > through a test script.
> >
> >> The truth of the matter is worlds are meaningless to mathematical
> >> abstractions.
> >
> > I'll take your word for it. But surely we are interested in a lot more
> > than mere abstraction here? We are interested in finding *just* those
> > abstractions whose behaviour is a good analogue to the real world. Or
to
> > put it differently, we are interested in how set theory and predicate
> > logic can be *applied* to real world data management. It's the
> > "applied-ness" that makes all the difference, and there is only one
world
> > where anything can be applied.
> >
> > Brian's assertion that some arbitrary update can cause a possible world
to
> > become an actual world is like something out of Gulliver's Travels.
> >
>
> The schema of a database describes a set of possible database values, each
> of which represents a complete description of a situation that may occur,
> and the body of a database, the actual database value, represents a
complete
> description of the situation that is actually occuring. In this context,
a
> world is a situation, an instance of the universe--a particular set of
> circumstances that can or do obtain. An update clearly designates which
of
> the possible database values is now the actual database value, and since
> there is a bijective mapping between the set of all possible database
values
> and the set of all possible worlds, it stands to reason that an update
> causes a possible world to become the actual world.
In the databases I have worked on, the cause and effect is reversed from what you said above. It's a change of state of the actual world that causes an update to be made to the database.