Re: Navigation vs Relational operators

"x" <x-false_at_yahoo.com> wrote in message news:40a8bc19_at_post.usenet.com...
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> "Mike Nicewarner" <psyclo_at_nospam_datamodel.org> wrote in message
> news:c8acu6$9ft$1_at_news.netins.net...
> > Interesting discussion.
> > The idea of suggesting a route to the taxi driver is like suggesting
index
> > usage to some DBMS. Sometimes we force a specific order of operation by
> > placement of subqueries. In any event, the DBMS/taxi driver makes the
> final
> > decision about how to get the data.
> > And remember that in many situations, the data is no where near where
you
> > think it is. Think "bufferpools" or other caching ideas. In other
words,
> > "LAX" isn't always at the same location, so for you to force the taxi
> driver
> > to take a specific route might be the worst idea possible.
>
> Well, the taxi driver know the names of the places and streets (our own
> names, not city hall names).
> When we tell him the "route", we use these names.
> What I have in mind , it is more like specifying attributes and join
> conditions.
>

Relational is still navigational, but logically. The relational model frees us from physical navigation and somewhat from logical navigation. To further provide users the capacity to have even greater freedom from logical navigation (e.g. specifying an attribute independent of knowledge and application of table names, relationships, and joins), Ullman and others (Fagin,Maier and Valdi) proposed and explored the concept of the Universal Relation. There was quite an interesting and uncharacteristically blunt public debate between proponents of the universal relaton assumption and William Kent.

Refererences:
Fagin, R., Mendelzon, A., & Ullman J.. (September, 1982). A simplified universal relation assumption and its properties. ACM Transactions on Database Systems (TODS) 7: (3).

Kent, W. (December, 1981). Consequences of assuming a universal relation. ACM Transactions on Databases (TODS), 6:(4).

Ullman, J. (December, 1983). On Kent's 'Consequences of assuming a universal relation'. ACM Transactions on Databases (TODS), 8:(4).

And others that can be found in references and citings of the listed references.

• Dan

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Received on Mon May 17 2004 - 16:50:36 CDT