ClickHouse

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#include <Functions/FunctionsDecimalArithmetics.h>
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#include <Functions/FunctionFactory.h>
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namespace DB
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{
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namespace ErrorCodes
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{
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    extern const int DECIMAL_OVERFLOW;
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}
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namespace
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{
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struct MultiplyDecimalsImpl
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{
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    static constexpr auto name = "multiplyDecimal";
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    template <typename FirstType, typename SecondType>
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    static inline Decimal256
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    execute(FirstType a, SecondType b, UInt16 scale_a, UInt16 scale_b, UInt16 result_scale)
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    {
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        if (a.value == 0 || b.value == 0)
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            return Decimal256(0);
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        Int256 sign_a = a.value < 0 ? -1 : 1;
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        Int256 sign_b = b.value < 0 ? -1 : 1;
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        std::vector<UInt8> a_digits = DecimalOpHelpers::toDigits(a.value * sign_a);
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        std::vector<UInt8> b_digits = DecimalOpHelpers::toDigits(b.value * sign_b);
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        std::vector<UInt8> multiplied = DecimalOpHelpers::multiply(a_digits, b_digits);
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        UInt16 product_scale = scale_a + scale_b;
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        while (product_scale < result_scale)
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        {
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            multiplied.push_back(0);
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            ++product_scale;
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        }
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        while (product_scale > result_scale&& !multiplied.empty())
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        {
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            multiplied.pop_back();
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            --product_scale;
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        }
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        if (multiplied.empty())
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            return Decimal256(0);
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        if (multiplied.size() > DecimalUtils::max_precision<Decimal256>)
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            throw DB::Exception(ErrorCodes::DECIMAL_OVERFLOW, "Numeric overflow: result bigger that Decimal256");
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        return Decimal256(sign_a * sign_b * DecimalOpHelpers::fromDigits(multiplied));
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    }
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};
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}
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REGISTER_FUNCTION(MultiplyDecimals)
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{
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    factory.registerFunction<FunctionsDecimalArithmetics<MultiplyDecimalsImpl>>(FunctionDocumentation{
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        .description=R"(
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Performs multiplication on two decimals. Result value will be of type [Decimal256](../../sql-reference/data-types/decimal.md).
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Result scale can be explicitly specified by `result_scale` argument (const Integer in range `[0, 76]`). If not specified, the result scale is the max scale of given arguments.
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:::note
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These functions work significantly slower than usual `multiply`.
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In case you don't really need controlled precision and/or need fast computation, consider using [multiply](#multiply)
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:::
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**Syntax**
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```sql
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multiplyDecimal(a, b[, result_scale])
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```
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**Arguments**
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-   `a` — First value: [Decimal](../../sql-reference/data-types/decimal.md).
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-   `b` — Second value: [Decimal](../../sql-reference/data-types/decimal.md).
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-   `result_scale` — Scale of result: [Int/UInt](../../sql-reference/data-types/int-uint.md).
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**Returned value**
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-   The result of multiplication with given scale.
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Type: [Decimal256](../../sql-reference/data-types/decimal.md).
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**Example**
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```text
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┌─multiplyDecimal(toDecimal256(-12, 0), toDecimal32(-2.1, 1), 1)─┐
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│                                                           25.2 │
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└────────────────────────────────────────────────────────────────┘
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```
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**Difference from regular multiplication:**
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```sql
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SELECT toDecimal64(-12.647, 3) * toDecimal32(2.1239, 4);
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SELECT toDecimal64(-12.647, 3) as a, toDecimal32(2.1239, 4) as b, multiplyDecimal(a, b);
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```
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```text
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┌─multiply(toDecimal64(-12.647, 3), toDecimal32(2.1239, 4))─┐
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│                                               -26.8609633 │
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└───────────────────────────────────────────────────────────┘
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┌─multiplyDecimal(toDecimal64(-12.647, 3), toDecimal32(2.1239, 4))─┐
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│                                                         -26.8609 │
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└──────────────────────────────────────────────────────────────────┘
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```
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```sql
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SELECT
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    toDecimal64(-12.647987876, 9) AS a,
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    toDecimal64(123.967645643, 9) AS b,
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    multiplyDecimal(a, b);
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SELECT
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    toDecimal64(-12.647987876, 9) AS a,
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    toDecimal64(123.967645643, 9) AS b,
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    a * b;
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```
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```text
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┌─────────────a─┬─────────────b─┬─multiplyDecimal(toDecimal64(-12.647987876, 9), toDecimal64(123.967645643, 9))─┐
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│ -12.647987876 │ 123.967645643 │                                                               -1567.941279108 │
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└───────────────┴───────────────┴───────────────────────────────────────────────────────────────────────────────┘
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Received exception from server (version 22.11.1):
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Code: 407. DB::Exception: Received from localhost:9000. DB::Exception: Decimal math overflow: While processing toDecimal64(-12.647987876, 9) AS a, toDecimal64(123.967645643, 9) AS b, a * b. (DECIMAL_OVERFLOW)
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```
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)"});
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}
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}
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