Date: Tue, 27 May 2008 13:46:13 -0300
paul c wrote:
>> paul c 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.) >> >> Goedel tells us there will always necessarily be some external >> intension. However, POOD asks us to minimize it and to render as much >> of the system as possible amenable to calculation. >> ...
> I think I can dig that. In other words, if one has decided to depend on
> a system, as far as that use is concerned it's pointless to worry about
> information the system is incapable of providing?
We are talking about design. If one has decided to design a system to depend on, it makes no sense to exclude necessary information from the system. If the system is missing necessary components, how can one depend on it?
A tractor doesn't need information about who owns what property or information about crop rotations to be dependable and useful for the farmer. As long as the tractor reliably pulls things where the farmer wants it to pull them the farmer can depend on it.
Similarly, a dbms doesn't need information about Congress for an organization to depend on it for legislative compliance.
Goedel tells us our formalisms can be complete or consistent but not both. We choose "consistent and incomplete" because inconsistent leads to all sorts of bad things like unreliable, unpredictable, unstable etc. And because inconsistency doesn't even guarantee completeness.
Codd's example assumes the insert must be disjoint absent the information about disjointedness and then concludes the dbms cannot update the view. The assumption is incorrect and hence the conclusion is unsound.
POOD attempts to capture what is necessary information for the dbms. Whether it entirely succeeds is another question. Received on Tue May 27 2008 - 18:46:13 CEST