Question

I have a query that works, but it’s taking at least 3 seconds to run so I think it can probably be faster. It’s used to populate a list of new threads and show how many unread posts there are in each thread. I generate the query string before throwing it into $db->query_read(). In order to only grab results from valid forums, $ids is string with up to 50 values separated by commas.

The userthreadviews table has existed for 1 week and there are roughly 9,500 rows in it. I’m not sure if I need to set up a cron job to regularly clear out thread views more than a week old, or if I will be fine letting it grow.

Here’s the query as it currently stands:

SELECT 
    `thread`.`title` AS 'r_title', 
    `thread`.`threadid` AS 'r_threadid', 
    `thread`.`forumid` AS 'r_forumid', 
    `thread`.`lastposter` AS 'r_lastposter', 
    `thread`.`lastposterid` AS 'r_lastposterid', 
    `forum`.`title` AS 'f_title', 
    `thread`.`replycount` AS 'r_replycount', 
    `thread`.`lastpost` AS 'r_lastpost', 
    `userthreadviews`.`replycount` AS 'u_replycount', 
    `userthreadviews`.`id` AS 'u_id',
    `thread`.`postusername` AS 'r_postusername', 
    `thread`.`postuserid` AS 'r_postuserid' 
FROM 
    `thread` 
INNER JOIN 
    `forum` 
    ON (`thread`.`forumid` = `forum`.`forumid`) 
LEFT JOIN 
    (`userthreadviews`) 
    ON (`thread`.`threadid` = `userthreadviews`.`threadid` 
    AND `userthreadviews`.`userid`=$userid) 
WHERE 
    `thread`.`forumid` IN($ids) 
    AND `thread`.`visible`=1 
    AND `thread`.`lastpost`> time() - 604800
ORDER BY `thread`.`lastpost` DESC LIMIT 0, 30

An alternate query that joins the post table (to only show threads where user has posted) is actually twice as fast, so I think there’s got to be something in here that could be changed to speed it up. Could someone provide some advice?

Edit: Sorry, I had put the EXPLAIN in front of the alternate query. Here is the correct output:
As Requested, here is the output generated by EXPLAIN SELECT:

Answer 1

To have no indexes on joins / where (used key = NULL on explain), this is the reason why your queries are slow. You should index them in such a way :

CREATE INDEX thread_forumid_index ON thread(forumid);
CREATE INDEX userthreadviews_forumid_index ON userthreadviews(forumid);

Documentation here