@Override
    public ImmutableList<RelCollation> getCollation(HiveJoin join) {
      return HiveAlgorithmsUtil.getJoinCollation(join.getJoinPredicateInfo(),
              MapJoinStreamingRelation.NONE);
    }
COMMON_JOIN is Sort Merge Join. Each parallel computation handles multiple
splits.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/cost/HiveOnTezCostModel.java