Re: Temporal database - no end date

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Wed, 24 Jan 2007 02:42:01 GMT
Message-ID: <ZHzth.4613$1x.78914@ursa-nb00s0.nbnet.nb.ca>

Marshall wrote:

> On Jan 22, 7:40 pm, "David" <davi..._at_iinet.net.au> wrote:
>

>>Marshall wrote:
>>
>>>There is one area in particular that is of the utmost
>>>importance. The one overriding limitation on the logical
>>>level that the physical level applies is that it must exist.
>>>Any logical model that is not implementable is not worth
>>>much.
>>
>>You need to qualify that, because as stated it implies that the reals
>>(as a logical model) is not worth much.
>>
>>It is common for logical models to ignore physical limitations, making
>>them impossible to implement. Proofs of correctness often depend on
>>pure logical models. For example, a stack that never overflows is a
>>useful abstraction. The Turing machine with its infinite tape is
>>another useful abstraction.

>
> Hmmm. Well, that is not how I use the terminology, but perhaps
> this is my mistake. I wouldn't call the reals a logical model.
> (What are they a model *of*?) Rather I would describe them
> as an abstraction, but I suppose "abstraction" and "model"
> are similar concepts.
>
> What say, c.d.t.? Are the reals a logical model?

No. Reals are an uncountably infinite set. Sets bear no resemblance whatsoever to logical data models.

> Anyone have a good pointer to a definition of conceptual,
> logical, physical?

The ISO/IEC (2382?) standard vocabularies for IT have a good start. Otherwise, one would have to go back to Codd's paper from the Great Debate.

>>My point is only that it's the underlying logical model of the reals
>>that allows us to make sense of what floating point numbers are all
>>about.

>
> Again, the phrasing is odd to me, but yeah, definitely.

You ascribe meaning to nonsense. It's a natural human flaw.

> Marshall
>
> PS. I would have replied sooner, but you know, Heroes was on.
Received on Tue Jan 23 2007 - 20:42:01 CST