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Re: Constraints and Functional Dependencies

From: paul c <toledobythesea_at_oohay.ac>
Date: Wed, 28 Feb 2007 18:06:18 GMT
Message-ID: <uwjFh.1164646$1T2.1156542@pd7urf2no>


Bob Badour wrote:
> Marshall wrote:
>

>> On Feb 27, 3:20 pm, "Tony D" <tonyisyour..._at_netscape.net> wrote:
>>
>>> I wish I could agree about the semantics, but I'm not yet convinced.
>>
>>
>> I am impressed with the degree to which they've thought through
>> every last detail of what needs to be possible with relations,
>> and how it should work. This is the main value it seems to me.
>> How they've organized it, choice of operators, syntax, etc.
>> is merely okay.
>>
>>
>>
>>> I'm also not 100% convinced about the utility of possreps.
>>
>>
>> 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.
>
> 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?

Although when it comes to possrep's, I'm an ignoramus, I think this is a darned good example of logical data independence, ie., lack of logical redundancy doesn't imply lack of physical redundancy.

I feel a vague question forming, but before trying to ask it, have I read the above wrong?

p Received on Wed Feb 28 2007 - 12:06:18 CST

Original text of this message

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