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Re: Why not reinvent the wheel?
- From: Vasileios Papadimos <firstname.lastname@example.org>
- To: Joe English <email@example.com>
- Date: Fri, 02 Mar 2001 18:27:13 -0800
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.