Re: Guessing?

paul c wrote:
> Bob Badour wrote:
> ...
>

>> ... (Not that I like anthropomorphizing dbmses.)

With POOD, any tuple satisfies the predicate of at most one relation in the dbms. Thus, with POOD, the dbms can calculate a unique relation to which to apply any insert, update or delete with the goal of avoiding anomalous behaviour.

In the case of the union view from Codd's book, POOD requires the base relations to have disjoint predicates. Because the predicates are disjoint, the dbms can calculate to which base relation to apply the insert. If one ignores POOD, the dbms still calculates the base relations to which to apply the insert, but the base relations need not be disjoint.

Symmetry requires the dbms apply the insert to all base relations in the union view for which the tuple satisfies the predicate.

In this respect, Codd's example is a straw man. In the example, he failed to declare the predicates to the dbms so they were unavailable for calculation. Received on Mon May 26 2008 - 16:08:42 CEST