# Re: header part of the value?

Date: Thu, 28 Feb 2008 13:27:47 -0800 (PST)

Message-ID: <ce2ad5f0-b72d-45d5-b92d-fe53ce965947_at_e10g2000prf.googlegroups.com>

On Feb 28, 1:16 pm, Jan Hidders <hidd..._at_gmail.com> wrote:

> On 28 feb, 21:58, Tegiri Nenashi <TegiriNena..._at_gmail.com> wrote:

*>
**>
**>
**> > On Feb 28, 11:35 am, Tegiri Nenashi <TegiriNena..._at_gmail.com> wrote:
**>
**> > > Let me reiterate the generalized relation idea one more time, on a
**> > > level perhaps more digestable for wider audience. Consider classic
**> > > relation
**>
**> > > "The person first name is ..."
**>
**> > > Normally, we don't write the whole sentence in the relation header (we
**> > > focus exclusively on named perspective, of course) and abbreviate it
**> > > to just
**>
**> > > Name
**> > > -----
**> > > Scott
**> > > Mike
**>
**> > > The concept of domain has been introduced to resolve questions weather
**> > > this relation is allowed to be joined with something like
**>
**> > > "The ship name is ..."
**>
**> > > All we do when allowing generalized relations is admitting predicates
**> > > like this:
**>
**> > > "The variable x is greater or equal than ..."
**>
**> > > and insisting that the whole sentence matters as a relation header.
**>
**> > Here is little more background. The inspirational paper is Grumbach&Su
**> > "Finitely Representable Databases". They introduced the concept of
**> > finite representativity, for example the relation {x:x>=0} is not
**> > finitely representable as a classic relation on infinite domain, but
**> > is finitely representable in more general sense. However, after few
**> > pages they lost me: I can't understand where are they heading with
**> > this idea, why compactness theorem, Ehrenfeuht-Frausse gaimse and
**> > PTIME matter. From practical perspective, one would think the first
**> > thing they should discuss is an algebra to conveniently operate these
**> > finite representations.
**>
**> The complexity and computability results indicate to which extent such
**> an algebra is possible and/or useful. Besides, why do you think such
**> an algebra is necessary? What is necessary is that you can ask queries
**> and that there are algorithms to compute them. An algebra is just one
**> possible solution for that.
**>
**> > Anyway, returning to the example
**>
**> > Q:
**> > x + 3 = y \/
**> > x + 5 = y
**>
**> > Is it binary or unary relation? Sure it is a binary relation Q(x,y) in
**> > classic sense, but it is not finitely representable!
**>
**> Why do you think it is not finitely representable?
*

Sure, there is infinite number of tuples in

Q(x,y) = {(x,y) | exists t in N such that x = t and (y = t + 3 or y = t + 5)} Received on Thu Feb 28 2008 - 22:27:47 CET