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我正在制作一个留言板，并尝试检索常规主题（即未粘贴的主题）并按最后发布消息的日期对它们进行排序。我能够做到这一点，但是当我有大约 10,000 条消息和 1500 个主题时，查询时间> 60 秒。

我的问题是，我可以对查询做些什么来提高性能，还是我的设计存在根本缺陷？

这是我正在使用的查询。

SELECT Messages.topic_id,
       Messages.posted,
       Topics.title,
       Topics.user_id,
       Users.username
FROM Messages
LEFT JOIN
  Topics USING(topic_id)
LEFT JOIN
   Users on Users.user_id = Topics.user_id
WHERE Messages.message_id IN (
    SELECT MAX(message_id)
    FROM Messages
    GROUP BY topic_id)
AND Messages.topic_id
NOT IN (
    SELECT topic_id
    FROM StickiedTopics)
AND Messages.posted IN (                           
    SELECT MIN(posted)
    FROM Messages 
    GROUP BY message_id)
AND Topics.board_id=1
ORDER BY Messages.posted DESC LIMIT 50

编辑这里是解释计划

+----+--------------------+----------------+----------------+------------------+----------+---------+-------------------------+------+----------------------------------------------+
| id | select_type        | table          | type           | possible_keys    | key      | key_len | ref                     | rows | Extra                                        |
+----+--------------------+----------------+----------------+------------------+----------+---------+-------------------------+------+----------------------------------------------+
|  1 | PRIMARY            | Topics         | ref            | PRIMARY,board_id | board_id | 4       | const                   |  641 | Using where; Using temporary; Using filesort |
|  1 | PRIMARY            | Users          | eq_ref         | PRIMARY          | PRIMARY  | 4       | spergs3.Topics.user_id  |    1 |                                               |
|  1 | PRIMARY            | Messages       | ref            | topic_id         | topic_id | 4       | spergs3.Topics.topic_id |    3 | Using where                                   |
|  4 | DEPENDENT SUBQUERY | Messages       | index          | NULL             | PRIMARY  | 8       | NULL                    |    1 |                                              |
|  3 | DEPENDENT SUBQUERY | StickiedTopics | index_subquery | topic_id         | topic_id | 4       | func                    |    1 | Using index                                  |
|  2 | DEPENDENT SUBQUERY | Messages       | index          | NULL             | topic_id | 4       | NULL                    |    3 | Using index                                  |
+----+--------------------+----------------+----------------+------------------+----------+---------+-------------------------+------+----------------------------------------------+

索引

+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table    | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Messages |          0 | PRIMARY  |            1 | message_id  | A         |        9956 |     NULL | NULL   |      | BTREE      |         |
| Messages |          0 | PRIMARY  |            2 | revision_no | A         |        9956 |     NULL | NULL   |      | BTREE      |         |
| Messages |          1 | user_id  |            1 | user_id     | A         |         432 |     NULL | NULL   |      | BTREE      |         |
| Messages |          1 | topic_id |            1 | topic_id    | A         |        3318 |     NULL | NULL   |      | BTREE      |         |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table  | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Topics |          0 | PRIMARY  |            1 | topic_id    | A         |        1205 |     NULL | NULL   |      | BTREE      |         |
| Topics |          1 | user_id  |            1 | user_id     | A         |         133 |     NULL | NULL   |      | BTREE      |         |
| Topics |          1 | board_id |            1 | board_id    | A         |           1 |     NULL | NULL   |      | BTREE      |         |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name        | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Users |          0 | PRIMARY         |            1 | user_id     | A         |        2051 |     NULL | NULL   |      | BTREE      |         |
| Users |          0 | username_UNIQUE |            1 | username    | A         |        2051 |     NULL | NULL   |      | BTREE      |         |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
4

2 回答 2

2

我将从合格主题的第一个基础开始，获取这些 ID，然后加入。我的内部第一个查询执行按 topic_id 和 max 消息分组的预限定，以获得预限定的不同 ID。我也对stickiesTopics 应用了左连接。为什么？通过进行左连接，我可以查找已找到的那些（您要排除的那些）。所以我为 Stickies 主题 ID 应用了 WHERE 子句为 NULL（即：未找到）。因此，通过这样做，我们已经在列表中显着配对，而无需执行多个嵌套子查询。根据该结果，我们可以加入消息、主题（包括 board_id = 1 的限定符）、用户并根据需要获取部件。最后，为您的 MIN(posted) 限定符应用一个 WHERE IN 子选择。不了解其基础，但将其作为原始查询的一部分保留。

SELECT STRAIGHT_JOIN
      M.topic_id,
      M.posted,
      T.title,
      T.user_id,
      U.username
   FROM 
      ( select 
              M1.Topic_ID, 
              MAX( M1.Message_id ) MaxMsgPerTopic
           from 
              Messages M1
                 LEFT Join StickiedTopics ST
                    ON M1.Topic_ID = ST.Topic_ID
           where
              ST.Topic_ID IS NULL
           group by 
              M1.Topic_ID ) PreQuery
        JOIN Messages M
           ON PreQuery.MaxMsgPerTopic = M.Message_ID
           JOIN Topics T
               ON M.Topic_ID = T.Topic_ID
              AND T.Board_ID = 1
              LEFT JOIN Users U
                 on T.User_ID = U.user_id 
   WHERE
      M.posted IN ( SELECT MIN(posted)
                       FROM Messages 
                       GROUP BY message_id)
   ORDER BY 
      M.posted DESC 
   LIMIT 50
于 2012-03-14T03:14:42.960 回答
1

我猜你的问题很大一部分在于你的子查询。尝试这样的事情：

SELECT Messages.topic_id,
       Messages.posted,
       Topics.title,
       Topics.user_id,
       Users.username
FROM Messages
LEFT JOIN
    Topics USING(topic_id)
LEFT JOIN
    StickiedTopics ON StickiedTopics.topic_id = Topics.topic_id 
                   AND StickedTopics.topic_id IS NULL
LEFT JOIN
    Users on Users.user_id = Topics.user_id
WHERE Messages.message_id IN (
    SELECT MAX(message_id)
    FROM Messages m1
    WHERE m1.topic_id = Messages.topic_id)
AND Messages.posted IN (                           
    SELECT MIN(posted)                                                                                           
    FROM Messages m2
    GROUP BY message_id)
AND Topics.board_id=1
ORDER BY Messages.posted DESC LIMIT 50

我通过删除分组优化了第一个子查询。第二个子查询是不必要的，因为它可以替换为JOIN.

我不太确定这第三个子查询应该做什么：

AND Messages.posted IN (                           
    SELECT MIN(posted)                                                                                           
    FROM Messages m2
    GROUP BY message_id)

如果我知道它应该做什么，我也许可以帮助优化它。究竟是什么posted- 日期、整数等？它代表什么？

于 2012-03-14T02:47:30.533 回答