public void timestampUpdate(Timestamp timestamp, int elementNum) {
    timestamp.setTime(time[elementNum]);
    timestamp.setNanos(nanos[elementNum]);
  }
Set a Timestamp object from a row of the column.
We assume the entry has already been NULL checked and isRepeated adjusted.
@param timestamp
@param elementNum
storage-api/src/java/org/apache/hadoop/hive/ql/exec/vector/TimestampColumnVector.java