Re: Relation Schemata vs. Relation Variables

Jon Heggland wrote:
> vc wrote:
> > Marshall wrote:
> > [...]
> >> The idea of transition constraints is inherrently non-set-theoretic,
> >> because it assumes tuple-identity.
> >
> > How come ? One can easily think about a transition constraint in set
> > terms by extending the original universe with a new attribute value.
> > Then declarative TC specification would be no different from any other
> > integrity constraint specification. That's, by the way, is what
> > Dataphor does with its 'old' and 'new' values.
>
> I don't understand your reference to Dataphor here. Dataphor's
> transition constraints are tuple-based,

So what ? The Dataphor row-level transition constraint is a special case of the more general relvar constraint. Are you criticizing Dataphor for failure to implement a more general relvar constraint mechanism ? I am sure they had a lot of reasons for not doing so.

> and aren't enforced usefully
> when using relvar assignment (as opposed to insert/update/delete).

I am not intimately familiar with Dataphor, but my impression was that they did not implement relational assignment at all, just the 'shorthand' delete/upadate/insert operators. Is that so ? In any case, they did what they could by implementing row-level declarative transition constraints 'set-theoretically', something the OP was wondering about.

> --
> Jon
Received on Wed Aug 30 2006 - 21:05:45 CEST