I've been studying basic usage and I found wrong minus result on DecimalXX type.

**Example Query**
`
select toDecimal64(4,4) - toDecimal32(2,2)
`

**Expected output**
2.0000

**Actual output**
3.9800

Is there anything wrong or maybe my misunderstanding of toDecimal ?

![image](https://user-images.githubusercontent.com/8475371/107513855-cdd57a00-6be3-11eb-841d-d425a198d172.png)


**How to reproduce**
1. OS: macOS Catalina 10.15.5
2. server: clickhouse-server, built from source, version 21.3.1.1
3. client:  clickhouse-client,  built from source, version 21.3.1.1

function_name = String(is_plus ? "add" : "subtract") + interval_data_type->getKind().toString() + 's';
        }
        else
        {
            if (isDate(type_time))
                function_name = is_plus ? "addDays" : "subtractDays";
            else
                function_name = is_plus ? "addSeconds" : "subtractSeconds";
        }
        return FunctionFactory::instance().get(function_name, context);
    }
    static bool isAggregateMultiply(const DataTypePtr & type0, const DataTypePtr & type1)
    {
        if constexpr (!is_multiply)
            return false;
        WhichDataType which0(type0);
        WhichDataType which1(type1);
        return (which0.isAggregateFunction() && which1.isNativeUInt())
            || (which0.isNativeUInt() && which1.isAggregateFunction());
    }
    static bool isAggregateAddition(const DataTypePtr & type0, const DataTypePtr & type1)
    {
        if constexpr (!is_plus)
            return false;
        WhichDataType which0(type0);
        WhichDataType which1(type1);
        return which0.isAggregateFunction() && which1.isAggregateFunction();
    }
    
    ColumnPtr executeAggregateMultiply(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t input_rows_count) const