ClickHouse

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#include <Functions/IFunction.h>
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#include <Functions/FunctionFactory.h>
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#include <DataTypes/DataTypeLowCardinality.h>
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#include <Columns/ColumnLowCardinality.h>
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#include <Common/typeid_cast.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_TYPE_OF_ARGUMENT;
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}
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namespace
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{
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class FunctionLowCardinalityKeys: public IFunction
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{
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public:
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    static constexpr auto name = "lowCardinalityKeys";
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    static FunctionPtr create(ContextPtr) { return std::make_shared<FunctionLowCardinalityKeys>(); }
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    String getName() const override { return name; }
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    size_t getNumberOfArguments() const override { return 1; }
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    bool useDefaultImplementationForNulls() const override { return false; }
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    bool useDefaultImplementationForConstants() const override { return true; }
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    bool useDefaultImplementationForLowCardinalityColumns() const override { return false; }
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    bool isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo & /*arguments*/) const override { return false; }
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    DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
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    {
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        const auto * type = typeid_cast<const DataTypeLowCardinality *>(arguments[0].get());
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        if (!type)
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            throw Exception(ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT,
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                            "First first argument of function lowCardinalityKeys must be ColumnLowCardinality, "
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                            "but got {}", arguments[0]->getName());
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        return type->getDictionaryType();
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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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        const auto & arg = arguments[0];
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        const auto * low_cardinality_column = typeid_cast<const ColumnLowCardinality *>(arg.column.get());
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        return low_cardinality_column->getDictionary().getNestedColumn()->cloneResized(arg.column->size());
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    }
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};
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}
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REGISTER_FUNCTION(LowCardinalityKeys)
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{
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    factory.registerFunction<FunctionLowCardinalityKeys>();
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}
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}
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