21. The computing device of claim 20 , wherein the processing unit is further configured to select a first tuple from the second set of tuples, wherein the first tuple locally dominates a second tuple from the second set of tuples.
21. The computing device of claim 20 , wherein the processing unit is further configured to select a first tuple from the second set of tuples, wherein the first tuple locally dominates a second tuple from the second set of tuples. 26. The computing device of claim 21 , wherein the processing unit is further configured to identify a dominance relationship among the First tuple and other tuples in the second relation based on a first local attribute of the second relation.