oracle中rownum效率低的原因以及解决办法

--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中要嵌套三层是因为最里面一层用于排序，倒数第二层用于查询排好序的记录并按顺序标记rownum