code: func (b *builtinAddDurationAndStringSig) evalDuration(row chunk.Row) (types.Duration, bool, error) {
  	arg0, isNull, err := b.args[0].EvalDuration(b.ctx, row)
  	if isNull || err != nil {
  		return types.ZeroDuration, isNull, err
  	}
  	s, isNull, err := b.args[1].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return types.ZeroDuration, isNull, err
  	}
  	if !isDuration(s) {
  		return types.ZeroDuration, true, nil
  	}
  	arg1, err := types.ParseDuration(b.ctx.GetSessionVars().StmtCtx, s, types.GetFsp(s))
  	if err != nil {
  		return types.ZeroDuration, true, err
  	}
  	result, err := arg0.Add(arg1)
  	if err != nil {
  		return types.ZeroDuration, true, err
  	}
  	return result, false, nil
  }
docs: // evalDuration evals a builtinAddDurationAndStringSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_addtime