mysql 5.7.15 union order by 子查询排序不生效

其实这么写理论上是对的，很多博客教程都是这样子写的。。

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

然后还是发现子查询的排序无效，尴尬，后来才发现如果order by 不带limit，会被优化器干掉，导致语句就是：

select * from (SELECT * FROM article WHERE is_top=0 ) as t1
union
select * from (SELECT * FROM article WHERE is_top=1) as t2

解决方案：

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