慢查詢。錯誤的數據庫結構？

Question

我有一個包含任務的表的數據庫。任務有一個生命週期。任務生命週期的狀態可能會改變。這些狀態轉換存儲在一個單獨的表tasktransitions中。 現在我寫了一個查詢來查找所有打開/重新打開的任務和最近更改的任務，但我已經看到執行時間變得很長（> 0.5s）的任務數量很少（< 1000）。

任務

+-------------+---------+------+-----+---------+----------------+ 
| Field  | Type | Null | Key | Default | Extra   | 
+-------------+---------+------+-----+---------+----------------+ 
| taskid  | int(11) | NO | PRI | NULL | auto_increment | 
| description | text | NO |  | NULL |    | 
+-------------+---------+------+-----+---------+----------------+ 

Tasktransitions

+------------------+-----------+------+-----+-------------------+----------------+ 
| Field   | Type  | Null | Key | Default   | Extra   | 
+------------------+-----------+------+-----+-------------------+----------------+ 
| tasktransitionid | int(11) | NO | PRI | NULL    | auto_increment | 
| taskid   | int(11) | NO | MUL | NULL    |    | 
| status   | int(11) | NO | MUL | NULL    |    | 
| description  | text  | NO |  | NULL    |    | 
| userid   | int(11) | NO |  | NULL    |    | 
| transitiondate | timestamp | NO |  | CURRENT_TIMESTAMP |    | 
+------------------+-----------+------+-----+-------------------+----------------+ 

查詢

SELECT tasks.taskid,tasks.description,tasklaststatus.status 
FROM tasks 
LEFT OUTER JOIN 
(
    SELECT tasktransitions.taskid,tasktransitions.transitiondate,tasktransitions.status 
    FROM tasktransitions 
    INNER JOIN 
    (
     SELECT taskid,MAX(transitiondate) AS lasttransitiondate 
     FROM tasktransitions 
     GROUP BY taskid 
    ) AS tasklasttransition ON tasklasttransition.lasttransitiondate=tasktransitions.transitiondate AND tasklasttransition.taskid=tasktransitions.taskid 
) AS tasklaststatus ON tasklaststatus.taskid=tasks.taskid 
WHERE tasklaststatus.status IS NULL OR tasklaststatus.status=0 or tasklaststatus.transitiondate>'2013-09-01'; 

更可讀的，上面的查詢的描述性較差版本：

SELECT t.taskid, t.description,ls.status 
FROM tasks AS t 
LEFT OUTER JOIN 
(
    SELECT tt.taskid, tt.transitiondate, tt.status 
    FROM tasktransitions AS tt 
    INNER JOIN 
    (
     SELECT taskid,MAX(transitiondate) AS transitiondate 
     FROM tasktransitions 
     GROUP BY taskid 
    ) AS lt USING (taskid,transitiondate) 
) AS ls USING (taskid) 
WHERE ls.status IS NULL OR ls.status=0 or ls.transitiondate>'2013-09-01'; 

我想知道如果數據庫結構是最好的選擇性能明智。可以添加索引幫助嗎？我已經嘗試添加一些，但我沒有看到很大的改進。

+-----------------+------------+----------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 
| Table   | Non_unique | Key_name  | Seq_in_index | Column_name  | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | 
+-----------------+------------+----------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 
| tasktransitions |   0 | PRIMARY  |   1 | tasktransitionid | A   |   896 |  NULL | NULL |  | BTREE  |   |    | 
| tasktransitions |   1 | taskid_date_ix |   1 | taskid   | A   |   896 |  NULL | NULL |  | BTREE  |   |    | 
| tasktransitions |   1 | taskid_date_ix |   2 | transitiondate | A   |   896 |  NULL | NULL |  | BTREE  |   |    | 
| tasktransitions |   1 | status_ix  |   1 | status   | A   |   3 |  NULL | NULL |  | BTREE  |   |    | 
+-----------------+------------+----------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 

其他建議？

Answer 1

未經測試，不確定這是否是您的目標，但只使用一個子查詢。

SELECT 
    tasks.taskid, tasks.description, 
    (SELECT MAX(transitiondate) 
    FROM tasktransitions WHERE tasktransitions.taskid=tasks.id) lasttransitiondate 
FROM tasks 
LEFT JOIN tasktransitions tt ON tt.taskid = tasks.id 
WHERE tt.tasktransitionid IS NULL 
    OR tt.tasktransitionid = 0 
    OR tt.transitiondate > '2013-09-01'; 

爲了避免子查詢的一切，你可以通過使用觸發器在插入到tasktransitions每跳變變化添加last_trasition_id外鍵tasks表並進行更新。