Getting Popular Topics on a Custom Made Forum - php

Question

For this website we're working on, we're trying to get the most popular topics (based on how many posts have been made in them within the last 24 hours). We have a medium to large based forum, and the current MySQL query looks like this:
SELECT `forums_topics`.`id`,`forums_topics`.`name`,
(
SELECT COUNT(`id`)
FROM `forums_posts`
WHERE `postdate` > (UNIX_TIMESTAMP()-60*60*24)
AND `topicid`=`forums_topics`.`id`
) AS `trendy_threads`
FROM `forums_topics`
WHERE `deleted`=0
AND `lastpost` > (UNIX_TIMESTAMP()-60*60*24)
ORDER BY `trendy_threads` DESC,`postdate` DESC
LIMIT 3
The SQL is quite sluggish.
How can we get this information as quickly and as efficiently as possible?
forums_topics
Field Type Null Key Default Extra
id int(50) NO PRI NULL auto_increment
uid varchar(255) NO NULL
flag int(1) NO 0
boardid varchar(255) NO NULL
postdate varchar(255) NO NULL
lastpost bigint(255) NO NULL
name varchar(50) NO NULL
description text NO NULL
body text NO NULL
author varchar(25) NO NULL
deleted tinyint(3) unsigned NO 0
deletememberid int(10) unsigned NO 0
pinned tinyint(1) NO 0
flagged text NO NULL
privateaccess text NO NULL
lastposter int(255) NO 1
replycount int(255) NO 0
viewcount int(255) NO 0
movedfrom int(255) NO 0
forums_posts
Field Type Null Key Default Extra
id int(50) NO PRI NULL auto_increment
topicid int(10) unsigned NO 0
author varchar(25) NO NULL
postdate varchar(255) NO NULL
body text NO NULL
lastedit varchar(255) NO NULL
postcount tinyint(1) NO NULL
invincible tinyint(1) NO 0
deleted tinyint(3) unsigned NO 0
deletememberid int(10) unsigned NO 0
thumbsup int(255) NO 0
thumbsdown int(255) NO 0
thumbsupuser text NO NULL
thumbsdownuser text NO NULL

Answer 1

The problem is probably that MySQL evaluates the subquery for every row. You can give MySQL a hint that that it should execute the subquery only once by moving the subquery into a join:
SELECT *
FROM forum_topics ft
JOIN (
SELECT topicid
, COUNT(*) as cnt
FROM forums_posts
WHERE postdate > UNIX_TIMESTAMP()-60*60*24
GROUP BY
topicid
) fpc
ON ft.topicid = fpc.topicid
WHERE ft.deleted = 0
ORDER BY
fpc.cnt DESC
, ft.postdate DESC
LIMIT 3
An index on forum_posts(postdate, topicid) would further improve performance.

Answer 2

I'm going to take a stab in the dark, and I'll edit further if needed. An EXPLAIN query would help.
SELECT `forums_topics`.*
FROM (
SELECT `topicid`, COUNT(*) as num
FROM `forums_posts`
WHERE `postdate` > (UNIX_TIMESTAMP()-60*60*24)
GROUP BY `topicid`
ORDER BY num DESC, `postdate` DESC
LIMIT 3
) `trendy`
LEFT JOIN `forums_topics` ON `id`=`topicid`
WHERE `deleted`=0