@Override
    public int compare(VectorHashKeyWrapperBase o1, VectorHashKeyWrapperBase o2) {
      boolean isNull1 = o1.isNull(keyIndex);
      boolean isNull2 = o2.isNull(keyIndex);

      if (isNull1 && isNull2) {
        return 0;
      }
      if (isNull1) {
        return -nullResult;
      }
      if (isNull2) {
        return nullResult;
      }
      return comparator.compare(o1, o2);
    }
Compare {@link VectorHashKeyWrapperBase} instances only by one column.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/wrapper/VectorHashKeyWrapperGeneralComparator.java