当系统、数据文件以及数据文件头这三个checkpoint_change#一致(只读、脱机表空间除外)时，数据库才能正常打开。
正常关库时，会生成新的检查点，写入上述三个checkpoint_change#，同时数据文件中的last_change#也会记录下该检查点，也就是说三个checkpoint_change#与last_change#记录着同一个值。
数据库打开前，先确定是否介质恢复，再确定是否实例恢复。介质恢复主要是更新旧的文件，而实例恢复主要是更新内存。如果last_change#值为空，则说明需要进行实例恢复，恢复后的数据库才能打开。
正常关闭的数据库是干净的，一致的
SQL> shutdown normal;
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.
Total System Global Area 1068937216 bytes
Fixed Size 2220200 bytes
Variable Size 629149528 bytes
Database Buffers 432013312 bytes
Redo Buffers 5554176 bytes
Database mounted.
SQL> select file#,status,name,checkpoint_change#,last_change# from v$datafile
FILE# STATUS NAME CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------- ------------------------- ------------------ ------------
1 SYSTEM +DATA/susu/datafile/syste 33003415 33003415
m.256.1029148521
2 ONLINE +DATA/susu/datafile/sysau 33003415 33003415
x.257.1029148523
3 ONLINE +DATA/susu/datafile/undot 33003415 33003415
bs1.258.1029148523
4 ONLINE +DATA/susu/datafile/users 33003415 33003415
.259.1029148523
5 ONLINE +DATA/susu/datafile/examp 33003415 33003415
le.269.1029149011
SQL> select checkpoint_change# from v$database;
CHECKPOINT_CHANGE#
------------------
33003415
SQL> select file#,checkpoint_change# from v$datafile_header;
FILE# CHECKPOINT_CHANGE#
---------- ------------------
1 33003415
2 33003415
3 33003415
4 33003415
5 33003415
SQL> alter database open;
Database altered.
SQL> select status from v$instance;
STATUS
------------
OPEN
非正常关闭
SQL> shutdown abort;
ORACLE instance shut down.
SQL> startup nomount;
ORACLE instance started.
Total System Global Area 1068937216 bytes
Fixed Size 2220200 bytes
Variable Size 629149528 bytes
Database Buffers 432013312 bytes
Redo Buffers 5554176 bytes
SQL> alter database mount;
Database altered.
SQL> select file#,status,name,checkpoint_change#,last_change# from v$datafile;
FILE# STATUS NAME CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------- ------------------------- ------------------ ------------
1 SYSTEM +DATA/susu/datafile/syste 33006255
m.256.1029148521
2 ONLINE +DATA/susu/datafile/sysau 33006255
x.257.1029148523
3 ONLINE +DATA/susu/datafile/undot 33006255
bs1.258.1029148523
4 ONLINE +DATA/susu/datafile/users 33006255
.259.1029148523
5 ONLINE +DATA/susu/datafile/examp 33006255
le.269.1029149011
SQL> select checkpoint_change# from v$database;
CHECKPOINT_CHANGE#
------------------
33003418
SQL>
SQL> select file#,checkpoint_change# from v$datafile_header;
FILE# CHECKPOINT_CHANGE#
---------- ------------------
1 33006255
2 33006255
3 33006255
4 33006255
5 33006255
SQL> recover database;
Media recovery complete.
SQL> alter database open;
Database altered.
SQL> select checkpoint_change# from v$database;
CHECKPOINT_CHANGE#
------------------
33026259
SQL>
SQL>
SQL>
SQL>
SQL> select file#,checkpoint_change# from v$datafile_header;
FILE# CHECKPOINT_CHANGE#
---------- ------------------
1 33026259
2 33026259
3 33026259
4 33026259
5 33026259