Re: By The Dawn's Normal Light
Date: Fri, 29 Oct 2004 13:13:44 +0100
Message-ID: <418233f8$0$80661$ed2619ec_at_ptn-nntp-reader03.plus.net>
Laconic2 wrote:
>>I can buy that atomicity is relative... that seems reasonable. So from >>the point of view of relational theory, sets and lists have no >>structure (their operators can do whatever they like, as with any other >>type), but relations do.
Only top-level relations have structure to the outer instance of the relational engine. The inner relations (part of a relation-valued type) have no structure to the outer relational engine (to which they are atomic), but do have structure to the inner relational engine.
> The trouble comes when a relation can have an attribute whose domain is a
> relation. Now there's an incestuous relationship between the relational
> engine and the type engine. The type engine has to know about relations,
> in order to be a type engine. The relational engine has to know about
> relations in order to be a relational engine. But do they each know that
> the other one also knows?
I think the crucial point is that each one shouldn't know about the other. They can share the same relational engine, sure, because that is a physical implementation detail. But at the logical level they must have different namespaces that are totally shielded from each other, otherwise you're mixing data and metadata and getting into second-order logic.
Paul. Received on Fri Oct 29 2004 - 14:13:44 CEST