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

From: Bob Badour <bbadour_at_golden.net>
Date: Wed, 12 Mar 2003 00:44:17 -0500
Message-ID: <3gAba.2$uA.159890@mantis.golden.net>


"Jan Hidders" <jan.hidders_at_REMOVE.THIS.ua.ac.be> wrote in message news:3e6e53bf.0_at_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.

Why would I have to simulation them? Received on Tue Mar 11 2003 - 23:44:17 CST

Original text of this message

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