code: func (b *builtinHourSig) evalInt(row chunk.Row) (int64, bool, error) {
  	dur, isNull, err := b.args[0].EvalDuration(b.ctx, row)
  	// ignore error and return NULL
  	if isNull || err != nil {
  		return 0, true, nil
  	}
  	return int64(dur.Hour()), false, nil
  }
docs: // evalInt evals HOUR(time).
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_hour