SQL query is taking a long time to execute - php

Question

I have researched other questions and tried their solutions, but none helped me. How can I lower the execution time of this query?
SELECT c.id,c.title,c.time,u.username, (SELECT time FROM comments_read as io WHERE io.id=c.id AND io.uid=$logged_userid LIMIT 1) as read_time
FROM comments as c
LEFT JOIN users u ON u.uid=c.uid
WHERE c.deleted=0
ORDER by c.time DESC
LIMIT 20
Thank you so much.

Answer 1

This is your query:
SELECT c.id, c.title, c.time, u.username,
(SELECT io.time
FROM comments_read as io
WHERE io.id = c.id AND
io.uid = $logged_userid
LIMIT 1
) as read_time
FROM comments as c LEFT JOIN
users u
ON u.uid = c.uid
WHERE c.deleted = 0
ORDER by c.time DESC
LIMIT 20;
Indexes can be used to improve performance. I would start with comments(deleted, time, uid, id), users(uid), and comments_read(id, uid, time).
Note: using LIMIT without an ORDER BY is usually discouraged, because the results are unstable. You can get any matching row.
EDIT:
If the above indexes do not help, they might be getting confused by the JOIN. You can do the same thing as:
SELECT c.id, c.title, c.time,
(SELECT u.username FROM users u WHERE u.uid = c.uid) as username,
(SELECT io.time
FROM comments_read as io
WHERE io.id = c.id AND
io.uid = $logged_userid
LIMIT 1
) as read_time
FROM comments as c
WHERE c.deleted = 0
ORDER by c.time DESC
LIMIT 20;
This should take advantage of the above indexes and have no "file sort" step to slow things down.