Re: Reminder, blatant ad

JOG wrote:
> Jan Hidders wrote:
> [snip]
> > > [...] Whenever I mention that I want to
> > > work with such data structures as ordered lists with associated insert
> > > and delete functions, I am informed that I am talking about
> > > representation and that what I want is possible with the data modeled
> > > via the RM and then handled with another product in front of that. I'm
> > > showing that the RM is all about representation. As soon as you decide
> > > to use another representation, you are moving away from the RM.
> >
> > I don't know who told you that lists are somehow only representation
> > while sets are not, but that is nonsense. The reason to avoid lists is
> > far more mundane and practical; it would make the DBMS more complex and
> > make tasks such as query optimization, concurrency control and integrity
> > maintenance harder. It would also make the theory more complex, which in
> > the long term usually also translates into practical problems.
>
> I do not agree with this. Conventional ordering (and hence listing) is
> mathematically impossible in the relational model period, due to the
> Codd's redefinition of RM-relations and the model's closure. Dawn is
> hence correct imo - lists must be interpreted by a process external to
> the RM, not for the sake of simplicity, or query optimization, but
> rather because the very the nature of that data model precludes their
> implementation.

I agree (I guess it makes sense that I would agree with you agreeing with me). While ordering functions (SORT) may be defined on sets, those are orderings on values. There is no ordering within the relational structure itself.

Of course you can simulate an ordered list using relations by adding in ordering attributes and related functions such as insert & delete. That could all be done behind the scenes without the programmer touching the ordering attribute at all. But once the interface to the user (programmer) has these features, it is no longer an RM interface because it supports ordered lists. --dawn Received on Fri Jan 27 2006 - 18:29:26 CET