@Explain(vectorization = Vectorization.OPERATOR, displayName = "Reduce Sink Vectorization",
      explainLevels = { Level.DEFAULT, Level.EXTENDED })
  public ReduceSinkOperatorExplainVectorization getReduceSinkVectorization() {
    VectorReduceSinkDesc vectorReduceSinkDesc = (VectorReduceSinkDesc) getVectorDesc();
    if (vectorReduceSinkDesc == null) {
      return null;
    }
    return new ReduceSinkOperatorExplainVectorization(this, vectorReduceSinkDesc);
  }
Returns the null order in the key columns.

@return null, which means default for all key columns, or a String
        of the same length as key columns, that consists of only "a"
        (null first) and "z" (null last).
ql/src/java/org/apache/hadoop/hive/ql/plan/ReduceSinkDesc.java