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
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{
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class FunctionToLowCardinality: public IFunction
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{
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public:
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    static constexpr auto name = "toLowCardinality";
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    static FunctionPtr create(ContextPtr) { return std::make_shared<FunctionToLowCardinality>(); }
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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 true; }
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    DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
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    {
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        if (arguments[0]->lowCardinality())
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            return arguments[0];
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        return std::make_shared<DataTypeLowCardinality>(arguments[0]);
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    }
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    ColumnPtr executeImpl(const ColumnsWithTypeAndName & arguments, const DataTypePtr & res_type, size_t /*input_rows_count*/) const override
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    {
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        const auto & arg = arguments[0];
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        if (arg.type->lowCardinality())
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            return arg.column;
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        else
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        {
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            auto column = res_type->createColumn();
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            typeid_cast<ColumnLowCardinality &>(*column).insertRangeFromFullColumn(*arg.column, 0, arg.column->size());
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            return column;
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        }
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    }
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};
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}
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REGISTER_FUNCTION(ToLowCardinality)
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{
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    factory.registerFunction<FunctionToLowCardinality>();
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}
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}
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