Re: Objects and Relations

On Feb 15, 2:24 am, "David BL" <davi..._at_iinet.net.au> wrote:
> On Feb 15, 12:22 am, "JOG" <j..._at_cs.nott.ac.uk> wrote:
>
> > On 14 Feb, 13:12, "Cimode" <cim..._at_hotmail.com> wrote:
>
> [snip]
>
> > > > 2) Hence their x,y,z position attribute always identifies them.
>
> > > False.
>
> > Location always identifies something. Two things cannot be in the same
> > place. It's sort of a law of physics, and I've got to be honest,
> > knowing it has tended to help me in daily life. Like if my tv is in
> > the corner of the room my car probably isn't there too.
>
> A quibble: saying "Location always identifies something" doesn't seem
> quite right. Not all things are physical things. For example
> location doesn't identity a company like Coca Cola.

Fair point.

> Furthermore some
> physical things aren't particularly well localised in space. Eg a
> railway line.
>
> Also, spatial location won't be adequate in a temporal DB. I presume
> you would then say that (x,y,z,t) identifies the thing. Would you be
> prepared to introduce a surrogate id that stands for (x,y,z,t), so
> that nothing keeps its identity from one moment to the next?

Yes exactly, if you so wished. Or a surrogate could represent its path over time, whatever gets the job done. None have any reason to be more valid than any other. Practically, it just matters what applies to *your* particular application, to get the results *you* need.

>
> Consider this example: There is a box containing a number of
> macroscopic particles. We use stereoscopic vision for a device that
> is able to record (x,y,z,t) of each particle. Over time we build up
> sampled data and record all measurements in a single relation. Are
> you happy to say that (x,y,z,t) (or else a surrogate standing for
> (x,y,z,t)) is the key and the particles as entities is illusionary?

I find it fun to witter on about the ways and whiles of reality, but I am henceforth restraining such self-indulgence as it is, after all, orthogonal to database theory.

>
> Curiously in QM exchange symmetry of the wave function is required for
> identical particles. However for macroscopic particles like above I
> would say their identity is "real" even though a suitable identifier
> must be invented in the sense of a surrogate id. I agree with the pov
> that all identifiers are merely names and whether you call them
> natural or surrogate isn't particularly interesting.
>
> I am struggling with your assertion that relations only are about
> roles and values, not entities.

A relation is a subset of the cross-product of n domains. Those domains do not contain 'entities' they contain values. I hope that clarifies it.

> You have yet to provide an
> explanation. I have the impression that you think in terms of
> entities but aren't prepared to admit it. Take for example your
> words like "something", "item" etc

Sure. My distinction with you is that I realise that entities are nebulous, overlapping, ill-defined things whose definition differs from person to person, and from one context to another. To constricitvely encapsulate something as an 'entity' is a mistake when it comes to data management.

While broadly irrelevant, if you have fun thinking about the philosophy behind it google for essentialism vs non-essentialism I don't know if that will help you - I fear you are entrenched in a cosy realist view of the world, but its worth pointing that Bhuddist Philosophers realised entities and 'things' are nebulous concepts thousands of years ago. It is hardly a new realisation.

Now, as I said, I'm not going to get into that epistemological debate. But what I *will* do instead (as soon I get time) is post some practical negative consequences of thinking in terms of entities and not propositions, and why we are better off getting out of that OO mindset when it comes to data management. I think this would be far more productive.

>
> [snip]
Received on Thu Feb 15 2007 - 13:44:31 CET