Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?
Date: 11 Mar 2003 21:21:27 +0100
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.
which is a simple straightforward rule in bag algebra. However, if you simulate that with sets the optimizer has to see that
SELECT f(x) AS fx, COUNT(*) AS cnt
FROM x IN
SELECT g(y) AS fy, COUNT(*) AS cnt
FROM y IN Y
GROUP BY fy
GROUP BY fx
is the same as
which is a lot less straightfoward.
- Jan Hidders