使用ORDER BY datetime時，MYSQL查詢非常慢

Question

我在社交網絡上運行以下查詢。 該查詢從數據庫中獲取帖子（例如Facebook帖子）。

SELECT P.*, 
   P.id_post id_p, 
   PM.meta_content video_title, 
   PM2.meta_content video_views, 
   PM3.meta_content racebooking_views, 
   Greatest(P.creation_date, Coalesce(Max(C.date), P.creation_date)) AS 
   last_activity, 
   P.creation_date creation_date, 
   (SELECT Count(*) 
    FROM   likes 
    WHERE  post_id = P.id_post 
           AND post_type = 'P') 
   likes_count, 
   (SELECT Count(*) 
    FROM   likes L 
    WHERE  post_id = P.id_post 
           AND post_type = 'P' 
           AND L.id_profile = 2796) 
   do_i_like 
FROM   posts P 
       LEFT JOIN comments C 
              ON P.id_post = C.post_id 
                 AND C.post_type = 'P' 
                 AND C.id_profile != P.id_profile 
       LEFT JOIN post_meta PM 
              ON PM.id_post = P.id_post 
                 AND PM.meta_type = 'T' 
       LEFT JOIN post_meta PM2 
              ON PM2.id_post = P.id_post 
                 AND PM2.meta_type = 'V' 
       LEFT JOIN post_meta PM3 
              ON PM3.id_post = P.id_post 
                 AND PM3.meta_type = 'W' 
GROUP  BY P.id_post 
ORDER  BY last_activity DESC 
LIMIT  41, 10 

每個帖子可能有或沒有評論。 我希望查詢首先獲取具有最新活動的帖子。 因此，如果帖子中有評論，我將使用最新評論的日期。 如果帖子沒有評論，我將以帖子的創建日期為准。

該工作由Greatest(P.creation_date, Coalesce(Max(C.date), P.creation_date)) ，該工作將在注釋日期（如果存在注釋）與創建帖子的日期之間獲取greates值。

然后， ORDER BY last_activity DESC進行排序。

問題

查詢真的很慢。 運行需要8秒鍾。 posts表具有8K行，comments表具有8K行。

我不明白的是，如果我用此ORDER BY P.id_post替換ORDER BY子句，則需要0.5秒才能運行。 但是，如果我再次用ORDER BY P.creation_date替換ORDER BY子句，則需要8秒鍾。 似乎不喜歡約會...

其他資訊

• posts表在creation_date上有一個索引。
• 評論表上有日期索引
• 服務器在CentOS Linux 6.6上運行LAMP
• 我像這樣嘗試了其他解決方案，但它們沒有用

如何解決該查詢以提高運行速度？

Answer 1

select子句中的相關子查詢可能會殺死您。 相反，請加入一個計算喜歡統計信息的子查詢：

SELECT P.*, 
    P.id_post id_p, 
    PM.meta_content video_title, 
    PM2.meta_content video_views, 
    PM3.meta_content racebooking_views, 
    GREATEST(P.creation_date, COALESCE(MAX(C.date), P.creation_date)) AS last_activity, 
    P.creation_date creation_date,
    t.likes_count,
    t.do_i_like
FROM posts P
LEFT JOIN
(
    SELECT
        post_id,
        SUM(CASE WHEN post_type = 'P' THEN 1 ELSE 0 END) AS likes_count,
        SUM(CASE WHEN post_type = 'P' AND L.id_profile = 2796
                 THEN 1 ELSE 0 END) AS do_i_like
    FROM likes
    GROUP BY post_id
) t
    ON t.post_id = P.id_post 
LEFT JOIN comments C 
    ON P.id_post = C.post_id AND
       C.post_type = 'P' AND
       C.id_profile != P.id_profile 
LEFT JOIN post_meta PM 
    ON PM.id_post = P.id_post AND
       PM.meta_type = 'T' 
LEFT JOIN post_meta PM2 
    ON PM2.id_post = P.id_post AND
       PM2.meta_type = 'V' 
LEFT JOIN post_meta PM3 
    ON PM3.id_post = P.id_post AND 
       PM3.meta_type = 'W' 
ORDER BY
    last_activity DESC 
LIMIT 41, 10

另外，在編輯查詢后，我看不到在外部查詢中使用GROUP BY的原因，因此我將其刪除。 您應該在適當的地方使用索引，盡管我的直覺是僅憑我的建議就可以顯着提高性能。

Answer 2

有一個MAX（C.Date）將需要一個group by子句，但是它也可以代替我認為的子查詢：

SELECT P.*, 
    P.id_post id_p, 
    PM.meta_content video_title, 
    PM2.meta_content video_views, 
    PM3.meta_content racebooking_views, 
    GREATEST(P.creation_date, COALESCE(max_c_date, P.creation_date)) AS last_activity, 
    P.creation_date creation_date,
    t.likes_count,
    t.do_i_like
FROM posts P
LEFT JOIN
(
    SELECT
        post_id,
        SUM(CASE WHEN post_type = 'P' THEN 1 ELSE 0 END) AS likes_count,
        SUM(CASE WHEN post_type = 'P' AND L.id_profile = 2796
                 THEN 1 ELSE 0 END) AS do_i_like
    FROM likes
    GROUP BY post_id
) t
    ON t.post_id = P.id_post 
LEFT JOIN (
      SELECT
        comments.post_id,
        MAX(comments.date) max_c_date
      FROM comments
      inner join posts ON comments.post_id = posts.id_post
      where  comments.post_type = 'P' AND
             comments.id_profile != posts.id_profile 
      GROUP BY comments.post_id
) C 
    ON P.id_post = C.post_id AND
LEFT JOIN post_meta PM 
    ON PM.id_post = P.id_post AND
       PM.meta_type = 'T' 
LEFT JOIN post_meta PM2 
    ON PM2.id_post = P.id_post AND
       PM2.meta_type = 'V' 
LEFT JOIN post_meta PM3 
    ON PM3.id_post = P.id_post AND 
       PM3.meta_type = 'W' 
ORDER BY
    last_activity DESC 
LIMIT 41, 10