Re: thinking about UPDATE
Date: Wed, 28 Jul 2004 16:11:50 GMT
Message-ID: <pan.2004.07.28.16.12.50.907297_at_REMOVETHIS.pandora.be>
On Wed, 28 Jul 2004 10:54:13 +0300, x wrote:
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
> news:pan.2004.07.27.17.33.30.451062_at_REMOVETHIS.pandora.be...
>
>> Of course, but I'm not starting with *any* relation. To show the type >> of completeness that we are dealing with here it is sufficient to show >> that for every set A of attributes that is not derived by the algorithm >> there is a relation that satisifies the initial set of candidate keys and >> in whose projection A is not a candidate key. Since the only restriction >> is that it satisfies the original set of candidate keys I'm free to assume >> that the only FDs that hold for it are those that are implied by these >> candidate keys.
>
> Oh, that completeness.
It's the only one that really makes sense here: every CK that logically follows from the original set of CKs is derived. The algorithm cannot be expected to derive what cannot be derived.
> It would have been easier if you stated that one need to know all the
> constraints on the relation to determine the candidate keys (not a
> superkey).
That should have been rather obvious because if you don't know all the constraints then not all CKs in the projection will logically follow.
> Are you aware of any algorithm that solve this problem ? What is
> the complexity of it ?
- Jan Hidders