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Re: Constraints and Functional Dependencies

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Wed, 28 Feb 2007 21:16:08 GMT
Message-ID: <simFh.2563$PV3.34963@ursa-nb00s0.nbnet.nb.ca>


paul c wrote:

> 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?

It is an example of both logical and physical independence; although, the specific question focused on physical performance. Received on Wed Feb 28 2007 - 15:16:08 CST

Original text of this message

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