Re: A pk is *both* a physical and a logical object.

"Brian Selzer" <brian_at_selzer-software.com> wrote in message news:_Nfni.26756$2v1.2010_at_newssvr14.news.prodigy.net...
>
>> 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 world.

Roy Received on Wed Jul 18 2007 - 10:41:40 CEST