@Explain(vectorization = Vectorization.OPERATOR, displayName = "nativeConditionsMet",
        explainLevels = { Level.DEFAULT, Level.EXTENDED })
    public List<String> getNativeConditionsMet() {
      if (nativeConditions == null) {
        nativeConditions = createNativeConditions();
      }
      return VectorizationCondition.getConditionsMet(nativeConditions);
    }
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