Re: SUPPORT FOR DECLARATIVE TRANSITION CONSTRAINTS

On Sep 27, 9:02 pm, paul c <toledobythe..._at_oohay.ac> wrote:
> On 27/09/2010 4:29 PM, Brian wrote:
>
>
>
>
>
> >> You have this irresistibe urge to regard tuples as mutable,
> > Not so.  Tuples are not mutable, but the referents of the propositions
> > they represent may be.
>
> > Consider a database in which every tuple has to have as one of its
> > components the time of the last transition.  Suppose now that there is
> > a database update.  In the update, every tuple is replaced, because
> > every tuple has to have as one of its components the time of the last
> > transition, which is now the time of the update under consideration.
>
> > The employee named paul c at time t1 was being paid $43/hour.
> > The employee named paul c at time t2 was being paid $37/hour.
>
> > Are "the employee named paul c at time t1" and "the employee named
> > paul c at time t2" the same employee at different times, or are they
> > different employees at different times.  In other words, did the
> > employee named paul c take a cut in pay, or are the employees named
> > paul c at t1 and t2 completelyh different employees?
>
> Either could be true.  We can't tell from either tuple.  It might be
> that some user could tell because, say, he knows that no one has quit or
> been fired lately or because of some other knowledge not recorded.  I
> don't think it would matter whether or not the transition was a key
> component, but in any event I'd still clean the toilets for 37 bucks per
> hour.- Hide quoted text -
>
> - Show quoted text -

You're absolutely right. Only the user that issues the update knows one way or the other. What I contend is that the system already provides a mechanism for the user to convey that information in such a way that it can be used to determine exactly which transition the user selected, thereby making it possible for transition constraints to be defined declaratively rather than procedurally, using set-based operations, spanning multiple relvars, without altering relvar headings, independent of whether or not any or even all keys are affected by a transition. Received on Tue Sep 28 2010 - 03:25:29 CEST