Re: separation of church and state?
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Sun, 07 Oct 2007 18:00:02 -0300
Message-ID: <4709489c$0$14857$9a566e8b_at_news.aliant.net>
>
> In order theory, an ordered set is a pair, consisting of a set and an
> order relation on that set. It's not a list or anything like that.
> The question of data structures only comes up when (in
> implementation land) we want to do the computation of putting
> the elements in some order. What that looks like is a design
> decision, and I don't see any reason why it can't look like a
> relation, at some level at least. In other words, it could be a
> relation {position, element} in the case of a total order, or
> {position, {element}} in a preorder. (It's not obvious what
> it should be for a partial order.)
>
> My sense is that Date's ideas about the solution space to
> this problem have been somewhat artificially constrained
> as a response to what SQL did.
Date: Sun, 07 Oct 2007 18:00:02 -0300
Message-ID: <4709489c$0$14857$9a566e8b_at_news.aliant.net>
Marshall wrote:
> On Oct 7, 8:40 am, paul c <toledobythe..._at_ooyah.ac> wrote:
>
>>Could be. Maybe one of Date's meanings is that no system that supports >>both ordering and some relational algebra is purely relational, even if >>the "pure" relational part of it could be isolated in some way from the >>rest! If so, calling the paragraph "doctrinaire" might be a bit of a slur.
>
> In order theory, an ordered set is a pair, consisting of a set and an
> order relation on that set. It's not a list or anything like that.
> The question of data structures only comes up when (in
> implementation land) we want to do the computation of putting
> the elements in some order. What that looks like is a design
> decision, and I don't see any reason why it can't look like a
> relation, at some level at least. In other words, it could be a
> relation {position, element} in the case of a total order, or
> {position, {element}} in a preorder. (It's not obvious what
> it should be for a partial order.)
>
> My sense is that Date's ideas about the solution space to
> this problem have been somewhat artificially constrained
> as a response to what SQL did.
Have you read the Lorentzos, Date, Darwen book on temporal data? It seems clear to me that Date understands the various orders: an interval type is defined by the interval type generator on the basis of a total order.
As a data type, a relation has no implicit order. Other data types can define one or more orders, but data types are generally orthogonal to the relational model.
Date is careful not to overspecify things. Leaving the RM as broadly applicable as possible is not a flaw or an oversight in my opinion. Received on Sun Oct 07 2007 - 23:00:02 CEST