scn是数据库对自身变化的一个标记。通过一个序列号能够反映出数据库在那个时间点正在进行的操作，scn没有选用时间来作为基准单位，可能也是因为时间的不确定性，比如当前时间为2014年3月14号晚上八点整，如果修改了系统时间，改为晚上七点钟，name重启数据库以后，那个时间段的操作就都乱套了。这个scn在一般的库上都看似比较大，根据数据库的设计角度来说，这个scn能够使用很长很长的时间。
scn的变化也基本分为四类，实例级别scn，数据文件scn，数据文件头scn,结束scn，为了测试scn的变化情况，准备了下面的场景
1)全局检查点更新  如alter system checkpoint，数据库级scn应该会发生变化。
2)resize 数据文件  把数据文件的大小进行改变
3)切换redo日志文件
4)归档当前日志文件
5)创建表
6)插入数据
7)插入一些数据，频繁的commit
8)插入一些数据，频繁的rollback

测试的结果如下,对于发生变化的部分都用黄色进行了标注。可以比对测试场景对比学习一下。有些场景没有涉及到，目的只是向大家分享一下日常的操作中scn的变化。

场景	database level					datafile&file header (header不包括last_change#列)		redo
	Chk_change#	Ctl_change#	Ctl_time	Curr_scn	Arch_change#	Chk_change#	Last_change#	First_change#	Nxt_change#
开始	3626097	3626320	2014-05-09 02:56:11	3626335	3626097	pool_data 3626318 others    3626097		504377	514666
1	3626336	3626336	2014-05-09 02:56:36	3626337	3626097	3626336		504377	514666
2	3626336	3626338	2014-05-09 02:56:38	3626346	3626097	3626336		504377	514666
3	3626336	3626349	2014-05-09 02:56:38	3626350	3626097	3626336		504377	514666
4	3626336	3626354	2014-05-09 02:56:39	3626355	3626354	3626336		504377	514666
5	3626336	3626354	2014-05-09 02:56:39	3626417	3626354	3626336		504377	514666
6	3626336	3626354	2014-05-09 02:56:39	3626442	3626354	3626336		504377	514666
7	3626336	3626354	2014-05-09 02:56:39	3626459	3626354	3626336		504377	514666
8	3626336	3626354	2014-05-09 02:56:39	3626462	3626354	3626336		504377	514666

生成scn快照的脚本如下,对于每一个操作，都可以使用下面的脚本从数据库级，数据文件，数据文件头，在线日志等维度进行scn的查验。
sqlplus -s n1/n1 < set pages 20
prompt ######scn from database level
col checkpoint_change# format 99999999999999999
col RESETLOGS_CHANGE# format 99999999999999999
col PRIOR_RESETLOGS_CHANGE# format 99999999999999999 
col CONTROLFILE_CHANGE# format 99999999999999999 
col ARCHIVELOG_CHANGE# format 99999999999999999 
col CURRENT_SCN format 99999999999999999
col control_time format a20
set linesize 200
select RESETLOGS_CHANGE# ,RESETLOGS_TIME,PRIOR_RESETLOGS_CHANGE#,CHECKPOINT_CHANGE#,CONTROLFILE_CHANGE# ,to_char(CONTROLFILE_TIME,'yyyy-mm-dd hh24:mi:ss') control_time,ARCHIVELOG_CHANGE#,CURRENT_SCN from v\$database;


prompt #####scn from datafile


col checkpoint_change# format 99999999999999999
col creation_change# format 99999999999999999
col checkpoint_change# format 99999999999999999
col last_change# format 99999999999999999
col online_change# format 99999999999999999
col online_time format a20
col last_time format a20
col checkpoint_time format a20
col creation_time format a9
col file# format 999
set linesize 200
select file#,creation_change#,creation_time,checkpoint_change#,to_char(checkpoint_time,'yyyy-mm-dd hh24:mi:ss') checkpoint_time,last_change#,to_char(last_time,'yyyy-mm-dd hh24:mi:ss')last_time,offline_change#,online_change#,to_char(online_time,'yyyy-mm-dd hh24:mi:ss')online_time from v\$datafile;




prompt #####scn from datafile header
col tablespace_name format a10
col resetlogs_change# format 99999999999999999
col creation_time format a9
--col undo_opt_current_change# format 99999999999999999
col checkpoint_time format a20
set linesize 200
select file#,creation_change#,creation_time,tablespace_name,resetlogs_change#,resetlogs_time,checkpoint_change#,checkpoint_time,checkpoint_count from v\$datafile_header;


prompt #####scn from redo   
col first_change# format 99999999999999999
col next_change# format 99999999999999999
col sequence# format 99999999999999999
select *from (select recid,sequence#,first_change#,next_change# from v\$log_history )
where rownum<20;


EOF
exit



对如上的测试场景中scn的变化进行总结，一共会生成9个快照。
ksh showscn.sh >beginning_snshowscn.shot.lst0
sqlplus -s n1/n1 <

alter system checkpoint;  --全局，数据文件，数据文件头部scn都递增，保持一致
EOF
ksh showscn.sh > checkpoint_snshowscn.shot.lst1
sqlplus  -s n1/n1 < alter database datafile '/u03/ora11g/oradata/TEST01/pool_data02.dbf' resize 160M;   -->只有数据库级scn递增
EOF
ksh showscn.sh > resize_datafile_snshowscn.shot.lst2
sqlplus  -s n1/n1 < alter system switch logfile;  -->日志文件scn递增
EOF
ksh showscn.sh > redo_switch_snshowscn.shot.lst3
sqlplus  -s n1/n1 < alter system archive log current;
EOF
ksh showscn.sh >archive_current_snshowscn.shot.lst4


sqlplus  -s n1/n1 < create table aaaa as select * from dba_objects;
EOF
ksh showscn.sh >create_tab_snshowscn.shot.lst5


sqlplus -s n1/n1 < insert into aaaa select *from aaaa;
commit;
EOF
ksh showscn.sh >insert_tab_snshowscn.shot.lst6


sqlplus  -s n1/n1 < insert into aaaa select *from aaaa where rownum<10;
commit;
insert into aaaa select *from aaaa where rownum<10;
commit;
insert into aaaa select *from aaaa where rownum<10;
commit;
insert into aaaa select *from aaaa where rownum<10;
commit;
insert into aaaa select *from aaaa where rownum<10;
commit;
insert into aaaa select *from aaaa where rownum<10;
commit;
insert into aaaa select *from aaaa where rownum<10;
commit;
insert into aaaa select *from aaaa where rownum<10;
commit;
EOF
ksh showscn.sh >over_commit_snshowscn.shot.lst7


sqlplus  -s n1/n1 < insert into aaaa select *from aaaa where rownum<10;
insert into aaaa select *from aaaa where rownum<10;
insert into aaaa select *from aaaa where rownum<10;
insert into aaaa select *from aaaa where rownum<10;
insert into aaaa select *from aaaa where rownum<10;
insert into aaaa select *from aaaa where rownum<10;
insert into aaaa select *from aaaa where rownum<10;
insert into aaaa select *from aaaa where rownum<10;
rollback;
EOF
ksh showscn.sh >rollback_snshowscn.shot.lst8

运行脚本的日志如下：
System altered.
Database altered.
System altered.
System altered.
Table created.
13470 rows created.
Commit complete.
9 rows created.
Commit complete.
9 rows created.
Commit complete.
9 rows created.
Commit complete.
9 rows created.
Commit complete.
9 rows created.
Commit complete.
9 rows created.
Commit complete.
9 rows created.
Commit complete.
9 rows created.
Commit complete
9 rows created.
9 rows created.
9 rows created.
9 rows created.
9 rows created.
9 rows created.
9 rows created.
9 rows created.
Rollback complete.
生成的快照如下：
-rw-r--r-- 1 ora11g dba 3144 May  9 02:56 beginning_snshowscn.shot.lst0
-rw-r--r-- 1 ora11g dba 3144 May  9 02:56 checkpoint_snshowscn.shot.lst1
-rw-r--r-- 1 ora11g dba 3144 May  9 02:56 resize_datafile_snshowscn.shot.lst2
-rw-r--r-- 1 ora11g dba 3144 May  9 02:56 redo_switch_snshowscn.shot.lst3
-rw-r--r-- 1 ora11g dba 3144 May  9 02:56 archive_current_snshowscn.shot.lst4
-rw-r--r-- 1 ora11g dba 3144 May  9 02:56 create_tab_snshowscn.shot.lst5
-rw-r--r-- 1 ora11g dba 3144 May  9 02:56 insert_tab_snshowscn.shot.lst6
-rw-r--r-- 1 ora11g dba 3144 May  9 02:56 over_commit_snshowscn.shot.lst7
-rw-r--r-- 1 ora11g dba 3144 May  9 02:56 rollback_snshowscn.shot.lst8