每页有 30 个帖子，逐一取最近 20 条回复，怎样效率最高？

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有一个访问频率非常高的版面，每页显示30个帖子，每个帖子都要按时间顺序显示最近的20条回复。表结构如下：

表A储存帖子，主键`ID`,字段`LAST_REPLIES`,字段`CONTENT`（`LAST_REPLIES`存储最近20条回复）
表B储存回复，主键`ID`,降序索引(`PARENT`,`TIME`),字段`CONTENT`（`PARENT`是被回复帖的ID）

我目前有三个想法

一个是从这30个帖子中取`LAST_REPLIES`，一共600个，然后

SELECT * FROM `B` WHERE `ID` IN (####一共六百条ID####)

另一个是从最近30个帖子中中取`THREAD_ID`,一共30个，然后

SELECT * FROM `B` WHERE `PARENT` = :ID01 ORDER BY `TIME` DESC LIMIT 0,20
UNION ALL
SELECT * FROM `B` WHERE `PARENT` = :ID02 ORDER BY `TIME` DESC LIMIT 0,20
UNION ALL
SELECT * FROM `B` WHERE `PARENT` = :ID03 ORDER BY `TIME` DESC LIMIT 0,20
UNION ALL
...... ......
SELECT * FROM `B` WHERE `PARENT` = :ID30 ORDER BY `TIME` DESC LIMIT 0,20

再一个也取`THREAD_ID`，语句是 [注意降序索引(`PARENT`,`TIME`)]

SET @NUM := 0, @PARENT := 0;
SELECT * , 
    @NUM := if(@PARENT = `PARENT`, @NUM + 1, 1) as "COUNT", 
    @PARENT = `PARENT` AS "DUMMY"
FROM `B` FORCE INDEX(`PARENT`)
GROUP BY `PARENT`
HAVING @NUM <= 20
WHERE `PARENT` IN (####一共三十条ID####)