Question

我正在开发一个论坛系统（mysql），我不确定在单个查询帖子，上下投票以及当前用户是否为每个帖子投票时，选择哪条路径可以获得更好的性能。第一个选项是：

SELECT posts.post_id, post_content, display_name, 
(SELECT COUNT(post_id) FROM post_votes WHERE post_votes.post_id=posts.post_id AND post_votes.user_id='+user_id+') voted,
(SELECT COUNT(post_id) FROM post_votes WHERE post_votes.post_id=posts.post_id AND up_vote=1) upvotes, 
(SELECT COUNT(post_id) FROM post_votes WHERE post_votes.post_id=posts.post_id AND up_vote=0) downvotes
FROM posts JOIN users ON users.user_id=posts.user_id WHERE parent_id ='+parent_id+' ORDER BY post_id DESC

第二个选项是用LEFT JOIN和count替换所有count子查询。一种方法对另一种方法有什么好处吗？

修改

由于我希望检索所有帖子而不是一个对帖子进行分组的行，我想出了这个查询（带有一些灵感from here）：

SELECT p.post_id, post_content, display_name,
COALESCE(v.upvotes, 0) AS upvotes, 
COALESCE(v.downvotes, 0) AS downvotes, 
COALESCE(v.voted, 0) AS voted 
FROM posts p 
LEFT JOIN ( 
        SELECT post_id, 
        SUM(vt.up_vote = 1) AS upvotes, 
        SUM(vt.up_vote = 0) AS downvotes, 
        MAX(IF(vt.user_id = ' + user_id + ', vt.up_vote, NULL)) voted 
        FROM post_votes vt 
        GROUP BY vt.post_id
        ) 
        v ON v.post_id = p.post_id 
JOIN users ON users.user_id=p.user_id 
WHERE parent_id =' + parent_id + ' ORDER BY post_id DESC

我已经在我的演示数据库上运行了两个解决方案（目前很小，每个表中包含不到100行），持续时间相同。问题是从长远来看哪一个会更快。

Answer 1

我很难想到子查询比连接更快的地方。

在这种情况下，您甚至不需要加入。在一个查询中完成所有操作：

SELECT 
p.post_id, 
p.post_content, 
u.display_name, 
COUNT(pv.post_id) AS voted,
SUM(pv.up_vote = 1) AS upvotes,
SUM(pv.up_vote = 0) downvotes
FROM posts p
JOIN users u ON u.user_id = p.user_id 
LEFT JOIN post_votes pv ON posts.post_id = pv.post_id AND pv.user_id ='whatever'
WHERE p.parent_id ='+parent_id+' 
GROUP BY p.post_id
ORDER BY p.post_id DESC

pv.up_vote = 'whatever'函数内的SUM()返回true或false，1或0.这就是我们在这里使用SUM()函数的原因。瞧，一切都在一个查询中。

计算子查询中的投票或使用连接 - 哪个更快？

1 个答案: