上次做了下基于scn恢复delete掉的数据后http://www.linuxidc.com/Linux/2012-08/69280.htm，觉得应该好好理解下scn的知识，今天在网上找了下相关的介绍，参考了某些文章，在此我通过实验总结一下。
SCN是当Oracle数据库更新后，由DBMS自动维护去累积递增的一个数字。当一笔交易commit 时，LGWR会将log buffer写入redo log file，同时也会将该笔交易的SCN同步写入到redo log file内（wait-until-completed）。因此当你commit transaction时，在交易成功的讯息返回之前，LGWR必须先完整的完成上述行为之后，否则你是看不到提交成功的回应讯息。
那系统是如何产生一个最新的
select dbms_flashback.get_system_change_number, SCN_TO_TIMESTAMP(dbms_flashback
2 .get_system_change_number) from dual;
GET_SYSTEM_CHANGE_NUMBER
------------------------
SCN_TO_TIMESTAMP(DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER)
---------------------------------------------------------------------------
117947354
24-MAR-11 11.32.22.000000000 AM
也可以用函数
SQL> select timestamp_to_scn(SYSTIMESTAMP) as scn from dual;
control中有三种SCN分别为，system SCN、datafile SCN、last SCN，数据文件头中有一种SCN start SCN
system scn从视图v$database中获得，对应checkpoint_change#字段，datafile scn、last scn分别对应视图v$datafile中的checkpoint_change#，last_change#，而 start scn则从v$datafile_header中checkpoint_change#得到。
数据库在正常启动后下，system scn,datafile scn,start scn会相等，而last scn会被置于无穷大，这里为null。
正常关闭后（immediate,noraml,translate），上面四个scn会应执行full checkpoint 而相等。
当系统在非正常关闭后，如shutdown abort，这个时候last scn依然为无穷大，那么当重新启动实例时，系统首先会比较start scn与system scn，如果一致，那么再比较start scn 与last scan是否一样大，因为是非正常关闭，这里会不一样大，那么就需要例程恢复。
如果打开数据库时发现system scn>datafile scn，那么以为着使用旧的备份数据文件，也就是需要介质恢复
如果是system scn<datafile scn，及控制文件scn是旧的，代表使用了老的控制文件，需要recover using backup controlfile进行恢复。
1、正常启动时
SQL> SELECT checkpoint_change# FROM v$database;
CHECKPOINT_CHANGE#
------------------
117866282
SQL> SELECT file#, checkpoint_change# FROM v$datafile_header;
FILE# CHECKPOINT_CHANGE#
---------- ------------------
1 117866282
2 117866282
3 117866282
4 117866282
5 117866282
6 117866282
7 117866282
8 117866282
9 117866282
10 117866282
11 117866282
FILE# CHECKPOINT_CHANGE#
---------- ------------------
12 117866282
13 117866282
14 117866282
14 rows selected.
SQL> SELECT file#, checkpoint_change#, last_change# FROM v$datafile;
FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
1 117866282
2 117866282
3 117866282
4 117866282
5 117866282
6 117866282
7 117866282
8 117866282
9 117866282
10 117866282
11 117866282
FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
12 117866282
13 117866282
14 117866282
14 rows selected.
2、正常关闭后，然后在startup mount;
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.
Total System Global Area 835104768 bytes
Fixed Size 2217952 bytes
Variable Size 624953376 bytes
Database Buffers 201326592 bytes
Redo Buffers 6606848 bytes
Database mounted.
SQL> SELECT file#, checkpoint_change# FROM v$datafile_header;
FILE# CHECKPOINT_CHANGE#
---------- ------------------
1 117925094
2 117925094
3 117925094
4 117925094
5 117925094
6 117925094
7 117925094
8 117925094
9 117925094
10 117925094
11 117925094
FILE# CHECKPOINT_CHANGE#
---------- ------------------
12 117925094
13 117925094
14 117925094
14 rows selected.
SQL> SELECT checkpoint_change# FROM v$database;
CHECKPOINT_CHANGE#
------------------
117925094
SQL> SELECT file#, checkpoint_change#, last_change# FROM v$datafile;
FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
1 117925094 117925094
2 117925094 117925094
3 117925094 117925094
4 117925094 117925094
5 117925094 117925094
6 117925094 117925094
7 117925094 117925094
8 117925094 117925094
9 117925094 117925094
10 117925094 117925094
11 117925094 117925094
FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
12 117925094 117925094
13 117925094 117925094
14 117925094 117925094
14 rows selected.
--发现start scn=last scn,证明系统是正常关闭
SQL> alter database open;
Database altered.
3、在正常打开状态下进行事务操作
SQL> CREATE TABLE w(a number);
Table created.
SQL> INSERT INTO w VALUES(1);
1 row created.
SQL> commit;
Commit complete.
SQL> INSERT INTO w VALUES(2);
1 row created.
4、非正常关闭
SQL> shutdown abort;
ORACLE instance shut down.
5、打开到mount状态下，观看scn
SQL> startup mount;
ORACLE instance started.
Total System Global Area 835104768 bytes
Fixed Size 2217952 bytes
Variable Size 624953376 bytes
Database Buffers 201326592 bytes
Redo Buffers 6606848 bytes
Database mounted.
SQL> SELECT file#,checkpoint_change#, last_change# FROM v$datafile;
FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
1 117925097
2 117925097
3 117925097
4 117925097
5 117925097
6 117925097
7 117925097
8 117925097
9 117925097
10 117925097
11 117925097
FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
12 117925097
13 117925097
14 117925097
14 rows selected.
SQL> SELECT checkpoint_change# FROM v$database;
CHECKPOINT_CHANGE#
------------------
117925097
SQL> SELECT file#,checkpoint_chnge# FROM v$datafile_header;
SELECT file#,checkpoint_chnge# FROM v$datafile_header
*
ERROR at line 1:
ORA-00904: "CHECKPOINT_CHNGE#": invalid identifier
SQL> SELECT file#, checkpoint_change# FROM v$datafile_header;
FILE# CHECKPOINT_CHANGE#
---------- ------------------
1 117925097
2 117925097
3 117925097
4 117925097
5 117925097
6 117925097
7 117925097
8 117925097
9 117925097
10 117925097
11 117925097
FILE# CHECKPOINT_CHANGE#
---------- ------------------
12 117925097
13 117925097
14 117925097
14 rows selected.
--这时发现start scn 与last scn不等，last scn为无穷大，需要例程恢复
6、改变数据库状态为open，并查看该阶段运行日志
SQL> ALTER DATABASE open;
Database altered.
