Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?
Date: Tue, 11 Mar 2003 15:45:47 -0500
"Jan Hidders" <jan.hidders_at_REMOVE.THIS.ua.ac.be> wrote in message
> Bob Badour wrote:
> >"Jan Hidders" <jan.hidders_at_REMOVE.THIS.ua.ac.be> wrote in message
> >> 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 Y is a relation, the same optimization is available and is just as easy to spot. Received on Tue Mar 11 2003 - 21:45:47 CET