or an Undo Segment"
See Also:
Oracle Database 2 Day DBA and Oracle Database Administrator's Guide to learn how to manage undo segments
Transaction Rollback
When a ROLLBACK statement is issued, the database uses undo records to roll back changes made to the database by the uncommitted transaction. During recovery, the database rolls back any uncommitted changes applied from the online redo log to the data files. Undo records provide read consistency by maintaining the before image of the data for users accessing data at the same time that another user is changing it.
至此undo基本原理介绍完毕。
另外对于undo retention 参数了解详见：http://blog.csdn.net/rhys_oracle/article/details/10162043
既然这样，那么就收集一下awr查看一下吧。
| Event |
Waits |
Time(s) |
Avg Wait(ms) |
% Total Call Time |
Wait Class |
| CPU time |
|
1,657 |
|
56.2 |
|
| db file sequential read |
562,614 |
998 |
2 |
33.8 |
User I/O |
| db file scattered read |
47,756 |
89 |
2 |
3.0 |
User I/O |
| inactive session |
68 |
66 |
977 |
2.3 |
Other |
| SQL*Net more data from client |
7,652 |
42 |
5 |
1.4 |
Network |
发现数据库确实存在一定的问题“INACTIVE SESSION"等待事件严重。
首先介绍v$session status状态信息：
那么目前数据库的会话状态是什么样的呢？
（信息做了相应处理）
SQL>
SQL> select A.SID,B.SPID,A.SERIAL#,a.lockwait,A.USERNAME,A.OSUSER,a.logon_time,a.last_call_et/3600 LAST_HOUR,A.STATUS,
2 'orakill '||sid||' '||spid HOST_COMMAND,
3 'alter system kill session '''||A.sid||','||A.SERIAL#||'''' SQL_COMMAND
4 from v$session A,V$PROCESS B where A.PADDR=B.ADDR AND SID>6
5 ;
SID SPID SERIAL# LOCKWAIT USERNAME OSUSER LOGON_TIME LAST_HOUR STATUS HOST_COMMAND SQL_COMMAND
---------- ------------ ---------- ---------------- ------------------------------ ------------------------------ ----------- ---------- -------- ------------------------------------------------------------- --------------------------------------------------------------------------------
1105 20355 1 orarep 2014-03-07 322.764166 ACTIVE orakill 1105 20355 alter system kill session '1105,1'
1099 20367 1 orarep 2014-03-07 322.764166 ACTIVE orakill 1099 20367 alter system kill session '1099,1'
1092 20381 1 orarep 2014-03-07 322.764166 ACTIVE orakill 1092 20381 alter system kill session '1092,1'
1100 20365 1 orarep 2014-03-07 322.764166 ACTIVE orakill 1100 20365 alter system kill session '1100,1'
1093 20379 1 orarep 2014-03-07 322.764166 ACTIVE orakill 1093 20379 alter system kill session '1093,1'
1101 20363 1 orarep 2014-03-07 322.764166 ACTIVE orakill 1101 20363 alter system kill session '1101,1'
1094 20377 1 orarep 2014-03-07 322.764166 ACTIVE orakill 1094 20377 alter system kill session '1094,1'
1087 20392 1 orarep 2014-03-07 322.761666 ACTIVE orakill 1087 20392 alter system kill session '1087,1'
1097 20371 1 orarep 2014-03-07 322.764166 ACTIVE orakill 1097 20371 alter system kill session '1097,1'
1098 20369 1 orarep 2014-03-07 322.764166 ACTIVE orakill 1098 20369 alter system kill session '1098,1'
1104 20357 1 o