Re: Reminder, blatant ad

From: x <x_at_not-exists.org>
Date: Mon, 13 Feb 2006 09:29:09 +0200
Message-ID: <dspcg8$ia8$1@emma.aioe.org>

"dawn" <dawnwolthuis_at_gmail.com> wrote in message news:1139620678.347430.274910_at_g43g2000cwa.googlegroups.com...

> > This is not necessarily true for all such systems but in many of those
> > the logical and physical model are closely tied together and their
> > efficiency in fact often depends on this close relationship.

> What is the test for determining whether they are too tightly coupled?
> What is it that changes (the "physical model" doesn't resonate with me,
> I want to know what software components would change) that should not
> require a change in the logical model but does in some non-relational
> products? I do not recall ever changing the logical data model in any
> software I have written when a new version of anything was released. I
> do recall changing COBOL code with a new release of Primos in 1977, but
> it was not the logical data model for the indexed sequential files that
> had to change.

Look at Codd 1970 ACM paper.
He describes various data dependencies:

- ordering dependence
- indexing dependence
- access path dependence

then he introduce "a relational view of data". Received on Mon Feb 13 2006 - 01:29:09 CST