Re: Guessing?

On May 27, 9:57 pm, paul c <toledoby...@xxxxxxxx> 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.

.

Relevant Pages

  • Re: Guessing?
    ... does the DBMS decide which of the two base relations SE and SW is to be ... predicate symbols P and Q. Isn't it true that under a first order logic ... updates to the base relvars. ... base relvar updates that are consistent with the derived relvar ...
    (comp.databases.theory)
  • Re: Guessing?
    ... Such a request must be reflected in some change applied to the base relations, ... it is not appropriate to assume that the DBMS or the database has ... updates to the base relvars. ... base relvar updates that are consistent with the derived relvar ...
    (comp.databases.theory)
  • Re: Guessing?
    ... That would suggest that the DBMS should ... updates to the base relvars. ... base relvar updates that are consistent with the derived relvar ... Regarding the insertions to the projection, I vaguely remember a book by a Russian guy where IIRC he was suggesting that all tuple components should be set-valued. ...
    (comp.databases.theory)
  • Re: Guessing?
    ... That would suggest that the DBMS should ... base relvar updates that are consistent with the derived relvar ... Let the following intensional definitions of predicates s,se,sw,su be ... s:- it is known that X is an supplier. ...
    (comp.databases.theory)
  • Re: Guessing?
    ... defined intensional definition outside the RM formalism and ... inaccessible to the DBMS. ... disjointness of the two base relvars isn't present and so won't be ...
    (comp.databases.theory)