text: #include <Functions/IFunction.h>
  #include <Functions/FunctionFactory.h>
  #include <Functions/FunctionHelpers.h>
  #include <DataTypes/DataTypesNumber.h>
  #include <DataTypes/DataTypeString.h>
  #include <Columns/ColumnString.h>
  #include <Columns/ColumnConst.h>
  #include <Core/Field.h>
  
  #include <unordered_map>
  #include <Poco/String.h>
  
  
  namespace DB
  {
  namespace ErrorCodes
  {
      extern const int BAD_ARGUMENTS;
  }
  
  namespace
  {
  
  /** globalVariable('name') - takes constant string argument and returns the value of global variable with that name.
    * It is intended for compatibility with MySQL.
    *
    * Currently it's a stub, no variables are implemented. Feel free to add more variables.
    */
  class FunctionGlobalVariable : public IFunction
  {
  public:
      static constexpr auto name = "globalVariable";
      static FunctionPtr create(ContextConstPtr)
      {
          return std::make_shared<FunctionGlobalVariable>();
      }
  
      String getName() const override
      {
          return name;
      }
  
      size_t getNumberOfArguments() const override
      {
          return 1;
      }
  
      DataTypePtr getReturnTypeImpl(const ColumnsWithTypeAndName & arguments) const override
      {
          if (!checkColumnConst<ColumnString>(arguments[0].column.get()))
              throw Exception("Argument of function " + getName() + " must be constant string", ErrorCodes::BAD_ARGUMENTS);
  
          String variable_name = assert_cast<const ColumnConst &>(*arguments[0].column).getValue<String>();
          auto variable = global_variable_map.find(Poco::toLower(variable_name));
          if (variable == global_variable_map.end())
              return std::make_shared<DataTypeInt32>();
          else
              return variable->second.type;
      }
  
      ColumnPtr executeImpl(const ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, size_t input_rows_count) const override
      {
          const ColumnWithTypeAndName & col = arguments[0];
          String variable_name = assert_cast<const ColumnConst &>(*col.column).getValue<String>();
          auto variable = global_variable_map.find(Poco::toLower(variable_name));
  
          Field val = 0;
          if (variable != global_variable_map.end())
              val = variable->second.value;
  
          return result_type->createColumnConst(input_rows_count, val);
      }
  
  private:
      struct TypeAndValue
      {
          DataTypePtr type;
          Field value;
      };
      std::unordered_map<String, TypeAndValue> global_variable_map
          = {{"max_allowed_packet", {std::make_shared<DataTypeInt32>(), 67108864}},
             {"version", {std::make_shared<DataTypeString>(), "5.7.30"}},
             {"version_comment", {std::make_shared<DataTypeString>(), ""}},
             {"transaction_isolation", {std::make_shared<DataTypeString>(), "READ-UNCOMMITTED"}}};
  };
  
  }
  
  void registerFunctionGlobalVariable(FunctionFactory & factory)
  {
      factory.registerFunction<FunctionGlobalVariable>();
  }
  
  }
  
  