code: func (b *builtinExpSig) evalReal(row chunk.Row) (float64, bool, error) {
  	val, isNull, err := b.args[0].EvalReal(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  	exp := math.Exp(val)
  	if math.IsInf(exp, 0) || math.IsNaN(exp) {
  		s := fmt.Sprintf("exp(%s)", strconv.FormatFloat(val, 'f', -1, 64))
  		return 0, false, types.ErrOverflow.GenWithStackByArgs("DOUBLE", s)
  	}
  	return exp, false, nil
  }
docs: // evalReal evals a builtinExpSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_exp