Re: Relation Schemata vs. Relation Variables

brian_at_selzer-software.com wrote:
> [snip]
> As has been made clear in earlier posts, keys can change, so unless you
> introduce object identifiers or immutable surrogates, you cannot
> *always* enforce transition constraints based only on key values.

oids are clearly no use as they are not observable identifiers.

However, you are correct - surrogates /can/ be used. The problem is your use of the term 'immutable' - it is the wrong perspective, starting from a db schema viewpoint, as opposed to correctly starting from the necesseties of real world identification. Its consistency of identification that matters there rather than some notion of immutability.

> (I percieve other problems with relations that have more than one
> candidate key, but I haven't drawn any conclusions.) My main objection
> is the injection of irrelevant information when it's not necessary.

A surrogate is not irrelevant. It may be vital information.

> If a change can be observed, then it doesn't matter if keys change, you're
> still looking at the same thing. That's why I think that the update
> semantics of the model should be augmented to allow the user to specify
> how facts correlate if needed. This does not break the model, it just
> makes it possible to enforce transition constraints when keys can
> change.
Received on Wed Aug 23 2006 - 18:16:48 CEST