G select * from t1 union select * from t2; (before rewrites) cluster0 Box0:Select cluster5 Box5:Union cluster3 Box3:Select cluster4 Box4:BaseTable T2 cluster1 Box1:Select cluster2 Box2:BaseTable T1 boxhead0 Distinct: Preserve 0: Q4.c0 as F1 1: Q4.c1 as F2 2: Q4.c2 as F3 Q4 Q4(F) boxhead5 Distinct: Enforce 0: Q2.c0 as F1 1: Q2.c1 as F2 2: Q2.c2 as F3 Q4->boxhead5 Q2 Q2(F) boxhead1 Distinct: Preserve 0: Q0.c0 as F1 1: Q0.c1 as F2 2: Q0.c2 as F3 Q2->boxhead1 Q3 Q3(F) boxhead3 Distinct: Preserve 0: Q1.c0 as F1 1: Q1.c1 as F2 2: Q1.c2 as F3 Q3->boxhead3 Q1 Q1(F) as T2 boxhead4 Distinct: Preserve 0: c0 as F1 1: c1 as F2 2: c2 as F3 Q1->boxhead4 Q0 Q0(F) as T1 boxhead2 Distinct: Preserve 0: c0 as F1 1: c1 as F2 2: c2 as F3 Q0->boxhead2