Re: what are keys and surrogates?

On Jan 11, 4:28 am, "David Cressey" <cresse..._at_verizon.net> wrote:
> "David BL" <davi..._at_iinet.net.au> wrote in message
>
> news:1d8bc808-c202-45bd-8d04-5ad80bb895ef_at_n22g2000prh.googlegroups.com...> On Jan 10, 5:05 pm, "David Cressey" <cresse..._at_verizon.net> wrote:
> > > "David BL" <davi..._at_iinet.net.au> wrote in message
>
> news:e6ba98c3-bc53-45a6-87c6-ea11e8c88616_at_p69g2000hsa.googlegroups.com...
>
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> > > > On Jan 10, 1:22 am, Marshall <marshall.spi..._at_gmail.com> wrote:
> > > > > On Jan 9, 8:07 am, David BL <davi..._at_iinet.net.au> wrote:
>
> > > > > > On Jan 9, 1:25 pm, Marshall <marshall.spi..._at_gmail.com> wrote:
>
> > > > > > > This issue goes away if we relax 1NF and allow attributes that
> are
> > > > > > > lists or relations. This gives us nested structures. (Nested
> > > relations
> > > > > > > are not particularly controversial around here.)
>
> > > > > > In addition to my previous post, I wish to add another comment
> > > > > > regarding my suspicion with RVAs. The tuples of a relation are
> > > > > > supposed to represent facts, but what does it mean when a relation
> > > > > > merely represents a value?
>
> > > > > The question is meaningless. The distinction you are drawing
> > > > > does not exist.
>
> > > > In what sense do tuples of an RVA represent propositions in *the* UoD?
>
> > > > > > Isn't the RM meant to have some close
> > > > > > association with FOPL?
>
> > > > > Yes.
>
> > > > > > It seems to me there is a fundamental difference between
>
> > > > > > a) a large collection of propositions relevant to a particular
> UoD;
> > > > > > and
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> > > > > > b) a composite data structure such as an AST which simply
> > > > > > "is what it is"
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> > > > > This is an illusion. There is no difference.
>
> > > > Hmmm. Unfortunately you didn't respond to my last paragraph which was
> > > > more tangible.
>
> > > > I don't believe the distinction is an illusion. I'll have a go at
> > > > providing an objective measure on a given relational database d...
>
> > > > Let B(d) equal some measure of the amount of information in d,
> > > > quantified as the total number of bits required to store all the data
> > > > (accounting for "compressibility").
>
> > > Off topic.
>
> > > I prefer quantified as the difference in entropy between the state that
> > > includes d and the state that excludes it. I believe that, except for a
> > > scale factor, the two measure boil down to the same thing, except for
> one
> > > subtle difference:
>
> > > Using entropy as the measure enables one to consider information content
> as
> > > being context sensitive. That is, if d is to be included in some other
> > > database e, then the information provided by d to e is the entropy
> > > difference between e and e+d (where "+" is suitably defined).
>
> > Are you suggesting that when d is included in e, there are less states
> > available for d?
>
> No. Did I say something that implies that?

Perhaps not. My understanding is that entropy is defined as a logarithm on the number of states available to a system, and tends to be proportional to the number of bits required to represent a particular state. When two *independent* systems s1,s2 are combined into a single overall system s = s1 + s2, the total number of states available to s is the product of the number of states available to s1 and s2, and by property of logarithms, the entropy is additive.

I thought your comment had something to do with coupling between d and e. ie there being less available states for d in the context of e, which is why you suggested an entropy measure of information content. Received on Fri Jan 11 2008 - 03:26:58 CET