mysql - 如何使用索引最好地连接表

Question

以下查询运行速度非常慢...

SELECT
   CONCAT(users.first_name, ' ', users.last_name) AS user_name,
   leads.first_name AS first_name,
   comments.*,
FROM comments
   INNER JOIN users ON 
      users.id = comments.user_id
   INNER JOIN leads ON 
      leads.id = comments.lead_id AND 
      leads.company_id = 1 
 ORDER BY 
    `sort` DESC, 
     reply ASC, 
     id ASC 
LIMIT 80,20

潜在客户表有约 8000 条记录 users 表有约 300 条记录 评论表有约 500,000 条记录

我有一个关于 Leads.id、comments.lead_id、leads.company_id、users.id 和 comments.user_id、comments.sort、comments.reply、comments.id 的索引

有人可以解释一下如何优化这个查询吗？

Screenshot of query EXPLAIN

Answer 1

将线索移至来源列表中的第一个位置:

SELECT
  CONCAT(users.first_name, ' ', users.last_name) AS user_name,
  leads.first_name AS first_name,
  comments.*
FROM leads
INNER JOIN comments ON 
  comments.lead_id = leads.id
INNER JOIN users ON 
  users.id = comments.user_id
WHERE leads.company_id = 1
ORDER BY 
  `sort` DESC, 
  reply ASC, 
  id ASC 
LIMIT 80,20

这允许 leads(company) 上的索引在加入其他表之前立即过滤掉尽可能多的行。

绩效提升应约为 1/(company_id = 1 的潜在客户比例)