UNION ALL MYSQL is really slow - mysql

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

Answer 2

UNION defaults to UNION DISTINCT. If you want UNION ALL, explictly say that (and you should, wherever possible, since UNION DISTINCT has to do a lot of work to make them distinct).
The other thing is that the order by and limit apply to the query as a whole. You may want to make each unioned query have them too. See https://dev.mysql.com/doc/refman/8.0/en/union.html:
ORDER BY and LIMIT in Unions
To apply an ORDER BY or LIMIT clause to an individual SELECT, parenthesize the SELECT and place the clause inside the parentheses:
(SELECT a FROM t1 WHERE a=10 AND B=1 ORDER BY a LIMIT 10)
UNION
(SELECT a FROM t2 WHERE a=11 AND B=2 ORDER BY a LIMIT 10);
If your fast individual versions did have those, that should help. So something like this:
(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)
order by created_at DESC
LIMIT 10