mysql - 改进 MySQL 查询性能

Question

我有一个查询，它在两个表 company 和 question 上运行得很好。这两个表的结构如下:

company   |  question
id        |  id
name      |  company_id
          |  sentiment

SELECT company.id as company_id, company.name,  question.sentiment,
(SELECT count(*) FROM question 
WHERE question.sentiment=0 AND question.company_id=company.id) AS count
FROM question
JOIN company ON company.id=question.company_id
 WHERE question.sentiment = 0 GROUP BY company_id ORDER BY count DESC LIMIT 5

我一直在试图弄清楚如何重写它，以避免子查询似乎减慢查询执行速度。 问题表有超过 1000 条记录。 另外，使用索引将执行时间从 545.037 秒减少到 180.228 秒，但我仍然需要改进。

谢谢

Answer 1

ALTER TABLE question ADD INDEX (sentiment, company_id);

SELECT c.id as company_id, c.name, q.sentiment, q.count
FROM company c
JOIN (
    SELECT company_id, MAX(sentiment) AS sentiment, COUNT(*) AS count
    FROM question
    WHERE sentiment = 0
    GROUP BY company_id
    ORDER BY NULL
) AS q ON c.id = q.company_id
ORDER BY count DESC LIMIT 5