# Re: Constraints and Functional Dependencies

From: paul c <toledobythesea_at_oohay.ac>

Date: Wed, 28 Feb 2007 18:57:31 GMT

Message-ID: <vgkFh.1172106$5R2.322326_at_pd7urf3no>

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

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

Date: Wed, 28 Feb 2007 18:57:31 GMT

Message-ID: <vgkFh.1172106$5R2.322326_at_pd7urf3no>

Marshall wrote:

> 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.

Surely a video application would not try to map sqrt(2) to an individual pixel without a notion of rounding or somesuch. Without somesuch, I presume a polar possrep would either disallow that polar value or equate it a cartesian coordinate that exists. Don't ask me how to implement it, though. I still don't understand how the RM all by itself can decide equality/identity.

p Received on Wed Feb 28 2007 - 19:57:31 CET