mysql 5.7.15 union order by 子查詢排序不生效

其實這麼寫理論上是對的，很多博客教程都是這樣子寫的。。

select * from (SELECT * FROM article WHERE is_top=0 ORDER BY id DESC) as t1

union

select * from (SELECT * FROM article WHERE is_top=1 ORDER BY start_time desc) as t2

然後還是發現子查詢的排序無效，尷尬，後來才發現如果order by 不帶limit，會被優化器幹掉，導致語句就是：

select * from (SELECT * FROM article WHERE is_top=0 ) as t1

union

select * from (SELECT * FROM article WHERE is_top=1) as t2

解決方案：

select * from (SELECT * FROM article WHERE is_top=0 ORDER BY id DESC limit 999999) as t1

union

select * from (SELECT * FROM article WHERE is_top=1 ORDER BY start_time desc limit 999999) as t2