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
如果你的表是MyISAM或id不是PRIMARY KEY，你需要创建一个复合ondex (topic_id, id)。
如果您的表是，InnoDB并且id是PRIMARY KEY，则仅创建索引就(topic_id)可以了（id将隐式添加到索引中）。
这个查询很可能会更加有效，前提是你必须对索引(topic_id, id)和(date, id)：
该查询完成了30 ms对100,000行的样本数据：
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
这两个索引都是有效查询所必需的。
如果您的表是，InnoDB id是PRIMARY KEY，则您可以从indexes上面省略id 。
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