[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] 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. Regards, Vassilis Papadimos.
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