code: func (s *session) GetAllSysVars() (map[string]string, error) {
  	if s.Value(sessionctx.Initing) != nil {
  		return nil, nil
  	}
  	sql := `SELECT VARIABLE_NAME, VARIABLE_VALUE FROM %s.%s;`
  	sql = fmt.Sprintf(sql, mysql.SystemDB, mysql.GlobalVariablesTable)
  	rows, _, err := s.ExecRestrictedSQL(s, sql)
  	if err != nil {
  		return nil, err
  	}
  	ret := make(map[string]string)
  	for _, r := range rows {
  		k, v := r.GetString(0), r.GetString(1)
  		ret[k] = v
  	}
  	return ret, nil
  }
docs: // GetAllSysVars implements GlobalVarAccessor.GetAllSysVars interface.