Slow mysql of a PHP Board

Question

I have multiple tables of a php board.

I need an effective query, to select all categories, all topics, last post of topics, with the posted user. With my query it takes 5-8 seconds to run it. I did an optimize with last_post_id field into topics table, but I need a better solution for it.

Structure

forum_categories ~ 15 lines
id|name|...

forum_topics ~ 150 lines
id|name|category_id|...

forum_posts ~ 1.000.000 lines
id|body|topic_id|user_id|...

users ~ 30.000 lines
id|username|...


category 1
 - topic 1
    - last post1 | user1
 - topic 2
    - last post2 | user2
 ...
category 2
 - topic 3
   - last post3 | user3
 ...
...

Last query (This was a help from my friend. But this also was so slow. )

SELECT c.NAME     AS category, 
       t.NAME     AS topic, 
       p.body     AS post, 
       p.username AS username 
FROM   forum_categories AS c 
       JOIN forum_topics AS t 
         ON t.category_id = c.id 
       JOIN (SELECT * 
             FROM   (SELECT p.body, 
                            p.topic_id, 
                            u.username 
                     FROM   forum_posts AS p 
                            JOIN users AS u 
                              ON u.id = p.user_id 
                     ORDER  BY p.id DESC) AS t 
             GROUP  BY topic_id) AS p 
         ON t.id = p.topic_id 

Exaplain query

Statistic of query

Headers are: sort, status, time | status, all time, pct. time, calls, time

Answer 1

I think "last post of topics" is key point of your query. That's why you used ORDER BY on most inner query but this makes two sub-queries.

Updated version

CREATE TEMPORARY TABLE last_post_per_topic_t ENGINE = MEMORY
SELECT topic_id, MAX(id) AS id -----+
FROM forum_posts                    --> find last post id per topic
GROUP BY topic_id; -----------------+

ALTER TABLE last_post_per_topic_t ADD INDEX (id, topic_id);

SELECT *
FROM forum_categories AS c INNER JOIN forum_topics t ON c.id = t.category_id
INNER JOIN forum_posts p ON p.topic_id = t.id
INNER JOIN last_post_per_topic_t ON last_post_per_topic_t.topic_id = t.id
  AND last_post_per_topic_t.id = p.id;
INNER JOIN users u ON p.user_id = u.id;

first version

SELECT *
FROM forum_categories AS c INNER JOIN forum_topics t ON c.id = t.category_id
INNER JOIN forum_posts p ON p.topic_id = t.id
INNER JOIN (
    SELECT topic_id, MAX(id) AS id -----+
    FROM forum_posts                    --- find last post_id per topic
    GROUP BY topic_id    ---------------+
) last_post_per_topic_t ON last_post_per_topic_t.topic_id = t.id
  AND last_post_per_topic_t.id = p.id;