MySQL Query占用时间超过6秒 [英] MySQL Query taking over 6 seconds

问题描述

一段时间后，我得到了一些特定查询的帮助。这是链接： SQL Group BY在新列中使用字符串

Awhile back I got some help with a specific query. Here's the link: SQL Group BY using strings in new columns

我的查询与此类似：

SELECT    event_data, class_40_winner, class_30_winner
FROM      events e
LEFT JOIN (SELECT result_event, name AS class_40_winner
       FROM   results 
       WHERE  class = 40 AND position = 1) c40 ON e.id = c40.result_event
LEFT JOIN (SELECT result_event, name AS class_30_winner
       FROM   results 
       WHERE  class = 30 AND position = 1) c30 ON e.id = c30.result_event

我现在在我的数据库中输入了足够的数据（22,000行），这个查询需要6秒才能完成。 （我的实际查询大于上面的内容，因为它现在有4个连接。）

I have now entered enough data in my database (22,000 rows) that this query is taking over 6 seconds to complete. (My actual query is bigger than the above, in that it now has 4 joins in it.)

我在查询中使用了Explain函数来查看。来自结果表的每个查询都会提取22,000行，所以这似乎是问题所在。

I used the "Explain" function on my query to take a look. Each of the queries from the "results" table is pulling in the 22,000 rows, so this seems to be the problem.

我做了一些研究，听起来像是我应该能够在结果表上索引相关列以帮助加快速度。但是当我这样做时，实际上我的查询速度降低了大约10秒。

I have done some research and it sounds like I should be able to INDEX the relevant column on the "results" table to help speed things up. But when I did that, it actually slowed my query down to about 10 seconds.

我可以采取哪些建议来改进此查询？

Any suggestions for what I can do to improve this query?

推荐答案

AFAIK，你正在转动你的数据，我认为使用 max（case ...）... group by 在旋转数据方面表现良好。

我建议您改用此查询：

AFAIK, you are pivoting your data and I think using max(case ...) ... group by has good performance in pivoting data.
I can suggest you to use this query instead:

select event_date
    , max(case when r.class = 40 then name end) `Class 40 Winner`
    , max(case when r.class = 30 then name end) `Class 30 Winner`
from events e
left join results r on e.event_id = r.result_event and r.position = 1
group by event_date;

[SQL小提琴演示]

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