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_at_ursa-nb00s0.nbnet.nb.ca>
>
> 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?
>
> Again, the phrasing is odd to me, but yeah, definitely.
Date: Wed, 24 Jan 2007 02:42:01 GMT
Message-ID: <ZHzth.4613$1x.78914_at_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 Wed Jan 24 2007 - 03:42:01 CET