Re: RM formalism supporting partial information

On 28 nov, 01:58, David BL <davi...@xxxxxxxxxxxx> wrote:
On Nov 27, 9:43 pm, Jan Hidders <hidd...@xxxxxxxxx> wrote:



On 26 nov, 15:06, David BL <davi...@xxxxxxxxxxxx> wrote:
On Nov 26, 7:47 pm, Jan Hidders <hidd...@xxxxxxxxx> wrote:
On 26 nov, 08:52, David BL <davi...@xxxxxxxxxxxx> wrote:

Firstly a minor nit pick: you can't say "possible answers", because
they don't actually represent an upper bound on the result in the
omniscient database.

?? They do so by definition.

What I meant was that unless CWA is available on an appropriate
projection there may be so much missing information (eg all
information about an entity) that the query purported to return the
"possible answers" does no such thing. ie it suffers a similar
problem to negation (it returns neither the certain nor the possible
answers).

I'm not sure what you mean by "the query purported to return the
'possible answers'". If the user formulates a query then this will now
include an indication of whether he or she wants the possible/certain
answers. It is up to the DBMS to efficiently compute the answer, and
this is not necessarily done by the usual translation of calculus to
algebra or even one very similar to it.

Consider a query to find all the 27 year old pilots from a census
recorded in an RDB. If the age or occupation is missing we could
think of the person as a possible answer. However we cannot say the
query returns all possible answers unless we assume every person took
part in the census.

Ok. Forget my other reply, for some reason I had missed something very
simple. Whether the suggested computation gives you all possible
answers or not depends on the query that is being asked. If it
concerned only the persons that took part in the census and you are
assuming the CWA for the value-unknown interpretation, then it does.
If you really meant all persons, then it doesn't, and you need another
computation if you want that answer.

-- Jan Hidders
.

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