code: func (s *session) GetGlobalSysVar(name string) (string, error) {
  	if s.Value(sessionctx.Initing) != nil {
  		// When running bootstrap or upgrade, we should not access global storage.
  		return "", nil
  	}
  	sql := fmt.Sprintf(`SELECT VARIABLE_VALUE FROM %s.%s WHERE VARIABLE_NAME="%s";`,
  		mysql.SystemDB, mysql.GlobalVariablesTable, name)
  	sysVar, err := s.getExecRet(s, sql)
  	if err != nil {
  		if executor.ErrResultIsEmpty.Equal(err) {
  			if sv, ok := variable.SysVars[name]; ok {
  				return sv.Value, nil
  			}
  			return "", variable.UnknownSystemVar.GenWithStackByArgs(name)
  		}
  		return "", err
  	}
  	return sysVar, nil
  }
docs: // GetGlobalSysVar implements GlobalVarAccessor.GetGlobalSysVar interface.