Re: Temporal operations

"Paul Vernon" <paul.vernon_at_ukk.ibmm.comm> wrote in message news:bj9mhg$1gna$1_at_gazette.almaden.ibm.com...
> "Paul G. Brown" <paul_geoffrey_brown_at_yahoo.com> wrote in message
> news:57da7b56.0309021140.e506e98_at_posting.google.com...
> [snip]
> > And note that, in order to support temporal operations of the kind
used
> > in Q3 elegantly, DD&L[2] find themselves obliged to add an entirely
new set
> > of relational operators to the data model (T_UNION, T_JOIN etc).
>
> You mean U_UNION, U_JOIN, etc and these are not (as far as I can see) 'an
entirely
> new set of operations', not at the logical level anyway. E.g. U_JOIN is
short hand
> for two UNPACKs, a JOIN then a PACK.
>
> At the implementaoin level these would be new operators. Well you would
want to
> reconise the UNPACKs, JOIN, PACK sequence anyhow and optimise them into
some physical
> operations.

It is interesting that someone recently brought up relativity. Just as relative mechanics predicts Newtonian mechanics at non-relativistic speeds, the U_ operations are exactly the relational operations we already know when operating on zero interval attributes.

> > (BTW: from a theory perspective this is an admirable work and the book
> > ought to be read.) But I would only observe that the widely quoted D&D
> > diktat "The relational model needs no extension, no correction, no
> > subsumption -- and above all, no perversion! -"[1], needs to be
qualified
> > with "except by us!".
>
> I guess it depends on what you consider to be an extension of the model. I
can see
> how their statement would rather too strong for some tastes. I for one
certainly
> don't think the model is perfect and want to improve/expand/clarify/deepen
the model.

Brown is taking a comment out of context. The above quotation refers to any perceived necessity to extend or to correct or to subsume the relational model to provide support for user-defined types and type inheritence. By combining orthogonal generic interval operations with generic relational operations, the U_ operations arguably extend the relational model, but they only do so as shorthands for things that are already valid within the pre-existing logical data model. Received on Fri Sep 05 2003 - 16:25:41 CEST