Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?

D Guntermann wrote:
>
>"Jan Hidders" <hidders_at_REMOVE.THIS.ua.ac.be> wrote in message
>news:3e4ad2b8.0_at_news.ruca.ua.ac.be...
>> Lauri Pietarinen wrote:
>> >
>> >what is your take on Garcia-Molina, Ullman and Jennifer Widom
>> >regarding their stand on duplicates?
>> >
>> >(see http://www.dbdebunk.com/cjddtdt.htm and cjddtdt2)
>> >
>[snipped for brevity]
>>
>> But how your algebra looks depends on how you
>> answer question 2, because query optimization is the main raison d'etre of
>> the algebra, and there it is a completely different story. It can for
>> example be more efficient to postpone duplicate elimination. If you don't
>> have a bag algebra you cannot express this in your algebra.
>
>I'll try not to sound too ignorant, but I'm afraid I will anyway, as I
>haven't had time to read the link to Mr. Date's comments yet.
>
>Why does query optimization have to expose bag algebra?

I don't think I said it has to.

>Can't structures be manipulated at a lower internal representation that is
>independent of a logical data model and logical manipulative aspects (e.g.
>relational model with relational algebra)? Isn't this the point of
>ANSI-SPARC architecture?

Yes, it can, and yes it is.

>I wonder what the ramifications are on data independence if relations,
>whether base, derived, or intermediate, are allowed to incorporate bags in
>order to accomodate optimization.

With "intermediate results" I meant the intermediate results of the steps in the query evaluation plan. These are usually not seen by the user.

• Jan Hidders