Optimize MySQL query to avoid “Using where; Using temporary; Using filesort”

Question

I built a custom forum for my site using MySQL. The listing page is essentially a table with the following columns: Topic, Last Updated, and

Answer 1

SELECT  id, name, last_reply, replies
FROM    (
        SELECT  topic_id, MAX(date) AS last_reply, COUNT(*) AS replies
        FROM    wp_pod_tbl_forum
        GROUP BY
                topic_id
        ) r
JOIN    wp_pod_tbl_forum t
ON      t.topic_id = 0
        AND t.id = r.topic_id
UNION ALL
SELECT  id, name, date, 0
FROM    wp_pod_tbl_forum t
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    wp_pod_tbl_forum r
        WHERE   r.topic_id = t.id
        )
        AND t.topic_id = 0
ORDER BY
       date DESC
LIMIT 0, 20

If your table is MyISAM or id is not a PRIMARY KEY, you need to create a composite ondex on (topic_id, id).

If your table is InnoDB and id is a PRIMARY KEY, an index just on (topic_id) will do (id will be implicitly added to the index).

Update

This query will most probably be even more efficient, provided that you have indexes on (topic_id, id) and (date, id):

See this article in my blog for performance details:

• Selecting last forum posts

This query completes in 30 ms on a 100,000 rows sample data:

SELECT  id, name, last_reply,
        (
        SELECT  COUNT(*)
        FROM    wp_pod_tbl_forum fc
        WHERE   fc.topic_id = fl.topic_id
        ) AS replies
FROM    (
        SELECT  topic_id, date AS last_reply
        FROM    wp_pod_tbl_forum fo
        WHERE   id = (
                SELECT  id
                FROM    wp_pod_tbl_forum fp
                WHERE   fp.topic_id = fo.topic_id
                ORDER BY
                        fp.date DESC, fp.id DESC
                LIMIT 1
                )
                AND fo.topic_id <> 0
        ORDER BY
                fo.date DESC, fo.id DESC
        LIMIT 20
        ) fl
JOIN    wp_pod_tbl_forum ft
ON      ft.id = fl.topic_id
UNION ALL
SELECT  id, name, date, 0
FROM    wp_pod_tbl_forum t
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    wp_pod_tbl_forum r
        WHERE   r.topic_id = t.id
        )
        AND t.topic_id = 0
ORDER BY
       last_reply DESC, id DESC
LIMIT  20

Both indexes are required for this query to be efficient.

If your table is InnoDB and id is a PRIMARY KEY, then you can omit id from the indexes above.