如何使用mysql中的order by和limit来优化查询

Question

explain extended 
SELECT 
      a.user_name AS userName, 
      crd.mdn_state_id AS mdnStateId, 
      a.alternate_number AS alternateNumber, 
      crd.temp_number AS tempMDN, 
      crd.esia_pin AS esiaPin,
      (crd.facevalue - crd.used)/10000 AS remainingBalance,
      a.email AS email, 
      a.created_datetime AS registrationDate, 
      hc.city_name AS homeCity, 
      cc.city_name AS currentCity, 
      crd.roaming_type AS roamingType, 
      m.expiry_datetime AS mdnExpiry, 
      a.first_name AS fullName 
   FROM 
      application_users a 
         LEFT JOIN cards crd 
            on a.id = crd.application_users_id 
            LEFT JOIN city hc 
               on crd.home_city_id = hc.id 
            LEFT JOIN city cc 
               on crd.current_city_id = cc.id 
            LEFT JOIN mdn m 
               ON crd.fullnumber = m.fullnumber 
   ORDER BY 
      a.user_name 
   LIMIT 
      22000,1000;

在解释此查询任何一个帮助时，我正面临额外列中的filesort？