ClickHouse

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#include <Functions/IFunction.h>
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#include <Functions/FunctionHelpers.h>
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#include <Functions/FunctionFactory.h>
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#include <DataTypes/DataTypesNumber.h>
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#include <DataTypes/DataTypeNullable.h>
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#include <DataTypes/getLeastSupertype.h>
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#include <Core/ColumnNumbers.h>
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#include <Columns/ColumnNullable.h>
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namespace DB
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{
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namespace
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{
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/// Implements the function ifNull which takes 2 arguments and returns
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/// the value of the 1st argument if it is not null. Otherwise it returns
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/// the value of the 2nd argument.
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class FunctionIfNull : public IFunction
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{
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public:
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    static constexpr auto name = "ifNull";
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    explicit FunctionIfNull(ContextPtr context_) : context(context_) {}
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    static FunctionPtr create(ContextPtr context)
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    {
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        return std::make_shared<FunctionIfNull>(context);
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    }
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    std::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 { return 2; }
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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 isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo & /*arguments*/) const override { return false; }
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    ColumnNumbers getArgumentsThatDontImplyNullableReturnType(size_t /*number_of_arguments*/) const override { return {0}; }
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    DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
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    {
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        if (arguments[0]->onlyNull())
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            return arguments[1];
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        if (!arguments[0]->isNullable())
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            return arguments[0];
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        return getLeastSupertype(DataTypes{removeNullable(arguments[0]), arguments[1]});
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    }
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    ColumnPtr executeImpl(const ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, size_t input_rows_count) const override
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    {
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        /// Always null.
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        if (arguments[0].type->onlyNull())
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            return arguments[1].column;
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        /// Could not contain nulls, so nullIf makes no sense.
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        if (!arguments[0].type->isNullable())
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            return arguments[0].column;
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        /// ifNull(col1, col2) == if(isNotNull(col1), assumeNotNull(col1), col2)
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        ColumnsWithTypeAndName columns{arguments[0]};
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        auto is_not_null = FunctionFactory::instance().get("isNotNull", context)->build(columns);
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        auto is_not_null_type = std::make_shared<DataTypeUInt8>();
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        auto is_not_null_res = is_not_null->execute(columns, is_not_null_type, input_rows_count);
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        auto assume_not_null = FunctionFactory::instance().get("assumeNotNull", context)->build(columns);
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        auto assume_not_null_type = removeNullable(arguments[0].type);
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        auto assume_nut_null_res = assume_not_null->execute(columns, assume_not_null_type, input_rows_count);
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        ColumnsWithTypeAndName if_columns
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        {
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                {is_not_null_res, is_not_null_type, ""},
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                {assume_nut_null_res, assume_not_null_type, ""},
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                arguments[1],
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        };
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        auto func_if = FunctionFactory::instance().get("if", context)->build(if_columns);
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        return func_if->execute(if_columns, result_type, input_rows_count);
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    }
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private:
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    ContextPtr context;
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};
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}
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REGISTER_FUNCTION(IfNull)
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{
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    factory.registerFunction<FunctionIfNull>({}, FunctionFactory::CaseInsensitive);
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}
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}
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