@Explain(vectorization = Vectorization.DETAIL, displayName = "bucketColumns",
        explainLevels = { Level.DEFAULT, Level.EXTENDED })
    public List<String> getBucketColumns() {
      if (!isNative) {
        return null;
      }
      int[] bucketColumnMap = vectorReduceSinkInfo.getReduceSinkBucketColumnMap();
      if (bucketColumnMap == null || bucketColumnMap.length == 0) {
        // Suppress empty column map.
        return null;
      }
      return outputColumnsAndTypesToStringList(
          vectorReduceSinkInfo.getReduceSinkBucketColumnMap(),
          vectorReduceSinkInfo.getReduceSinkBucketTypeInfos());
    }
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