Re: more on delete from join

From: paul c <>
Date: Wed, 02 Sep 2009 18:35:24 GMT
Message-ID: <MVynm.43519$PH1.41930_at_edtnps82>

Walter Mitty wrote:
> "Marshall" <> wrote in message
> On Sep 2, 2:44 am, "Joe Thurbon" <> wrote:
> <quote:>
>> So, to update a view, rather than update the conclusion directly, one must
>> update (one of) the base relvars that are used to derive the
>> conclusion/view. That is abductive.

> I guess I see what you mean, but strictly speaking it doesn't seem
> to fit the definition. In abductive reasoning, one knows a -> b, and
> b, but one doesn't know that it was in fact a that implied b. Whereas
> with a view, we do know.
> </quote:>
> I think I see what joe is saying, perhaps by analogy. When we give an
> imperative to update a view or assign an after update state to a view, it
> is like knowing the conclusion (b). The rule that defines the view is like
> knowing the implication (a -> b). And what has to be derived (by the DBMS)
> is the update required to the base relvars in order to adhere to both the
> rule in the view definition and the conclusion we have supplied. That's
> like abducting a from b and a -> b.
> This might be only an anlogy, or it might be more than an analogy. I'm not
> sure.
> BTW, when I was young they used to call this "converse reasoning".

The typical database doesn't record analogies! To say a view is different from a base is to say that it records something the base doesn't. Over and over, in all kinds of contexts, not just this one people, insist on comparing views to bases that don't record the same things the bases do! Why, why, why? Why don't they compare a view to a base that records the same thing? Or do they think views can record things bases can't? Again, why? Eg., if a view involves three relations, why do people insist on comparing it to a single base relation? Received on Wed Sep 02 2009 - 13:35:24 CDT

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