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我正在尝试优化我使用 MySQL 编写的最长查询，EXPLAIN但由于这是我的第一个查询，我似乎无法理解结果。这是查询和运行EXPLAIN命令的结果：

EXPLAIN SELECT pb.name, s1.MessageFrom, s1.MessageText, s1.SendTime, s1.is_unread, s1.Id, s1.autoreply_sent FROM sol_inbound s1
    JOIN sol_contactnum c ON s1.MessageFrom = c.number
    JOIN sol_phonebk_contactnum USING (contactnum_id)
    JOIN sol_phonebk pb USING (phonebk_id)
    JOIN sol_message_folder mf ON s1.Id = mf.message_id
    WHERE (MessageFrom, SendTime) IN (SELECT MessageFrom, MAX(SendTime) FROM sol_inbound inb
        JOIN sol_message_folder mf WHERE inb.Id = mf.message_id
        AND mf.folder_id=1 AND mf.direction='inbound' AND mf.user_id=1
        GROUP BY MessageFrom)
    AND mf.folder_id=1 AND mf.direction='inbound' AND mf.user_id=1
    UNION
    SELECT NULL `name`, s1.MessageFrom, s1.MessageText, s1.SendTime, s1.is_unread, s1.Id, s1.autoreply_sent FROM sol_inbound s1
    LEFT JOIN sol_contactnum c ON s1.MessageFrom = c.number
    JOIN sol_message_folder mf ON s1.Id = mf.message_id
    WHERE c.number IS NULL
    AND mf.folder_id=1 AND mf.direction='inbound' AND mf.user_id=1
    AND (MessageFrom, SendTime) IN (SELECT MessageFrom, MAX(SendTime) FROM sol_inbound inb
        JOIN sol_message_folder mf WHERE inb.Id = mf.message_id
        AND mf.folder_id=1 AND mf.direction='inbound' AND mf.user_id=1
        GROUP BY MessageFrom)
    ORDER BY SendTime DESC LIMIT 100

结果EXPLAIN：

 id  select_type         table                   type    possible_keys                                                  key               key_len  ref                                                     rows  Extra                   
------  ------------------  ----------------------  ------  -------------------------------------------------------------  ----------------  -------  ----------------------------------------------------  ------  ------------------------
     1  PRIMARY             pb                      ALL     PRIMARY                                                        (NULL)            (NULL)   (NULL)                                                   303                          
     1  PRIMARY             sol_phonebk_contactnum  ref     PRIMARY,phonebk_id1_idx,contactnum_id1_idx,phonebk_contactnum  PRIMARY           4        googlep1_solane.pb.phonebk_id                              1  Using index             
     1  PRIMARY             c                       eq_ref  PRIMARY,number_idx                                             PRIMARY           4        googlep1_solane.sol_phonebk_contactnum.contactnum_id       1                          
     1  PRIMARY             s1                      ref     PRIMARY,message_from_idx                                       message_from_idx  243      googlep1_solane.c.number                                   1  Using where             
     1  PRIMARY             mf                      eq_ref  PRIMARY                                                        PRIMARY           22       const,googlep1_solane.s1.Id,const,const                    1  Using where; Using index
     2  DEPENDENT SUBQUERY  inb                     index   PRIMARY                                                        message_from_idx  243      (NULL)                                                     1                          
     2  DEPENDENT SUBQUERY  mf                      eq_ref  PRIMARY                                                        PRIMARY           22       const,googlep1_solane.inb.Id,const,const                   1  Using where; Using index
     3  UNION               s1                      ALL     PRIMARY                                                        (NULL)            (NULL)   (NULL)                                                   877  Using where             
     3  UNION               c                       ref     number_idx                                                     number_idx        243      googlep1_solane.s1.MessageFrom                             1  Using where; Using index
     3  UNION               mf                      eq_ref  PRIMARY                                                        PRIMARY           22       const,googlep1_solane.s1.Id,const,const                    1  Using where; Using index
     4  DEPENDENT SUBQUERY  inb                     index   PRIMARY                                                        message_from_idx  243      (NULL)                                                     1                          
     4  DEPENDENT SUBQUERY  mf                      eq_ref  PRIMARY                                                        PRIMARY           22       const,googlep1_solane.inb.Id,const,const                   1  Using where; Using index
(NULL)  UNION RESULT        <union1,3>              ALL     (NULL)                                                         (NULL)            (NULL)   (NULL)                                                (NULL)  Using filesort

查询中间的UNION将那些号码出现在电话簿中的人与那些没有出现在电话簿中的人连接起来（因此是LEFT JOIN）。

编辑：

此查询的作用是获取每个号码的最新入站消息并将其返回。我可以使用GROUP BY，因为它返回最旧的消息......我需要最新的。然后它将电话簿中不存在的那些号码加入其中，这就是我检查的原因WHERE c.number IS NULL.

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2 回答 2

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在可能的情况下，使用不相关的子查询，如下所示...

FROM... x
JOIN 
   ( SELECT MessageFrom
          , MAX(SendTime) max_sendtime
       FROM sol_inbound inb
       JOIN sol_message_folder mf 
         ON inb.Id = mf.message_id
      WHERE mf.folder_id=1 
        AND mf.direction='inbound' 
        AND mf.user_id=1
      GROUP 
         BY MessageFrom 
   ) y
  ON y.messagefrom = x.messagefrom
 AND y.max_sendtime = x.sendtime
于 2013-05-06T16:15:27.037 回答
0

看来您已经使用 2sub-query's来过滤结果，这些并不是真正需要的。我假设您需要显示latest message来自每个MessageFromid 的。

试试这个以获得更快的结果

SELECT 
  *
FROM 
  ( SELECT 
    pb.name, 
    s1.MessageFrom, 
    s1.MessageText, 
    s1.SendTime, 
    s1.is_unread, 
    s1.Id, 
    s1.autoreply_sent,
    @row_num := IF(@prev_value=s1.MessageFrom,@row_num+1,1) AS row_num,
    @prev_value := s1.MessageFrom
  FROM 
    sol_inbound s1
    JOIN sol_contactnum c ON s1.MessageFrom = c.number
    JOIN sol_phonebk_contactnum USING (contactnum_id)
    JOIN sol_phonebk pb USING (phonebk_id)
    JOIN sol_message_folder mf ON s1.Id = mf.message_id
  WHERE 
      mf.folder_id=1 
      AND mf.direction='inbound' 
      AND mf.user_id=1
  ORDER BY
    s1.MessageFrom,
    s1.sendTime desc ) temp WHERE temp.row_num = 1

UNION

SELECT 
  *
FROM 
  (     
    SELECT 
      NULL `name`, 
      s1.MessageFrom, 
      s1.MessageText, 
      s1.SendTime, 
      s1.is_unread, 
      s1.Id, 
      s1.autoreply_sent,
      @row_num := IF(@prev_value=s1.MessageFrom,@row_num+1,1) AS row_num,
      @prev_value := s1.MessageFrom 
    FROM 
      sol_inbound s1
      LEFT JOIN sol_contactnum c ON s1.MessageFrom = c.number
      JOIN sol_message_folder mf ON s1.Id = mf.message_id
    WHERE 
        c.number IS NULL
        AND mf.folder_id=1 
        AND mf.direction='inbound' 
        AND mf.user_id=1
    ORDER BY
      s1.MessageFrom,
      s1.sendTime desc      
        ) temp2 WHERE temp2.row_num = 1

ORDER BY 
  SendTime DESC 
LIMIT 100

我没有使用 asub-query来过滤结果，而是session vars在 Mysql 中使用来对 a 的所有消息进行排名MessageFrom。有的latest sendtime有rankas1

于 2013-05-06T17:06:17.367 回答