Re: Possible problems with Date & McGoveran View Updating

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