Re: Date and McGoveran comments on view updating 'problem'

On Dec 8, 3:16 pm, paul c <toledobythe..._at_oohay.ac> wrote:
> paul c wrote:
> > ... so I wonder
> > what happens with "delete" to JOIN (which I believe is the operation
> > that many people find controversial)?
> > ...
>
> Just as they find "insert" to UNION.  The main argument seems to be that
> the operation is not deterministic when trying to decide on a base table
> by base table basis what to change, ie., there are three combinations of
> base values that could produce the same tuple in a view.  McGoveran
> seemed to be saying that this argument is the wrong one, doesn't take in
> all available information because it manipulates extensions without
> concern as to the implications of the predicates and values involved
> before the insert or delete is tried.

OK, the union view update. Assume the folowing relation

M - Males
F - Femails
D - Delta of M v F

The assertions:
1. M disjunct to F
(M ^ F) ^ R00 = R00.
This may be not necessary, although we'll demonstrate that we won't be able to derive view update even under this additional assertion. 2. M and F having the same headers
M ^ R00 = F ^ R00.
3. D and F having the same headers
D ^ R00 = F ^ R00.
4. Added tuples (D) are disjoint with F
(D ^ F) ^ R00 = R00.
5. Added tuples are disjoint with M
(D ^ M) ^ R00 = R00.

It *doesn't* follow that we can insert D into M M v D = (M v F) v D.
This is, again, established by Mace4 finding a two element counterexample.

> It just seemed to me that if one started with the algebra and could
> somehow gauge all possible expressions as to what their resulting
> relations would be, one might find that McGoveran is right and if not,
> show that the problem is more complicated than he suggested.  It also
> seemed to me that for such an exercise, if one had the right mental
> machinery, one could ignore the practical restrictions that are usually
> followed, such as common headings for union and run-time exceptions that
> some think will confuse dumb users.

IMO D&D ad-hock rules approach for view update problem is futile. Received on Tue Dec 09 2008 - 01:36:15 CET