Re: Guessing?

On May 27, 9:57 pm, paul c <toledoby..._at_ac.ooyah> wrote:
> David BL wrote:
>
> > It seems to me that every base relvar will in practice have some
> > defined intensional definition outside the RM formalism and
> > inaccessible to the DBMS.
>
> A practitioner who (knowingly) tolerates or suggests that is likely
> either a sucker or a charlatan. (Bob B called Codd's example a straw-man.)

Consider the intensional definition :

S = set of surnames of UK prime-ministers after Thatcher

with (current) extension

        S = { Major, Blair, Brown }

Only the intensional definition (which is outside the RM formalism) is able to tell us how to apply updates over time.

Are you suggesting intensional definitions never exist or sometimes don't exist or what?

>
> That would suggest that the DBMS should
>
>
>
>
>
> > only permit updates to derived relvars that map uniquely to associated
> > updates to the base relvars. Without any need to anthropomorphize the
> > DBMS, it is mathematically well defined whether there are alternative
> > base relvar updates that are consistent with the derived relvar
> > update. In such a case the DBMS should indicate an ambiguity error.
>
> > In the example from Codd, I think it is incompatible with the
> > Interchangeability Principle. The problem is that the database schema
> > doesn't allow for missing information about whether a given supplier
> > is east or west of the Mississippi. Since the DB cannot represent
> > that kind of partial information it cannot support updates from a view
> > (ie derived relvar) with the missing information. The problem is
> > analogous to attempting insertions to a derived relvar that has
> > projected away an attribute.
>
> Regarding the insertions to the projection, I vaguely remember a (thin)
> book by a Russian guy where IIRC he was suggesting that all tuple
> components should be set-valued. Lost the book and can't remember the
> name. Apologies for the mysticism, I'm imagining he might have been
> suggesting that empty sets could be used, ie., that non-specification of
> an attribute value would be interpreted as 'no value'. I guess this
> would require re-thinking of projection as a basic operator, otherwise
> some people would start thinking about NULL's again.

Ha Ha. Received on Wed May 28 2008 - 04:25:22 CEST