code: func GetGlobalSystemVar(s *SessionVars, key string) (string, error) {
  	key = strings.ToLower(key)
  	gVal, ok, err := GetScopeNoneSystemVar(key)
  	if err != nil || ok {
  		return gVal, err
  	}
  	gVal, err = s.GlobalVarsAccessor.GetGlobalSysVar(key)
  	if err != nil {
  		return "", err
  	}
  	return gVal, nil
  }
docs: // GetGlobalSystemVar gets a global system variable.