# Re: Constraints and Functional Dependencies

Date: 28 Feb 2007 10:31:51 -0800

Message-ID: <1172687511.893551.256600_at_q2g2000cwa.googlegroups.com>

On Feb 28, 9:20 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:

*> Marshall wrote:
**>
*

> > For myself, I completely fail to see the point of possreps.

*>
**> Data independence.
**>
**> Consider a Complex number type. It has two possible representations:
**> cartesian and polar.
**>
**> Consider the following chart of performance characteristics for
**> combinations of operations and representations:
**>
**> | Cart | Polr |
**> -----------------
**> + | Fast | Slow |
**> -----------------
**> * | Slow | Fast |
**> -----------------
**>
**> Consider three similar relations that have a Complex attribute. The
**> first application mostly uses the attribute for addition. The second
**> application mostly uses the attribute for multiplication. The third uses
**> the attribute for a balanced mix of operations.
**>
**> The first application will perform better if the dbms physically stores
**> the attribute in cartesian coordinates. The second application will
**> perform better if the dbms physically stores the attribute in polar
**> coordinates. The third application performs better if it avoids
**> unecessary conversions.
**>
**> Now, suppose it is the same relvar in all three cases and a dba has to
**> achieve specific performance goals. How does the dba do that without
**> disrupting any of the applications.
*

My problem with this examples has always been the limitations of floating point numbers. Consider (1,1). It is exactly representable in cartesian form, but not in polar form; the radius is sqrt(2), an irrational.

However, I had not considered: "How does the dba do that without disrupting any of the applications" previously; I'll have to think about that some more.

> Finally, consider a Video data type where each conversion results in a

*> loss of picture quality. What is the best way to support multiple
**> players and formats?
*

In the case of lossy compression, I feel strongly that it is a logical matter, and cannot be abstracted away. The choice of compression ratios and algorithms needs to be under user/application control, not dba control.

Marshall Received on Wed Feb 28 2007 - 19:31:51 CET