code: func loadSystemTZ(se *session) (string, error) {
  	sql := `select variable_value from mysql.tidb where variable_name = 'system_tz'`
  	rss, errLoad := se.Execute(context.Background(), sql)
  	if errLoad != nil {
  		return "", errLoad
  	}
  	// the record of mysql.tidb under where condition: variable_name = "system_tz" should shall only be one.
  	defer func() {
  		if err := rss[0].Close(); err != nil {
  			logutil.Logger(context.Background()).Error("close result set error", zap.Error(err))
  		}
  	}()
  	req := rss[0].NewRecordBatch()
  	if err := rss[0].Next(context.Background(), req); err != nil {
  		return "", err
  	}
  	return req.GetRow(0).GetString(0), nil
  }
docs: // loadSystemTZ loads systemTZ from mysql.tidb