带有子查询和连接的 MYSQL 查询优化

Question

嗨，我有 800 万行数据，需要优化 Mysql 查询以从该数据中获取一行。 我正在使用以下查询，但它的服务器响应时间太长，导致页面加载速度出现问题

SELECT q.id
     , q.title
     , q.question
     , q.price
     , q.created
     , q.userid
     , q.duedate
     , q.tags
     , s.id subjectid
     , sc.id subcategoryid
     , s.title subject
     , sc.title subcategory
     , q.statusid
     , (SELECT  COUNT(id) FROM tbl_answers a WHERE a.questionid = q.id AND a.statusid = 1 AND a.deleted = 'N') q_num_answers
     , u.username
     , u.image
     , u.gender
     , (SELECT  COUNT(id) FROM tbl_answers a WHERE a.userid = q.userid AND a.statusid = 1 AND a.deleted = 'N') num_answers
     , (SELECT  COUNT(id) FROM tbl_questions WHERE userid = q.userid AND statusid = 1 AND deleted = 'N') AS num_questions
     , 0 amt_earned
     , 0 amt_spent
     , 0 num_sold
     , (SELECT  COUNT(ur.id) FROM tbl_users_ratings ur WHERE ur.userid = q.userid AND ur.deleted = 'N') u_num_ratings
     , (SELECT  COALESCE(SUM(ur.rating), 0) FROM tbl_users_ratings ur WHERE ur.userid = q.userid AND ur.deleted = 'N') u_score
  FROM tbl_questions q
  JOIN tbl_subjects s 
    ON q.subject = s.id
  JOIN tbl_subjects sc 
    ON q.subcategory = sc.id
  LEFT 
  JOIN tbl_users u 
    ON u.id = q.userid
 WHERE q.deleted = '$show_deleted' 
   AND q.id = ? 
 LIMIT 1

Answer 1

这些索引可能会有所帮助。 （我假设id是每个表的PRIMARY KEY 。）

ur:  (deleted, userid, rating)
a:  (deleted, statusid, userid)
a:  (deleted, statusid, questionid)

请提供EXPLAIN SELECT ... .

不要使用COUNT(id)除非您需要检查id不是NULL 。 通常的写法是COUNT(*) 。

在一个地方，您正在检查为deleted提供的值。 另一方面，您对其进行硬编码。 也许错了？

AND  ur.deleted = 'N'

如果q的PRIMARY KEY是id ，那么这将导致 1 行或没有行。 我错过了什么？

    WHERE  q.deleted = '$show_deleted'
      AND  q.id = ?

（可能还有更多提示，请把我建议的东西改一下，按照别人的建议去做，然后再看。）