选择在联盟中花费大量时间所有按日期排序desc？

Question

我有一个选择来获取警报。

select 'comments' prefix, c.foto, c.data as data, c.user, concat(k.user, ' comments your post') as logs from comentarios c 
inner join posts p on c.foto = p.id 
inner join cadastro k on c.user = k.id 
where p.user = 1 and p.user <> c.user and c.delete = 0

union all

select 'like' prefix, l.post, l.data as data, l.user, concat(k.user, ' liked your post') as logs from likes l 
inner join posts p on l.post = p.id 
inner join cadastro k on l.user = k.id 
where p.user = 1 and l.user <> p.user

order by data desc
limit 5

问题是如果我以这种方式运行选择，则需要2.4871秒。

如果我删除了select 'like'... = 0.0024，但如果我将此选择分开，则只有0.010而没有数据描述的顺序。

任何想法为什么？我不知道问题是工会还是订单...... 它们都被很好地编入索引。