Question

I have the above models in my MySQL database:

Blogs (id: integer, name: varchar)
Posts (id: integer, name: varchar, blog_id: integer, created_at: date)

I want to retrieve a list of all the blogs, ordered by the ones that have the newests posts.

I’ve reached that with the following query:

SELECT b.*, (SELECT p.created_at FROM posts p WHERE p.blog_id = b.id ORDER BY p.created_at DESC LIMIT 1) AS last_post_created_at FROM blogs b ORDER BY last_post_created_at DESC;

But this query is too slow and I’m unable to use it on my application.

Do you guys have a good solution for that?

Thank you.

Answer 1

A rewriting of the query:

SELECT b.*, 
       p.last_post_created_at 
FROM blogs b 
  LEFT JOIN
    ( SELECT blog_id,
             MAX(created_at) AS last_post_created_at
      FROM posts  
      GROUP BY blog_id 
    ) AS p
    ON  p.blog_id = b.id  
ORDER BY last_post_created_at DESC;

An index on (blog_id, created_at) will help both this and your version.

If you want to limit the number of blogs returned, you should add the ORDER BY in the subquery and put the LIMIT there:

SELECT b.*, 
       p.last_post_created_at 
FROM blogs b 
  LEFT JOIN
    ( SELECT blog_id,
             MAX(created_at) AS last_post_created_at
      FROM posts  
      GROUP BY blog_id 
      ORDER BY last_post_created_at DESC
        LIMIT 100
    ) AS p
    ON  p.blog_id = b.id  
ORDER BY last_post_created_at DESC;