# Re: On Formal IS-A definition

From: Erwin <e.smout_at_myonline.be>
Date: Thu, 6 May 2010 02:39:05 -0700 (PDT)

On 5 mei, 22:56, Tegiri Nenashi <tegirinena..._at_gmail.com> wrote:
> On May 5, 12:41 pm, Erwin <e.sm..._at_myonline.be> wrote:
>
> > ... Not in the same sense that relation algebra prevents
> > graph databases because relation algebra does not understand graphs.
>
> This is very imprecise statement. I'll embark upon your typo and
> suggest that relation algebra (http://en.wikipedia.org/wiki/
> Relation_algebra) does exactly that. It manipulates binary relations
> that are essentially adjacency matrices representing direct acyclic
> graphs.

Yes. And I know about transitive closure and the concept of generalized transitive closure too, thank you.

The point is, the relational model supports graphs by moulding them / casting them in a relational form that can be proven equivalent.

Or, iow : relations are sufficient to "mimic" graphs, and graphs are not themselves an "unmissable" component of the model.

Yet iow : The IP in relationland states that all information must be conveyed as the presence of some tuple in some relation that is the current value of some relvar. Graph databases allow information to be conveyed as either the presence of a node or else as the presence of an edge. Imo, the IP is the single one principle that "proves" (note the quotes) (a) that the relational model can be used to represent _ANY_ information, iow that the relational model has a certain quality of "completeness", and (b) that it allows a simpler form of manipulation than would be possible in any model that is based on some type of "graph algebra".

And you say this is obsolete ? Honestly, I have a hard time taking that serious.

Another small point is that there is a school of thought which says something about a connection between relation algebra and first order logic, thereby excluding transitive closure as a possible operator of RA because transitive closure requires recursion and recursion is excluded by FOL. I don't really understand their point, or why they are trying to make that point, because imo (generalized) transitive closure is a sine qua non for "expressive completeness". Without transitive closure, certain queries are provably unanswerable, and so TC is needed. I find that statement about the link with FOL rather vacuous for that reason. Received on Thu May 06 2010 - 11:39:05 CEST

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