Re: views of binary operations
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Mon, 17 Jul 2006 13:17:23 GMT
Message-ID: <D5Mug.11522$pu3.265968_at_ursa-nb00s0.nbnet.nb.ca>
>
> I think there are certainly relationships between the concepts. The
> effect of an assignment in a relational languages hinges on views and
> snapshots. A view can at any time, in any statement or expression, be
> replaced by the relational expression it designates. It's nothing but a
> shortcut. Snapshots in assignments are different, of course.
>
>
>
> I agree, but I don't think Marshall was saying that. I think part of
> the confusion is that r and view(r) are different types entirely;
> view(r) is not a view OF r, but simply a different variable r that
> happens to be a view. It would probably have been better to call it v
> to avoid the confusion.
>
> Marshall, am I mistaken in the above?
Date: Mon, 17 Jul 2006 13:17:23 GMT
Message-ID: <D5Mug.11522$pu3.265968_at_ursa-nb00s0.nbnet.nb.ca>
erk wrote:
> Cimode wrote:
>
>>[snipped] >>erk wrote: >> >>>Views and snapshots aren't unique to SQL. They're fundamental to >>>relational. >> >>I understand and agree but views and snapshots are not *always* >>necessary to understand RM issues. Marshall's question seems to >>directly point out directly to inter relation assignment rather than >>views or snapshots.
>
> I think there are certainly relationships between the concepts. The
> effect of an assignment in a relational languages hinges on views and
> snapshots. A view can at any time, in any statement or expression, be
> replaced by the relational expression it designates. It's nothing but a
> shortcut. Snapshots in assignments are different, of course.
>
>
>>>A simple rule of thumb might be this: A view is an expression,
>>>unevaluated, while an assignment necessarily requires a value, thus a
>>>snapshot. Special syntax is needed to differentiate a view creation
>>>("view(r) = A op B" or perhaps "view r = A op B") from a simple
>>>assignment of a relval to a relvar ("r = A op B").
>>
>>As stated previously, I am not convinced that r = A op B can not
>>logically lead to view(r) = A op B because it supposes r = view(r)
>>which is a shorcut leading to an absurdity which is that a relation is
>>the same as a view defined on the relation. OTOH view(r) = view(A op
>>B) would be acceptable.
>
> I agree, but I don't think Marshall was saying that. I think part of
> the confusion is that r and view(r) are different types entirely;
> view(r) is not a view OF r, but simply a different variable r that
> happens to be a view. It would probably have been better to call it v
> to avoid the confusion.
>
> Marshall, am I mistaken in the above?
I think it would be better to admit to oneself that one cannot make sense of what Marshall wrote and then hope he clarifies it. Received on Mon Jul 17 2006 - 15:17:23 CEST