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
Si su tabla es MyISAM o id no es una PRIMARY KEY , debe crear un índice compuesto en (topic_id, id) .
Si su tabla es InnoDB y id es una PRIMARY KEY , un índice justo en (topic_id) lo hará (id se agregará implícitamente al índice).
Actualizar
Esta consulta probablemente será aún más eficiente, siempre que tenga índices en (topic_id, id) y (date, id) :
Consulte este artículo en mi blog para obtener detalles sobre el rendimiento:
Esta consulta se completa en 30 ms en un 100,000 datos de muestra de filas:
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
Se requieren ambos índices para que esta consulta sea eficiente.
Si su tabla es InnoDB y id es una PRIMARY KEY , entonces puede omitir la identificación de los indexes arriba.
