SQL exists 和not exists 替代in not in 问题

项目中遇到个超级长的sql。拷贝出来,用java 程序判断了下.总计字符长度 3464.里里外外的看的头晕.用oracle监测工具这里消耗时间特长.需要优化.
看了半天,牵涉业务逻辑,还是先改简单的吧.用exists 和not exists 替代in not in 。
A:

select uu.user_id
  from tab_uum_users uu
 where uu.company_id in
       (select u.company_id from tab_uum_users u where u.user_id =21800001900)
   and uu.user_id not in (21800001900)

改成
B:

select a.user_id
  from tab_uum_users a
 where exists (select u.company_id
          from tab_uum_users u
         where u.user_id = 21800001900
           and a.company_id = u.company_id)
   and not exists (select uuu.user_id
          from tab_uum_users uuu
         where a.user_id = uuu.user_id
           and uuu.user_id = 21800001900)

在表 tab_uum_users记录23933条的情况下.
开始执行时间B比A快0.01
后续A和B几乎一样0.14秒左右.貌似没看出效果么.