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