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 # Replies.

The DB table has the following columns:

id
name
body
date
topic_id
email

A topic has the topic_id of “0”, and replies have the topic_id of their parent topic.

SELECT SQL_CALC_FOUND_ROWS
    t.id, t.name, MAX(COALESCE(r.date, t.date)) AS date, COUNT(r.id) AS replies
FROM
    wp_pod_tbl_forum t
LEFT OUTER JOIN
    wp_pod_tbl_forum r ON (r.topic_id = t.id)
WHERE
    t.topic_id = 0
GROUP BY
    t.id
ORDER BY
    date DESC LIMIT 0,20;

There are about 2,100 total items in this table, and queries usually take a whopping 6 seconds. I added an INDEX to the “topic_id” column, but that didn’t help much. Are there any ways of speeding up this query w/out doing significant restructuring?

EDIT: not quite working yet. I can’t seem to get the examples below to work properly.

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.