Re: Why bother with Logical data model?

Brian Selzer wrote:

>

It is not my definition. It is the one defined by Codd, by industry standards (ISO 2382 and also the ISO SQL standards) and all good textbooks that I know. Representation of data in an RDBMS as relations or in a SQL DBMS as tables is a LOGICAL representation, by definition. A standard SQL CREATE TABLE statement defines the Logical Model not the physical one. The Physical Model is always implementation defined and typically consists of files, devices, volumes and other structures (perhaps data pages but not usually "tables") that map the logical level onto storage.

So your statements about the logical model being absent and the physical model being denormalized don't make any sense. How can you use a database without tables? How can a file or device be denormalized?

> On the other hand, the decision to use a partitioned view to improve
> performance does affect the physical model because additional database
> objects are needed (tables and views)--not because the underlying tables and
> indexes will reside on different storage media.

No. The decision to create additional tables and views affects the *logical* model because by definition those are objects in the database's logical model. Your stated intention is to change the physical model as well, however there is no need to assume that this *must* happen or that such a logical change will *always* affect the physical model. For example, multiple tables could easily have exactly the same physical representation as a single table.

As Kenneth mentioned, your remarks and the OP's question do make some sense if you read "logical model" as "conceptual model" or "business model". This is a fairly common attitude but it is technically a big mistake.

-- 
David Portas