Re: RM formalism supporting partial information

On 29 nov, 03:54, David BL <davi...@xxxxxxxxxxxx> wrote:
On Nov 29, 12:15 am, Jan Hidders <hidd...@xxxxxxxxx> wrote:



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.

The concept of "possible answers" isn't universally applicable, and
therefore seems to represent quite a problem for any model of partial
information that emphasises that concept as fundamental.

The concept of 'possible answers' applies and is well defined for all
databases where you have precisely defined what it means if certain
data is missing, and note that his includes the definition that says
that it means nothing. So what you mean by "isn't universally
applicable" is completely beyond my comprehension.

Did you read my response to Brian regarding the approach to absorb the
CWA/OWA distinction into the intensional definitions?

Yes, I did. Typical case of "let's redefine our terminology to make
the problem go away". :-) It won't do.

What do you think of the suggestion that the formalism (which is
concerned with extensions rather than intensions)

1) ignores the CWA/OWA distinction;

2) assumes the CWA applies everywhere; and

3) null is *always* interpreted as non-existence w.r.t.
the (carefully worded) intensional definitions?

This approach seems simple and self consistent.

If I ignore for the moment 1) (because 1) and 2) seem contradictory
because I cannot assume there is no difference between X and Y and at
the same time assume that only Y applies everywhere) this is just the
classical value-does-not-apply interpretation.

It doesn't however, attempt to model the case of "value exists but is
unknown". IMO that case should be modeled *explicitly* with a
different predicate.Of

Sure, the value-does-not-apply interpretation can always also be
represented without null values.

The thing is that you have now fully ignored the real problem of
incomplete information which is that in practice the CWA does not
always fully apply. Your main solution seems to be to redefine the
meaning of the relations such that it does, which, of course, doesn't
solve anything at all and simply puts the problem back on the plate of
the user.

-- Jan Hidders
.

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