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Re: views of binary operations

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Mon, 17 Jul 2006 15:47:36 GMT
Message-ID: <siOug.11622$pu3.267177@ursa-nb00s0.nbnet.nb.ca>


paul c wrote:

> Bob Badour wrote:
>

>> Aloha Kakuikanu wrote:
>>
>>> Marshall wrote:
>>>
>>>> Aloha Kakuikanu wrote:
>>>>
>>>>> Marshall wrote:
>>>>>
>>>>>> Consider named views of binary operations on relations.
>>>>>>
>>>>>> Given a relational operator "op" and relation variables A and B,
>>>>>> and a declaration of:
>>>>>>
>>>>>> r = A op B
>>>>>>
>>>>>> the language evaluates the expression "A op B" and assigns the
>>>>>> result to r.
>>>>>>
>>>>>> However, if we declare this as a view, ...
>>>>>
>>>>>
>>>>> View is a named expression. r is a view.
>>>>
>>>>
>>>> Is it necessarily named? Might we find use for an anonymous view?
>>>
>>>
>>> Well, the point is that the term "view" is redundant and SQL-ish.
>>> Relations and relational expressions are the fundamental concepts, and
>>> view is some bastardized idea.
>>
>>
>> I disagree. Views and snapshots are fundamental and are the source of 
>> logical independence.

>
>
> Not familiar with the term 'logical independence' as written - does it
> refer to logical-physical independence and/or lack of redundancy?
>
> (I notice that Codd 1970 mentioned the word 'view' several times, I
> think once as a synonym for model and also in non-specific terms but not
> in language terms, say as a named relational expression or catalogue
> macro, ie. not in today's sense, if I've got it right. As a 'macro', a
> view already defers some evaluation, if that's what Marshall S. is
> driving at. Maybe that's not important, after all what we call
> relations today he called relationships then.)

Data independence comprises two distinct types of independence. Physical independence allows one to adjust performance characteristics without disturbing a given logical design. Logical independence allows one to adjust the logical design without disturbing existing applications. One achieves the latter form of data independence through application views -- especially updatable views.

Physical independence gives independence between the base relations and the physical storage. Logical independence gives independence between the base relations and the relations that applications interact with.

Data independence is a relative concept and a principle both for designing dbmses and for evaluating the quality of dbmses. Thus, while one could create a product using relations and the relational algebra but lacking named views, one would be hard-pressed to establish that the product meets the requirements for a dbms.

I am not sure exactly when the name 'view' was introduced, but it was certainly already well-established when Codd published his '12 rules'. Received on Mon Jul 17 2006 - 10:47:36 CDT

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