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Re: Why not reinvent the wheel?

On Fri, Mar 02, 2001 at 01:02:48PM -0800, Joe English wrote:

> Ordered and unordered joins are both associative, in the sense that:
>     [(x1,x2,x3) | x1 <- e1, (x2,x3) <- [(x2,x3) | x2 <- e2, x3 <- e3]]
> ==
>     [(x1,x2,x3) | (x1,x2) <- [(x1,x2) | x1 <- e1, x2 <- e2], x3 <- e3]
> Unordered joins however are commutative:

Of course Joe is right! Commutativity is what we gain from "unordering" 
(disordering!?) joins. Commutativity + associativity makes for the 
exponential search space in join optimization. 


Vassilis Papadimos.