Re: Guessing?
From: paul c <toledobysea_at_ac.ooyah>
Date: Sat, 21 Jun 2008 22:48:27 GMT
Message-ID: <%yf7k.23319$Jx.3424_at_pd7urf1no>
>
> Because one involves two database states and the other involves only one.
> Any mutating operation necessarily involves two database states (or
> instances, or values--whichever nomenclature you prefer): there is what was
> then supposed to be the case and what is now supposed to be the case. But a
> view simply presents information contained in one database state--what is
> now supposed to be the case--in a different way. The connection between
> what is presented by a view and what is in the underlying relations is the
> expression that defines the view. If that happens to be a union, then the
> predicate of the view is the disjunction of the predicates of the two
> operands of the union. This is inescapable.
> ...
Date: Sat, 21 Jun 2008 22:48:27 GMT
Message-ID: <%yf7k.23319$Jx.3424_at_pd7urf1no>
Brian Selzer wrote:
> "paul c" <toledobysea_at_ac.ooyah> wrote in message
...
>>> ... In a first order language, you have constant symbols and >>> predicate symbols, and under an interpretation, meaning is assigned not >>> only to the constant symbols but also to the predicate symbols. The way >>> I see it, the only way you can have overlapping meanings is if a relation >>> has a disjunctive predicate. ... >> >> Not sure if that's so, but willing to assume it is for now. It reminds me >> that I have doubts about whether any relation should be allowed to >> disjunctions within individual propositions, at least if we want >> 'interchangeability'. For example, a base relvar that has been inserted >> to with "union" certainly doesn't have a disjunctive predicate so why >> should a view that is manifested the same way be different?
>
> Because one involves two database states and the other involves only one.
> Any mutating operation necessarily involves two database states (or
> instances, or values--whichever nomenclature you prefer): there is what was
> then supposed to be the case and what is now supposed to be the case. But a
> view simply presents information contained in one database state--what is
> now supposed to be the case--in a different way. The connection between
> what is presented by a view and what is in the underlying relations is the
> expression that defines the view. If that happens to be a union, then the
> predicate of the view is the disjunction of the predicates of the two
> operands of the union. This is inescapable.
> ...
One may choose one's starting point so as to conclude that it is 'inescapable' (sorry, not trying to mimic Bob B's high tones), but I feel that escape from this consequence is rather necessary! (seems a pedantic consequence to me which usually signals to me that something basic is just wrong, also I'm not saying I have the IQ wherewithal to show how, but am wondering whether a more complete POOD might argue that the presence of any non-disjunctive tuple in a db, no matter whether it's in a base or 'virtual' relation, trumps any disjunctive operator that might have produced/manifested it.) Received on Sun Jun 22 2008 - 00:48:27 CEST