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