Re: foreign key constraint versus referential integrity constraint

On Oct 27, 9:33 am, paul c <toledobythe..._at_oohay.ac> wrote:
> Marshall wrote:
>
> ...
>
> > Consider relations A and B each with a single, common attribute.
> > Natural join and inner union will behave much like intersection
> > and union in this case. If the result type of the join isn't an
> > intersection type, then we lose the property:
>
> >     A = A join (A union B)
>
> > because the type of the attribute of the expression is different
> > than the type of the attribute of A.
> > ...
>
> Marshall, I probably am diverging from your purpose but let me ask if
> that property is important because without it you don't have a
> relational lattice or is it important because without it some practical
> use is lost?

Heh. I think that's a fair question.

I think absorption, (the above property) is in fact something with practical use, to the optimizer anyway. But I guess my point is more general, which is to say that the behavior of the type system ought to match the behavior of the runtime system. If join puts attribute values together with AND, then the type system should put type values together with AND.

Marshall Received on Wed Oct 28 2009 - 14:26:27 CET