code: func (b *builtinTimeToSecSig) evalInt(row chunk.Row) (int64, bool, error) {
  	duration, isNull, err := b.args[0].EvalDuration(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  	var sign int
  	if duration.Duration >= 0 {
  		sign = 1
  	} else {
  		sign = -1
  	}
  	return int64(sign * (duration.Hour()*3600 + duration.Minute()*60 + duration.Second())), false, nil
  }
docs: // evalInt evals TIME_TO_SEC(time).
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_time-to-sec