我正在制作留言板，我试图检索常规主题（即没有粘贴的主题），并按最后发布消息的日期对其进行排序。我能够实现这一点，但是当我有大约10,000条消息和1500个主题时，查询时间大于60秒。
我的问题是，我的查询有什么可以提高性能的，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
编辑以下是解释计划
+----+--------------------+----------------+----------------+------------------+----------+---------+-------------------------+------+----------------------------------------------+
| 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 | |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
答案 0 :(得分：2)
我会先从合格主题的第一个基础开始，获取这些ID，然后再加入。 我的内部第一个查询执行按topic_id和max消息分组的预先限定，以获得预先限定的不同ID。我也对stickiesTopics应用了LEFT JOIN。为什么？通过执行左连接，我可以查找那些FOUND（您要排除的那些）。所以我已经为Stickies主题ID应用了一个WHERE子句为NULL（即：未找到）。因此，通过执行此操作，我们已经ALREADY在列表中显着配对而不执行多个嵌套子查询。从这个结果，我们可以加入消息，主题（包括board_id = 1的限定符），用户和根据需要获取部分。最后，为MIN（已发布）限定符应用单个WHERE IN子选择。不要理解它的基础，但将其作为原始查询的一部分。然后按顺序和限制。
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
答案 1 :(得分：1)
我猜你问题的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
我通过删除分组来优化第一个子查询。第二个子查询是不必要的，因为它可以用JOIN替换。
我不太确定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，我或许可以帮助优化它。究竟是什么posted - 日期，整数等？它代表什么？