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Re: Constraints and Functional Dependencies

From: Marshall <marshall.spight_at_gmail.com>
Date: 28 Feb 2007 10:31:51 -0800
Message-ID: <1172687511.893551.256600@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 - 12:31:51 CST

Original text of this message

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