Re: Interesting article: In the Beginning: An RDBMS history

JOG wrote:
> dawn wrote:
> > David Cressey wrote:
> > > "dawn" <dawnwolthuis_at_gmail.com> wrote in message
> > > news:1144354101.878068.281070_at_z34g2000cwc.googlegroups.com...
> > > >
> > > > David Cressey wrote:
> > > > > "x" <x_at_not-exists.org> wrote in message
> > > news:e12djb$ha2$1_at_emma.aioe.org...
> > > > > > > I don't think so. I think he was making the distinction between
> > > > > > attributes
> > > > > > > specified by name and attributes specified by position.
> > > > > >
> > > > > > What is the difference between a "name" and a "position" from a
> > > > > > mathematically point of view ?
> > > > >
> > > > > I can't speak for Codd on this, and I don't choose to speak for myself.
> > > >
> > > > The only difference is the domain for the function, whether it is a set
> > > > of counting numbers or a set of attribute names. If counting numbers,
> > > > then there is an obvious order (function), represented as the order of
> > > > a tuple.
> > >
> > > This is not true.
> > >
> > > If the numbers were explictily used as if they were names, in every place
> > > where a specific attribute is specified, your statement would be true.
> >
> > I don't see how a function being explicitly written or not makes any
> > difference. In some systems you can refer to an attribute either by
> > name or number. They serve as two different names for the attributes.
> >

>
>

I don't dispute that there is a difference. I just don't see what is so problematic about it.

> - given a mathematical relation is a list of elements (whose order is
> specified by the order of its domains), when one refers to the 'first'
> or 'second' element one is using knowledge external to the model

unless you include it in the model, in which case it is part of the model. A reason why it might be an advantage in a model is if you did not want to fix the attribute name or want to have synonyms without designating one as most important. These are both viable for a data model, even if not part of the RM.

> in
> order to ascertain which domain each element corresponds to. My
> impression was that Codd believed the cognitive load of maintaining
> this meta-knowledge to be deleterious,

That could be. I would also agree it is easier to remember an attribute name than number.

> and so added attribute naming.
> Being aware of this subtle distinction he proposed a name change to
> 'relationship' (which I learnt from yourself in fact).

Yes, he writes that in the 1970 paper. In what Pascal calls "Dawn's Proof" or something like that, I made a joke about the fact that the MV model, which does have an ordering (which relates to a physical ordering, admittedly) actually models the data in relations (rather than relationships) so (I claimed, tongue in cheek) it is more relational than the RM. While Fabian claims that to be my proof that the MV is relational, I have always held that the Pick model clearly pre-dates and is different from the RM.

Outside of the issue of how coupled a logical model is to the physical model, I see no harm in numbering the attributes provided you also name them. I have seen statements about how the attributes must be unordered written by several RM authors. It just seems to be an unimportant and unnecessary point. Cheers! --dawn Received on Fri Apr 07 2006 - 04:37:25 CEST