@Override
  public String vectorExpressionParameters() {
    if (field == -1) {
      return "col " + inputColumnNum[0];
    } else {
      return "col " + inputColumnNum[0] + ", field " + DateUtils.getFieldName(field);
    }
  }
Abstract class to return various fields from a Timestamp or Date.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/VectorUDFTimestampFieldDate.java