Re: Idempotence and "Replication Insensitivity" are equivalent ?

David Cressey wrote:
> "paul c" <toledobythesea_at_oohay.ac> wrote in message
> news:od0Rg.12923$1T2.10076_at_pd7urf2no...
>> David Cressey wrote:
>>> "Chris Smith" <cdsmith_at_twu.net> wrote in message ...
> It's probably me. I'm trying to use the term "domain" the way some of the
> mathematicians seem to be using it.
>
> Here's what I think I got by making inferences from other posts: the domain
> of a relation is the cartesian product of the domains of each of its
> attributes.
>
> I probably misinterpreted something I read in here. That'll teach me to
> learn things in c.d.t.!

Seems okay to me, but this got me to thinking in a different line. As far as RT is concerned, I tend to think of a domain as being equivalent to a type, eg., a set of values plus identity operator plus maybe some other ops. (Maybe this isn't strictly correct but my reason is that I haven't thought of a situation where there might be some difference that mattered.)

If you just mean project in some mathematical sense that is apart from the RM, then I suppose the domain formed that way could still have the same name as the relation (at least that would be convenient).

But I'm also thinking that when you say 'project a relation onto its attributes', if such a thing were permitted by some RM impl'n, what *could* actually happen is that a relation with a single relation-valued attribute would be formed and I suppose that attribute's 'type' would be the name of the relation. But join is usually the operator we expect to be able to undo a projection, so if an impl'n did this, then I suppose it might want to undo the rva-creating projection, and that might entail that it also have a way of equating a relation with several attributes against a single-attribute rva equivalent.

In this admittedly oddball view of things, I wonder if the name of an rva really matters? That's as far as I've got.

p Received on Mon Sep 25 2006 - 04:10:46 CEST