. "constructing a sample set WC of an induced set ??C, wherein P is a join predicate between tables R and S and ??C={r.a1 ??? ra2:r ??? R} when the pairing rule P is a trivial rule ???R and ??C={r.a1 ??? s.a2:r ??? R, s ??? S, and (r,s) satisfies P};" . . .