Re: RM VERY STRONG SUGGESTION 4: TRANSITION CONSTRAINTS

On Sep 4, 6:10 am, Erwin <e.sm..._at_myonline.be> wrote:
> On 4 sep, 05:21, Brian <br..._at_selzer-software.com> wrote:
> > On Sep 3, 2:29 pm, Erwin <e.sm..._at_myonline.be> wrote:
> > > On 3 sep, 17:35, Brian <br..._at_selzer-software.com> wrote:
> > <snip>
> > > Please explain what the difference is between this update and a
> > > multiple assignment consisting of the delete of t1 and the insert of
> > > t2.
>
> > In the update, the referent of t1 is the referent of t2, but in the
> > multiple assignment, the referent of t1 ceased to exist and the
> > referent of t2 came into existence.  The "meaning" of the fact "t2" is
> > therefore different.  For example,
>
> So the meaning of t2 does not depend exclusively on the attribute
> values within it, but also on its history of how it came into
> existence ?
>
> The fact documented by a tuple t2 in a relvar R is nothing more and
> nothing less than the instantiation of the relvar's external predicate
> with the attribute values of t2.  You claim to the contrary, that it
> also includes "something" to do with the history of how t2 came to
> be.

I do not claim to the contrary. I claim that it can be determined whether t1 and t2 refer to the same thing at different times or to different things at different times. That distinction is critical to the definition of transition constraints. Again, unlike abstract objects that are independent of time, concrete objects have lifetimes, so what t1 refers to at one time may be different than what t1 refers to at another, and what t1 refers to at one time may be what t2 refers to at another. The transition marks the boundary between the interval during which the referent of t1 exemplifies the relvar's predicate and the interval during which the referent of t2 does. If the transition is UPDATE, then the thing that both t1 and t2 refer to has a lifetime that spans both intervals; if the transition is DELETE-then-INSERT, then the thing that t1 refers to has a lifetime that ends at the point in time of the transition, and the thing that t2 refers to has a lifetime that begins at the point in time of the transition.

> You are free to think as you like.  But my position (which is
> also Date's) allows me to use the CWA, and the CWA alone, to determine
> whether or not some tuple should or should not appear in some relvar.

The CWA has absolutely nothing to do with whether or not some tuple should or should not appear in some relvar. The CWA applies to how the tuples that don't appear in a relvar should be interpreted: false, or unknown. Received on Sat Sep 04 2010 - 16:32:20 CEST