Re: Possible problems with Date & McGoveran View Updating
From: Costin Cozianu <c_cozianu_at_hotmail.com>
Date: Thu, 11 Sep 2003 13:43:15 -0700
Message-ID: <bjqmkq$mburj$1_at_ID-152540.news.uni-berlin.de>
>
>
> Ok. In the meantime let me try to give some intuition behind the definitions
> I gave. It's not really mine btw., just one I know from the literature and
> that is IMO the best semantical definition of updatable.
>
> If you update the view by giving some additional tuples then you are telling
> the database about a few new facts. Logically speaking a view is just
> another relation that happens to be fully determined by a database
> constraint in the form of an equation V = Q(D) with Q some query over the
> database D. So we can use this constraint (and other constraints that might
> hold) to derive other facts in other relations that then also must hold. If
> there is a unique smallest set of facts that we can add to make the
> database consistent again then it is clear that this set is both necessary
> in the sense that they logically follow from the update on the view and
> sufficient in the the sense that they are enough to make the database
> consistent. In other words, the database knows exactly what other facts
> should also be added to the instance.
>
> A small example. Suppose we have a table Emps(name,dept) and a view TrEmps
> defined by
>
> SELECT name
> FROM Emps
> WHERE dept = "sales";
>
> then if you add a tuple to this view it is perfectly clear which tuple
> should be added to Emps and so this is a well-defined update.
>
> -- Jan Hidders
>
Date: Thu, 11 Sep 2003 13:43:15 -0700
Message-ID: <bjqmkq$mburj$1_at_ID-152540.news.uni-berlin.de>
Jan Hidders wrote:
> Mikito Harakiri wrote:
>
>>>Definition #0: >>> >>>Given D and V such that >>> >>>Q( D ) = V >>> >>>and some deltaV, Q is called well-defined if there exists deltaD such >>>that >>> >>>Q( D + deltaD ) = V + deltaV >>> >>>The alternative term I might prefer is "locally invertible". >> >>Sorry, I overlooked minimality here again. Time to think more...
>
>
> Ok. In the meantime let me try to give some intuition behind the definitions
> I gave. It's not really mine btw., just one I know from the literature and
> that is IMO the best semantical definition of updatable.
>
> If you update the view by giving some additional tuples then you are telling
> the database about a few new facts. Logically speaking a view is just
> another relation that happens to be fully determined by a database
> constraint in the form of an equation V = Q(D) with Q some query over the
> database D. So we can use this constraint (and other constraints that might
> hold) to derive other facts in other relations that then also must hold. If
> there is a unique smallest set of facts that we can add to make the
> database consistent again then it is clear that this set is both necessary
> in the sense that they logically follow from the update on the view and
> sufficient in the the sense that they are enough to make the database
> consistent. In other words, the database knows exactly what other facts
> should also be added to the instance.
>
> A small example. Suppose we have a table Emps(name,dept) and a view TrEmps
> defined by
>
> SELECT name
> FROM Emps
> WHERE dept = "sales";
>
> then if you add a tuple to this view it is perfectly clear which tuple
> should be added to Emps and so this is a well-defined update.
>
> -- Jan Hidders
>
Best,
Costin
Received on Thu Sep 11 2003 - 22:43:15 CEST