How to make JOINS faster? - mysql

Question

I had this query to start out with:
SELECT DISTINCT spentits.*
FROM `spentits`
WHERE (spentits.user_id IN
(SELECT following_id
FROM `follows`
WHERE `follows`.`follower_id` = '44'
AND `follows`.`accepted` = 1)
OR spentits.user_id = '44')
ORDER BY id DESC
LIMIT 15 OFFSET 0
This query takes 10ms to execute.
But once I add a simple join in:
SELECT DISTINCT spentits.*
FROM `spentits`
LEFT JOIN wishlist_items ON wishlist_items.user_id = 44 AND wishlist_items.spentit_id = spentits.id
WHERE (spentits.user_id IN
(SELECT following_id
FROM `follows`
WHERE `follows`.`follower_id` = '44'
AND `follows`.`accepted` = 1)
OR spentits.user_id = '44')
ORDER BY id DESC
LIMIT 15 OFFSET 0
This execute time increased by 11x. Now it takes around 120ms to execute. What's interesting is that if I remove either the LEFT JOIN clause or the ORDER BY id DESC , the time goes back to 10ms.
I am new to databases so I don't understand this. Why is it that removing either one of these clauses speeds it up 11x ? And how can I keep it as is but make it faster?
I have indexes on spentits.user_id, follows.follower_id, follows.accepted, and on primary ids of each table.
EXPLAIN:
1 PRIMARY spentits index index_spentits_on_user_id PRIMARY 4 NULL 15 Using where; Using temporary
1 PRIMARY wishlist_items ref index_wishlist_items_on_user_id,index_wishlist_items_on_spentit_id index_wishlist_items_on_spentit_id 5 spentit.spentits.id 1 Using where; Distinct
2 SUBQUERY follows index_merge index_follows_on_follower_id,index_follows_on_following_id,index_follows_on_accepted
index_follows_on_follower_id,index_follows_on_accepted 5,2 NULL 566 Using intersect(index_follows_on_follower_id,index_follows_on_accepted); Using where

Answer 1

You should have index also on:
wishlist_items.spentit_id
Because you are joining over that column

Answer 2

The LEFT JOIN is easy to explain: A cross product of all entries against all other entries is made. The conditions of the join (in your case: Take all entries on the left and find fitting ones on the right) are applied afterwards. So if your spentits table is large it will take the server some time. Would suggest you get rid of your subquery and make three joins. Start with the smallest table to avoid big amounts of data.

Answer 3

In the 2nd example the subselect runs for every spentits.user_id.
If you write is like this it will be faster because the subselect runs once:
SELECT DISTINCT spentits.*
FROM `spentits`, (SELECT following_id
FROM `follows`
WHERE `follows`.`follower_id` = '44'
AND `follows`.`accepted` = 1)
OR spentits.user_id = '44') as `follow`
LEFT JOIN wishlist_items ON wishlist_items.user_id = 44 AND wishlist_items.spentit_id = spentits.id
WHERE (spentits.user_id IN
(follow)
ORDER BY id DESC
LIMIT 15 OFFSET 0
As you can see the subselect moved to the FROM-part of the query and creates a imaginary tabel (or view).
This imaginary tabel is a inline-view.
JOINs and inline-views are faster every time than a subselect in the WHERE-part.