在約束查詢慢？

Question

我有這個疑問這需要時間。下面是查詢 -

SELECT 
    m1.mes_id, 
    m1.body, 
    m1.subject, 
    m1.frm_id AS mem_id, 
    m1.date 
FROM 
    messages_system_new m1 
WHERE 
    m1.mes_id IN 
     (SELECT 
      MAX(mn1.mes_id) AS mes_id 
     FROM messages_system_new mn1 
     WHERE 
      mn1.mem_id = '401' 
      AND mn1.frm_id != '401' 
      AND mn1.trashed_user NOT LIKE '%401%' 
      AND mn1.type = 'message' 
      AND mn1.folder = 'inbox' AND mn1.is_spam='N' 
     GROUP BY mn1.frm_id 
     ) 

發射時解釋它給

id select_type table type possible_keys key key_len ref rows Extra 
1 PRIMARY m1 ALL (NULL) (NULL) (NULL) (NULL) 6091 Using where 
2 DEPENDENT SUBQUERY mn1 ref mem_id,frm_id mem_id 8 const 59 Using where; Using temporary; Using filesort 

這是什麼意思，我怎樣可以提高查詢？

Answer 1

可能嘗試使用JOIN來代替。像這樣的東西

SELECT 
    m1.mes_id, 
    m1.body, 
    m1.subject, 
    m1.frm_id AS mem_id, 
    m1.date 
FROM messages_system_new m1 
INNER JOIN (SELECT mn1.frm_id MAX(mn1.mes_id) AS mes_id 
     FROM messages_system_new mn1 
     WHERE 
      mn1.mem_id = '401' 
      AND mn1.frm_id != '401' 
      AND mn1.trashed_user NOT LIKE '%401%' 
      AND mn1.type = 'message' 
      AND mn1.folder = 'inbox' AND mn1.is_spam='N' 
     GROUP BY mn1.frm_id 
     ) Sub1 
ON m1.mes_id = Sub1.mes_id 

messafes_system_new上的索引是什麼？