public int compareTo(int elementNum, Timestamp timestamp) {
    return asScratchTimestamp(elementNum).compareTo(timestamp);
  }
Compare row to Timestamp.
We assume the entry has already been NULL checked and isRepeated adjusted.
@param elementNum
@param timestamp
@return -1, 0, 1 standard compareTo values.
storage-api/src/java/org/apache/hadoop/hive/ql/exec/vector/TimestampColumnVector.java