code: func GetSessionSystemVar(s *SessionVars, key string) (string, error) {
  	key = strings.ToLower(key)
  	gVal, ok, err := GetSessionOnlySysVars(s, key)
  	if err != nil || ok {
  		return gVal, err
  	}
  	gVal, err = s.GlobalVarsAccessor.GetGlobalSysVar(key)
  	if err != nil {
  		return "", err
  	}
  	s.systems[key] = gVal
  	return gVal, nil
  }
docs: // GetSessionSystemVar gets a system variable.
  // If it is a session only variable, use the default value defined in code.
  // Returns error if there is no such variable.