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/DataTypesNumber.h>
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#include <DataTypes/DataTypeString.h>
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#include <Columns/ColumnString.h>
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#include <Columns/ColumnConst.h>
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#include <Core/Field.h>
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#include <unordered_map>
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#include <Poco/String.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 BAD_ARGUMENTS;
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}
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namespace
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{
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/** globalVariable('name') - takes constant string argument and returns the value of global variable with that name.
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  * It is intended for compatibility with MySQL.
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  *
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  * Currently it's a stub, no variables are implemented. Feel free to add more variables.
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  */
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class FunctionGlobalVariable : public IFunction
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{
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public:
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    static constexpr auto name = "globalVariable";
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    static FunctionPtr create(ContextPtr)
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    {
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        return std::make_shared<FunctionGlobalVariable>();
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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 false; }
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    DataTypePtr getReturnTypeImpl(const ColumnsWithTypeAndName & arguments) const override
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    {
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        if (!checkColumnConst<ColumnString>(arguments[0].column.get()))
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            throw Exception(ErrorCodes::BAD_ARGUMENTS, "Argument of function {} must be constant string", getName());
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        String variable_name = assert_cast<const ColumnConst &>(*arguments[0].column).getValue<String>();
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        auto variable = global_variable_map.find(Poco::toLower(variable_name));
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        if (variable == global_variable_map.end())
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            return std::make_shared<DataTypeInt32>();
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        else
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            return variable->second.type;
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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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        const ColumnWithTypeAndName & col = arguments[0];
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        String variable_name = assert_cast<const ColumnConst &>(*col.column).getValue<String>();
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        auto variable = global_variable_map.find(Poco::toLower(variable_name));
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        Field val = 0;
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        if (variable != global_variable_map.end())
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            val = variable->second.value;
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        return result_type->createColumnConst(input_rows_count, val);
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    }
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private:
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    struct TypeAndValue
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    {
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        DataTypePtr type;
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        Field value;
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    };
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    std::unordered_map<String, TypeAndValue> global_variable_map
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        = {{"max_allowed_packet", {std::make_shared<DataTypeInt32>(), 67108864}},
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           {"version", {std::make_shared<DataTypeString>(), "5.7.30"}},
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           {"version_comment", {std::make_shared<DataTypeString>(), ""}},
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           {"transaction_isolation", {std::make_shared<DataTypeString>(), "READ-UNCOMMITTED"}}};
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};
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}
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REGISTER_FUNCTION(GlobalVariable)
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{
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    factory.registerFunction<FunctionGlobalVariable>();
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}
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}
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