public void add(int from, int to) {
      if (vector[from] == null) {
        vector[from] = new HashSet<Integer>();
      }
      vector[from].add(to);
    }
creates synthetic predicates that represent "IN (keylist other table)"
ql/src/java/org/apache/hadoop/hive/ql/ppd/SyntheticJoinPredicate.java