Question

以下查询的运行速度非常慢...

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

销售线索表具有约8000条记录用户表有300条记录评论表有约500,000条记录

我在Leads.id，comments.lead_id，leads.company_id，users.id和comments.user_id，comments.sort，comments.reply，comments.id上都有一个索引

有人可以解释如何优化此查询吗？

Screenshot of query EXPLAIN

Answer 1

将leads移至from列表的第一位：

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)上的索引在连接到其他表之前立即过滤出尽可能多的行。

效果应提高约1/(fraction of leads with company_id = 1)

如何使用索引最好地联接表

1 个答案: