Oracle undo镜像数据探究
我们都知道，undo是为了保证一致性读的，也就是说你在对更改的数据但是没有提交，那么其他session是无法查看到你更改的内容的，如果对方
进行读取数据，依然是之前的数据，undo就是保存了数据的前镜像。看完我写的这篇文章之后，你就知道undo是怎么保存前镜像的以及它的内部机
制是什么。现在开始挖掘undo是怎么保存前镜像的。
1）首先更新一条数据，但是不进行commit如下：
SQL> col name for a50
SQL> select obj#,name from t where rownum<5;
OBJ# NAME
---------- --------------------------------------------------
46 I_USER1
28 Rhys1
15 RHYS20
29 RHYS3
SQL> update t set name='rhys1' where obj#=46;
1 row updated.
SQL> update t set name='rhys2' where obj#=28;
1 row updated.
SQL> update t set name='rhys3' where obj#=15;
1 row updated.
SQL> update t set name='rhys4' where obj#=29;
1 row updated.
SQL> select xidusn,xidslot,ubafil,ubablk from v$transaction;
XIDUSN XIDSLOT UBAFIL UBABLK
---------- ---------- ---------- ----------
12 7 8 1493
SQL> select * from v$rollname where usn=12;
USN NAME
---------- ------------------------------
12 _SYSSMU12_584745277$
SQL> select usn,status,curext from v$rollstat;
USN STATUS CUREXT
---------- --------------- ----------
0 ONLINE 5
11 ONLINE 4
12 ONLINE 3
13 ONLINE 2
14 ONLINE 3
15 ONLINE 2
16 ONLINE 2
17 ONLINE 14
18 ONLINE 4
19 ONLINE 3
20 ONLINE 2
11 rows selected.
SQL> select usn,status,curext,xacts from v$rollstat where xacts>0;
USN STATUS CUREXT XACTS
---------- --------------- ---------- ----------
12 ONLINE 3 1
SQL> alter system dump undo header '_SYSSMU12_584745277$';
System altered.
SQL> alter system dump datafile 8 block 1493;
System altered.
SQL> col name for a80
SQL> col inst_id for 9999
SQL> col value for a80
SQL> select * from v$diag_info;
INST_ID NAME VALUE
------- -------------------------------------------------------------------------------- --------------------------------------------------------------------------------
1 Diag Enabled TRUE
1 ADR Base /opt/app/oracle
1 ADR Home /opt/app/oracle/diag/rdbms/rhys/RHYS
1 Diag Trace /opt/app/oracle/diag/rdbms/rhys/RHYS/trace
1 Diag Alert /opt/app/oracle/diag/rdbms/rhys/RHYS/alert
1 Diag Incident /opt/app/oracle/diag/rdbms/rhys/RHYS/incident
1 Diag Cdump /opt/app/oracle/diag/rdbms/rhys/RHYS/cdump
1 Health Monitor /opt/app/oracle/diag/rdbms/rhys/RHYS/hm
1 Default Trace File /opt/app/oracle/diag/rdbms/rhys/RHYS/trace/RHYS_ora_5284.trc
1
