需要强制选择的索引才能更快地运行？

Question

我有一个很大的选择，当我使用解释它显示：

我需要使用force index在pp表上使用(user)作为索引（键）。

强制索引后，它将删除pp2表中的用户（密钥）。所以我也需要在(user)上使用强制索引pp2。

强制在pp和pp2上建立索引后，表格p将自动使用primary作为索引，并且一切正常。

我的问题是，问题是什么？为什么mysql没有在pp表中使用possible_keys user作为键？

为什么在我强制pp使用user密钥后，它会弄乱pp2上的密钥？在它上面添加key = null。 为什么在强制pp和pp2使用用户作为键之后，表p将使用primary作为键并且所有都可以正常工作？

SQL：

select c.nome, p.foto, c.user, p.user, p.id, p.data, p.titulo, p.youtube, pp.foto, count(DISTINCT likes.user) as likes_count, count(distinct comentarios.id) as comentarios_count, count(DISTINCT l2.user) as count2, 

linked.id as shared_id, linked.titulo as shared_titulo, linked.user as shared_user_id, c2.user as shared_nick, linked.foto as shared_foto, pp2.foto as shared_perfil,
count(DISTINCT share_count.id) as shares_count

from posts p 

join cadastro c on p.user=c.id 
left join profile_picture pp force index (user) on p.user = pp.user
left join likes on likes.post = p.id
left join comentarios on comentarios.foto = p.id and comentarios.delete = 0  
left join likes l2 on l2.post = p.id and l2.user = 1

left join posts linked on linked.id = p.post_share
left join cadastro c2 on linked.user=c2.id
left join profile_picture pp2 force index (user) on linked.user = pp2.user
left join posts share_count on share_count.post_share = p.id and share_count.delete=0

where (p.user in (select following from following where user =1 and block=0) or p.user=1) and p.delete=0
group by p.id
order by p.id desc limit 15

个人资料图片：

CREATE TABLE `profile_picture` (
  `user` int(11) UNSIGNED NOT NULL,
  `foto` varchar(400) NOT NULL,
  UNIQUE KEY `user` (`user`)
) ENGINE=InnoDB ;

感谢。

Answer 1

为了提高性能，你也可以避免使用IN子句并使用内部连接，例如:(我希望）

  select c.nome, p.foto, c.user, p.user, p.id, p.data, p.titulo, p.youtube, pp.foto, count(DISTINCT likes.user) as likes_count, count(distinct comentarios.id) as comentarios_count, count(DISTINCT l2.user) as count2, 

  linked.id as shared_id, linked.titulo as shared_titulo, linked.user as shared_user_id, c2.user as shared_nick, linked.foto as shared_foto, pp2.foto as shared_perfil,
  count(DISTINCT share_count.id) as shares_count

  from posts p 

  join cadastro c on p.user=c.id 
  left join profile_picture pp force index (user) on p.user = pp.user
  left join likes on likes.post = p.id
  left join comentarios on comentarios.foto = p.id and comentarios.delete = 0  
  left join likes l2 on l2.post = p.id and l2.user = 1

  left join posts linked on linked.id = p.post_share
  left join cadastro c2 on linked.user=c2.id
  left join profile_picture pp2 force index (user) on linked.user = pp2.user
  left join posts share_count on share_count.post_share = p.id and share_count.delete=0

  inner join following f on (f.following = p.user and f.user=1 and f.block = 0 and p.delete=0)  
      OR  ( p.user=1 and p.delete =0)

  group by p.id
  order by p.id desc limit 15

并查看您的pp表，其中您在用户中有一个唯一索引，您可以尝试将其更改为创建中的pk条件

  CREATE TABLE `profile_picture` (
    `user` int(11) UNSIGNED NOT NULL,
    `foto` varchar(400) NOT NULL,
   PRIMARY KEY   (`user`)
  ) ENGINE=InnoDB ;