今天是2013-10-15，论坛有个朋友提这个问题：

简单介绍下情景： DELETE 一张大表数据，未执行完，就把session 杀掉了。
kill session 后。smon占CPU 仍然超过60%，难道是在回滚数据？

我的测试过程：

session 1：
SQL> select count(*) from amy;

COUNT(*)
----------
1047140

SQL> select * from v$mystat where rownum<2;

SID STATISTIC# VALUE
---------- ---------- ----------
46 0 0
session 2：
SQL> select sid,serial#,username from v$session where username is not null;

SID SERIAL# USERNAME
---------- ---------- ------------------------------
37 127 SYS
46 15 RHYS
48 11 SYS
50 23 SYS
然后再rhys下删除amy表：
SQL> delete from amy;
delete from amy
*
ERROR at line 1:
ORA-00028: your session has been killed
ORA-00028: your session has been killed


SQL>
在session 2 然后kill 该session：
SQL> alter system kill session '46,15';

System altered.

SQL> select usn,xacts,latch,extents,gets,waits,hwmsize,status,curext,curblk from v$rollstat where xacts>0;

USN XACTS LATCH EXTENTS GETS WAITS HWMSIZE STATUS CUREXT CURBLK
---------- ---------- ---------- ---------- ---------- ---------- ---------- --------------- ---------- ----------
3 1 0 106 82251 10 402776064 ONLINE 102 393

SQL>
select usn,xid,slt,state,undoblocksdone,undoblockstotal,cputime,parentslt,rcvserSQL> vers from v$fast_start_transactions;

USN XID SLT STATE UNDOBLOCKSDONE UNDOBLOCKSTOTAL CPUTIME PARENTSLT RCVSERVERS
---------- ---------------- ---------- ---------------- -------------- --------------- ---------- ---------- ----------
3 0300000068050000 0 RECOVERING 4479 14624 16 0 1
3 03001F0065050000 31 RECOVERED 3953 3953 58 1

SQL> r
1* select usn,xid,slt,state,undoblocksdone,undoblockstotal,cputime,parentslt,rcvservers from v$fast_start_transactions

USN XID SLT STATE UNDOBLOCKSDONE UNDOBLOCKSTOTAL CPUTIME PARENTSLT RCVSERVERS
---------- ---------------- ---------- ---------------- -------------- --------------- ---------- ---------- ----------
3 0300000068050000 0 RECOVERING 4479 14624 17 0 1
3 03001F0065050000 31 RECOVERED 3953 3953 58 1
结论：没错，是需要回滚，有时候大的事务回滚导致很严重的性能问题，有时候为了加快速度，设置fast_start_parallel_rollback 为high