UNION ALL MYSQL is really slow

Question

I have a query in mysql with 2 select from's. When I run these queries individually they run quick within 1 second. But when I combine them with union all, the website freeze's and it takes atleast 20 seconds for the same query to execute in union all.

Any idea why this happens? Can't figure it out.

See the query below:

SELECT p.user_id, p.description, p.post_id, p.created_at, ps.username AS size_name, ps.username AS user_name, ps.avatar AS avatar, pz.title AS title, pz.slug AS slug 
FROM comments p 
JOIN users ps ON ps.id = p.user_id 
JOIN posts pz ON pz.id = p.post_id 

UNION ALL

SELECT p2.user_id, p2.description, p2.post_id, p2.created_at, ps2.username AS size_name, ps2.username AS user_name, ps2.avatar AS avatar, pz2.title AS title, pz2.slug AS slug 
FROM reply p2 
JOIN users ps2 ON ps2.id = p2.user_id 
JOIN posts pz2 ON pz2.id = p2.post_id 

order by created_at DESC 
LIMIT 10

Answer 1

Before merging with UNION, reduce the size of the tables returned by each subquery. Since you only want the top 10, you only need the top 10 of each subquery.

SELECT *
FROM (
    (SELECT p.user_id, p.description, p.post_id, p.created_at, ps.username AS size_name, ps.username AS user_name, ps.avatar AS avatar, pz.title AS title, pz.slug AS slug 
    FROM comments p 
    JOIN users ps ON ps.id = p.user_id 
    JOIN posts pz ON pz.id = p.post_id 
    ORDER BY created_at DESC
    LIMIT 10) 
    
    UNION ALL
    
    (
    SELECT p2.user_id, p2.description, p2.post_id, p2.created_at, ps2.username AS size_name, ps2.username AS user_name, ps2.avatar AS avatar, pz2.title AS title, pz2.slug AS slug 
    FROM reply p2 
    JOIN users ps2 ON ps2.id = p2.user_id 
    JOIN posts pz2 ON pz2.id = p2.post_id 
    ORDER BY created_at DESC
    LIMIT 10)
) AS x
order by created_at DESC 
LIMIT 10