public static SemanticNodeProcessor getJoinReducerProc() {
      return new JoinReducerProc();
    }
If two reducer sink operators share the same partition/sort columns and order,
they can be merged. This should happen after map join optimization because map
join optimization will remove reduce sink operators.

This optimizer removes/replaces child-RS (not parent) which is safer way for DefaultGraphWalker.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/correlation/ReduceSinkDeDuplication.java