Re: How can I proove associtivity of natural in relational algebra?

In article <1132608953.263060.39140_at_g44g2000cwa.googlegroups.com>, mikharakiri_nospaum_at_yahoo.com says...
> "Extend", is the opposite of "project". E.g. the relation with a single
> attribute
>
> A = {(a=1), (a=2)} is exended to the attribute set {a,b} with the
> domain of attribute b being {7,8,9} as
>
> A' = {(a=1, b=7),(a=1, b=8),(a=1, b=9),(a=2, b=7),(a=2, b=8),(a=2,
> b=9))}

This looks like a cartesian product to me. Why do you call it "extend"?

-- 
Jon