ClickHouse

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#include <Functions/IFunction.h>
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#include <Functions/FunctionFactory.h>
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#include <Functions/FunctionHelpers.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 ILLEGAL_COLUMN;
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}
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namespace
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{
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class FunctionGetSubcolumn : public IFunction
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{
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public:
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    static constexpr auto name = "getSubcolumn";
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    static FunctionPtr create(ContextPtr) { return std::make_shared<FunctionGetSubcolumn>(); }
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    String getName() const override { return name; }
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    size_t getNumberOfArguments() const override { return 2; }
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    bool isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo &) const override { return true; }
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    bool useDefaultImplementationForConstants() const override { return true; }
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    ColumnNumbers getArgumentsThatAreAlwaysConstant() const override { return {1}; }
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    DataTypePtr getReturnTypeImpl(const ColumnsWithTypeAndName & arguments) const override
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    {
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        auto subcolumn_name = getSubcolumnName(arguments);
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        return arguments[0].type->getSubcolumnType(subcolumn_name);
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    }
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    ColumnPtr executeImpl(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t /*input_rows_count*/) const override
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    {
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        auto subcolumn_name = getSubcolumnName(arguments);
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        return arguments[0].type->getSubcolumn(subcolumn_name, arguments[0].column);
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    }
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private:
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    static std::string_view getSubcolumnName(const ColumnsWithTypeAndName & arguments)
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    {
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        const auto * column = arguments[1].column.get();
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        if (!isString(arguments[1].type) || !column || !checkAndGetColumnConstStringOrFixedString(column))
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            throw Exception(ErrorCodes::ILLEGAL_COLUMN,
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                "The second argument of function {} should be a constant string with the name of a subcolumn", name);
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        return column->getDataAt(0).toView();
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    }
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};
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}
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REGISTER_FUNCTION(GetSubcolumn)
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{
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    factory.registerFunction<FunctionGetSubcolumn>(FunctionDocumentation{
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        .description=R"(
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Receives the expression or identifier and constant string with the name of subcolumn.
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Returns requested subcolumn extracted from the expression.
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)",
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        .examples{{"getSubcolumn", "SELECT getSubcolumn(array_col, 'size0'), getSubcolumn(tuple_col, 'elem_name')", ""}},
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        .categories{"OtherFunctions"}
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    });
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}
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}
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