oracle中rownum效率低的原因以及解決辦法

--A低效率

select t.*
from (select *
from GET_ENTER_EXAM_SCORE t
where signupyear = '2013'
and signupseason = '春季') t
where rownum < 100

--B高效率

select t.*
from (select b.*, rownum r
from (select *
from GET_ENTER_EXAM_SCORE a
where signupyear = '2013'
and signupseason = '春季'
order by zkzh) b) t
where r < 100

其中B中要嵌套三層是因為最里面一層用於排序，倒數第二層用於查詢排好序的記錄並按順序標記rownum