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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#include <DataTypes/DataTypeAggregateFunction.h>
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#include <Columns/ColumnAggregateFunction.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_COLUMN;
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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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/** finalizeAggregation(agg_state) - get the result from the aggregation state.
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  * Takes state of aggregate function. Returns result of aggregation (finalized state).
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  */
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class FunctionFinalizeAggregation : public IFunction
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{
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public:
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    static constexpr auto name = "finalizeAggregation";
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    static FunctionPtr create(ContextPtr)
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    {
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        return std::make_shared<FunctionFinalizeAggregation>();
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    }
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    String getName() const override
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    {
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        return name;
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    }
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    size_t getNumberOfArguments() const override
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    {
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        return 1;
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    }
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    bool isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo & /*arguments*/) const override { return true; }
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    bool useDefaultImplementationForConstants() const override { return true; }
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    DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
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    {
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        const DataTypeAggregateFunction * type = checkAndGetDataType<DataTypeAggregateFunction>(arguments[0].get());
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        if (!type)
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        {
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            throw Exception(ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT,
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                "Argument for function '{}' must have type AggregateFunction - state of aggregate function."
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                " Got '{}' instead", getName(), arguments[0]->getName());
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        }
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        return type->getReturnType();
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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 column = arguments.at(0).column;
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        if (!typeid_cast<const ColumnAggregateFunction *>(column.get()))
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            throw Exception(ErrorCodes::ILLEGAL_COLUMN, "Illegal column {} of first argument of function {}",
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                    arguments.at(0).column->getName(), getName());
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        /// Column is copied here, because there is no guarantee that we own it.
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        auto mut_column = IColumn::mutate(std::move(column));
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        return ColumnAggregateFunction::convertToValues(std::move(mut_column));
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    }
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};
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}
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REGISTER_FUNCTION(FinalizeAggregation)
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{
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    factory.registerFunction<FunctionFinalizeAggregation>();
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}
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}
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