code: func (b *builtinRoundWithFracRealSig) 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
  	}
  	frac, isNull, err := b.args[1].EvalInt(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  	return types.Round(val, int(frac)), false, nil
  }
docs: // evalReal evals ROUND(value, frac).
  // See https://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_round