Subselect in WHERE or FROM clause? - mysql

Question

I have wondered about general performance of a query if specific subselect (subquery) is located in WHERE or FROM clause. I didn't find sufficient explanation which way is better. Are there some rules how we should apply subselect in this kind of queries?
I prepared following example
Query FROM
SELECT name
FROM users a
JOIN (SELECT user_id, AVG(score) as score
FROM scores GROUP BY user_id
) b ON a.id=b.user_id
WHERE b.score > 15;
Query WHERE
SELECT name
FROM users
WHERE
(SELECT AVG(score) as score
FROM scores WHERE scores.user_id=users.id GROUP BY user_id
) > 15;
Tables:
CREATE TABLE users (
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(30));
CREATE TABLE scores (
id INT PRIMARY KEY AUTO_INCREMENT,
user_id INT,
score INT);
INSERT INTO users(name)
VALUES ('John'), ('Max'), ('Dan'), ('Alex');
INSERT INTO scores(user_id, score)
VALUES
(1, 20),
(1, 19),
(2, 15),
(2, 10),
(3, 20),
(3, 18),
(4, 13),
(4, 16),
(4, 15);

Answer 1

In both cases, scores needs INDEX(user_id, score) for performance.
It is hard to predict which will run faster.
There are times when a query similar to the first formulation is excellent. This is because it goes off an focuses on b and efficiently calculates all the AVGs all at once. Then it reaches over to the other table for the final info.
Let's tweak the second version slightly by adding some other test in the WHERE clause. Now the second one might be faster.
This may be even better:
SELECT name
FROM ( SELECT user_id -- Don't fetch AVG if not needed
FROM scores GROUP BY user_id
HAVING AVG(score) > 15; -- Note
) b
JOIN users a ON a.id = b.user_id
(The swapping of FROM and JOIN is not an optimization; it is just to show what order the Optimizer will perform the steps.)
In some other situations, EXISTS( SELECT ... ) is beneficial. (But I don't see such in your case.
Your question was about general optimization. I'm trying to emphasize that there is no general answer.

Answer 2

I think this request is faster than what you give above, as it has no subqueries.
SELECT u.name
FROM users u
JOIN scores s
ON (s.user_id = u.id)
GROUP BY s.user_id
HAVING AVG(s.score) > 15
You can see it on this link: http://sqlfiddle.com/#!9/b050f9/16
it shows the execution time of the next 3 Select queries:
SELECT name
FROM users a
JOIN (SELECT user_id, AVG(score) as score
FROM scores GROUP BY user_id
) b ON a.id=b.user_id
WHERE b.score > 15;
SELECT name
FROM users
WHERE
(SELECT AVG(score) as score
FROM scores WHERE scores.user_id=users.id GROUP BY user_id
) > 15;
SELECT u.name
FROM users u
JOIN scores s
ON (s.user_id = u.id)
GROUP BY s.user_id
HAVING AVG(s.score) > 15