Re: A pk is *both* a physical and a logical object.
Date: Wed, 18 Jul 2007 09:25:04 -0300
Roy Hann wrote:
> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
>>>Under the closed world hypothesis, the only sensible reason to do the >>>update you describe to a row in a relation in which the entire header is >>>the key would be to retract a falsehood. I have no interest in >>>falsehoods and I don't see how they are related to "individuals" >>>(whatever they are). What am I not getting? >> >>What does the closed world assumption have to do with it? The closed >>world assumption simply requires that if a tuple that does not violate the >>predicate of a relation is not contained within the particular relation >>value at a particular world, then the atomic formula represented by the >>tuple is false at that particular world.
> Yes, exactly so. But a database exists only to represent what is said to be
> true about the real world. The real world is the only "particular" world of
> interest. If any particular database represents a falsehood about the real
> world or is updated to represent a falsehood about the real world, it is of
> no use and no interest.
>>An update selects which possible world is actual;
> In practice, the only useful update is one that selects a world we are
> provisionally asserting is currently like the real world.
>>therefore, it operates independent of the closed world assumption.
> Notice I used the phrase "the only sensible reason" above. We are not
> interested in random updates.
>>Any possible world can become the actual world, so it follows that each >>possible world should be closed with respect to itself.
> Get a grip. We are talking about databases. Databases are not abstract
> collections of symbols that can be manipulated willy-nilly. They are
> collections of symbols that we seek to manipulate in the safe knowledge that
> we always end up with a representation of what we think is true in the real
Unless we are doing a simulation. In which case, we seek to manipulate in the safe knowledge that we end up with a representation of what we think the real world would have been had it started with the same boundary conditions.
The truth of the matter is worlds are meaningless to mathematical abstractions. Received on Wed Jul 18 2007 - 14:25:04 CEST