我有一个文章表，查询多条文章记录
id是主键，还有若干像title,content,author之类的字段（post表记录300W+）

select * from post  where id in (id1,id2,id3,id4)   

和

select * from post where id = id1 union select * from post where id = id2 union select * from post where id = id3 union select * from post where id = id4  

的效率哪一个更高？(id的个数从1-20条不等)