Re: two nasty schemata, union types and surrogate keys

On Nov 8, 6:37 pm, r...@xxxxxxxxxxx (Reinier Post) wrote:
Brian wrote:
Under the closed world interpretation, there are no unknown truth
values, but under the open world interpretation, only what has been
explicitly asserted is known to be true, and it is known to be true

Yes ...

even if the user that made the assertion is mistaken.

Huh?!  So if I have a database relation 'X works at Y', with the open
world assumption, if someone updates the relation to say that Jane Doe
works at Acme Corp., then I must assume this to be true even if that
person is mistaken?  How does that make any sense?

Yes. If you don't also record who uttered the assertion, then the
supposition implicit in its representation in the database under the
closed world interpretation is that the assertion is true or under the
open world interpretation is that whoever uttered the assertion knows
that it is true. This makes sense because it is not known that the
person is mistaken. If a bank teller counts his drawer at the end of
the day and is neither over nor short, then the assumption must be
that every customer received correct change, but it could be that one
customer received a dollar more than he should have and another
received a dollar less.


[...]  Just because something
is known to be true doesn't mean that it actually is true.  Even under
the closed world interpretation, what is supposed to be true may not
actually be true.

Exactly.  There is no difference between OWA and CWA in this respect,
regarding the assertions that correspond to tuples in the relation.
The difference is in what they imply for assertions that correspond
to tuples not in the relatoion.

--
Reinier

.

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