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

From: Jan Hidders <jan.hidders_at_REMOVE.THIS.ua.ac.be>
Date: 11 Mar 2003 22:23:11 +0100
Message-ID: <3e6e53bf.0@news.ruca.ua.ac.be>


Bob Badour wrote:
>"Jan Hidders" <jan.hidders_at_REMOVE.THIS.ua.ac.be> wrote in message
>news:3e6e4547.0_at_news.ruca.ua.ac.be...
>> Bob Badour wrote:
>> >"Jan Hidders" <jan.hidders_at_REMOVE.THIS.ua.ac.be> wrote in message
>> >news:3e6da66b.0_at_news.ruca.ua.ac.be...
>> >>
>> >> I'm not convinced that bags are never needed. [...] And if you are then
>> >> going to simulate them with sets then the cost might become negative
>> >> because the set-based optimizer might miss certain optimizations that
>> >> would have been easier to spot for a bag-based optimizer.
>> >
>> >Such as?
>>
>> Such as combining two iterations over the same bag into one. It's pretty
>> easy to see that
>>
>> SELECT f(x)
>> FROM x IN
>> SELECT g(y)
>> FROM y IN Y
>>
>> is the same as
>>
>> SELECT f(g(y))
>> FROM y in Y
>
>If we don't have bags, it is not an issue. Is it?

Yes, it is. Because then you have to simulate them and what the example showed is that in that case the same optimization becomes very hard to spot.

Received on Tue Mar 11 2003 - 15:23:11 CST

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