Re: Seeking Database Design Guide

["Kahn, David" <KAHN@xxxxxxxxxxxxxxxxxx>]

John Carr (74711.77@CompuServe.COM) wrote:

>I can't see ever designing a file in that manner.

I'm also not sure what they're hoping to achieve. It sounds like an
unnecessary complication with a shaky or half-understood theoretical
basis.

>I'm (I hope I don't start another DOU/DOW discussion)

Mea culpa, John.  :-)

>from the school of thought that every file (with very rare exceptions)
>requires a unique key(as per CODD). And if that is true, it should be
>on the physical file itself. (As for the arguements for having a non-keyed
>PF and a unique key LF,  I personally never have had either of the two
>scenerios happen (or heard of them) that justifies that)

I used always to do the opposite. The unique key would be on a logical
and no program would ever touch the physical at all, although the same
unique key could just as easily have been on the unkeyed physical. My
main reason for doing it this way was the idea that programs should not
be concerned with the physical representation of the data base, only
with their own views of it. Even if the physical and logical files had
the same definition initially, you could still make changes to the
physical and keep the logical the same. This was purely theoretical. In
practice I never found occasion to add a field or otherwise change the
physical layout without changing the main logicals also.

More recently I have thrown all that away and begun writing systems
where the unique key is defined on the physical file and the physical is
defined directly in programs. I'm happy to report that the sky has not
fallen in. Again without starting a war, I would appreciate it if
someone who is thoroughly familiar with the theoretical arguments on
both sides could summarise them for us.


Dave Kahn - TCO, Tengiz, Kazakstan
=========

e-mail:  kahn@tengizchevroil.com    (until August 5th)
         dkahn@cix.compulink.co.uk  (from  August 6th)

Note new e-mail address in Kazakstan
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