Oracle数据库中的4种SCN号
一个SCN号就是一个oracle数据库中使用的逻辑的、内部的时间戳。这个SCN号可以表明一个事件在oracle数据库中发生的精确时间。
Oracle数据库中的SCN号分为四种，分别为：系统检查点SCN、数据文件scn、结束scn：、数据文件头scn。
系统检查点SCN：
系统检查点SCN位于控制文件中，当检查点进程启动时(ckpt)，Oracle就把系统检查点的SCN存储到控制文件中。
查询系统检查点SCN的命令如下SQL> select CHECKPOINT_CHANGE# from v$database; CHECKPOINT_CHANGE# ------------------ 2017133
数据文件SCN：
当ckpt进程启动时，包括全局范围的（比如日志切换）以及文件级别的检查点（将表空间置为只读、begin backup或将某个数据文件设置为offline等），这时会在控制文件中记录的scn。
查询数据文件SCN的命令如下
SQL> select a.TS#,a.name,b.name,b.checkpoint_change# from v$tablespace a, v$datafile b where a.ts#=b.ts#(+) order by ts#;
TS# NAME NAME CHECKPOINT_CHANGE#
---------- -------------------------------------------------- -------------------------------------------------- ------------------
0 SYSTEM /u01/app/oracle/oradata/PROD1/system01.dbf 2028292
1 SYSAUX /u01/app/oracle/oradata/PROD1/sysaux01.dbf 2028292
2 UNDOTBS1 /u01/app/oracle/oradata/PROD1/undotbs01.dbf 2028292
3 TEMP
4 USERS /u01/app/oracle/oradata/PROD1/users01.dbf 2028292
6 EXAMPLE /u01/app/oracle/oradata/PROD1/example01.dbf 2028292
7 TEST /u01/app/oracle/oradata/PROD1/test.tbs 2028292
8 DEV_MDS /u01/app/oracle/oradata/PROD1/DEV_mds.dbf 2028292
9 DEV_IAS_TEMP
10 DEV_BIPLATFORM /u01/app/oracle/oradata/PROD1/DEV_biplatform.dbf 2028292
11 DEV1_ODI_TEMP
13 DEV2_ODI_TEMP
14 DEV2_ODI_USER /u01/app/oracle/oradata/PROD1/DEV2_odi_user.dbf 2028292
13 rows selected.
SQL> alter tablespace test read only;
Tablespace altered.
SQL> select a.TS#,a.name,b.name,b.checkpoint_change# from v$tablespace a, v$datafile b where a.ts#=b.ts#(+) order by ts#;
TS# NAME NAME CHECKPOINT_CHANGE#
---------- -------------------------------------------------- -------------------------------------------------- ------------------
0 SYSTEM /u01/app/oracle/oradata/PROD1/system01.dbf 2028292
1 SYSAUX /u01/app/oracle/oradata/PROD1/sysaux01.dbf 2028292
2 UNDOTBS1 /u01/app/oracle/oradata/PROD1/undotbs01.dbf 2028292
3 TEMP
4 USERS /u01/app/oracle/oradata/PROD1/users01.dbf 2028292
6 EXAMPLE /u01/app/oracle/oradata/PROD1/example01.dbf 2028292
7 TEST /u01/app/oracle/oradata/PROD1/test.tbs 2028314
8 DEV_MDS /u01/app/oracle/oradata/PROD1/DEV_mds.dbf 2028292
9 DEV_IAS_TEMP
10 DEV_BIPLATFORM /u01/app/oracle/oradata/PROD1/DEV_biplatform.dbf 2028292
11 DEV1_ODI_TEMP
13 DEV2_ODI_TEMP
14 DEV2_ODI_USER /u01/app/oracle/oradata/PROD1/DEV2_odi_user.dbf 2028292
13 rows selected.
SQL> alter tablespace test read write;
Tablespace altered.
SQL> select a.TS#,a.name,b.name,b.checkpoint_change# from v$tablespace a, v$datafile b where a.ts#=b.ts#(+) order by ts#;
TS# NAME NAME CHECKPOINT_CHANGE#
---------- -------------------------------------------------- -------------------------------------------------- ------------------
0 SYSTEM /u01/app/oracle/oradata/PROD1/system01.dbf 2028292
1 SYSAUX /u01/app/oracle/oradata/PROD1/sysaux01.dbf 2028292
2 UNDOTBS1 /u01/app/oracle/oradata/PROD1/undotbs01.dbf 2028292
3 TEMP
4 USERS /u01/app/oracle/oradata/PROD1/users01.dbf 2028292
6 EXAMPLE /u01/app/oracle/oradata/PROD1/example01.dbf 2028292
7 TEST /u01/app/oracle/oradata/PROD1/test.tbs 2028338
8 DEV_MDS /u01/app/oracle/oradata/PROD1/DEV_mds.dbf 2028292
9 DEV_IAS_TEMP
10 DEV_BIPLATFORM /u01/app/oracle/oradata/PROD1/DEV_biplatform.dbf 2028292
11 DEV1_ODI_TEMP
13 DEV2_ODI_TEMP
14 DEV2_ODI_USER /u01/app/oracle/oradata/PROD1/DEV2_odi_user.dbf 2028292
13 rows selected.
SQL> alter system checkpoint;
System altered.
SQL> select a.TS#,a.name,b.name,b.checkpoint_change# from v$tablespace a, v$datafile b where a.ts#=b.ts#(+) order by ts#;
TS# NAME NAME CHECKPOINT_CHANGE#
---------- -------------------------------------------------- -------------------------------------------------- ------------------
0 SYSTEM /u01/app/oracle/oradata/PROD1/system01.dbf 2028370
1 SYSAUX /u01/app/oracle/oradata/PROD1/sysaux01.dbf 2028370
2 UNDOTBS1 /u01/app/oracle/oradata/PROD1/undotbs01.dbf 2028370
3 TEMP
4 USERS /u01/app/oracle/oradata/PROD1/users01.dbf 2028370
6 EXAMPLE /u01/app/oracle/oradata/PROD1/example01.dbf 2028370
7 TEST /u01/app/oracle/oradata/PROD1/test.tbs 2028370
8 DEV_MDS /u01/app/oracle/oradata/PROD1/DEV_mds.dbf 2028370
9 DEV_IAS_TEMP
10 DEV_BIPLATFORM /u01/app/oracle/oradata/PROD1/DEV_biplatform.dbf 2028370
11 DEV1_ODI_TEMP
13 DEV2_ODI_TEMP
14 DEV2_ODI_USER /u01/app/oracle/oradata/PROD1/DEV2_odi_user.dbf 2028370
13 rows selected.从上述结果中可以看到当我们将test表空间转换为read only状态后，他的scn号比其他数据未见SCN号要大。
结束SCN：
每个数据文件都有一个结束scn，在数据库的正常运行中，只要数据文件在线且是可读写的，结束scn为null。否则则存在具体的scn值。结束scn也记录在控制文件中。
SQL> select a.TS#,a.name,b.name,b.checkpoint_change#,b.LAST_CHANGE# from v$tablespace a, v$datafile b where a.ts#=b.ts#(+) order by ts#;
TS# NAME NAME CHECKPOINT_CHANGE# LAST_CHANGE#
---------- -------------------------------------------------- -------------------------------------------------- ------------------ ------------
0 SYSTEM /u01/app/oracle/oradata/PROD1/system01.dbf 2028370
1 SYSAUX /u01/app/oracle/oradata/PROD1/sysaux01.dbf 2028370
2 UNDOTBS1 /u01/app/oracle/oradata/PROD1/undotbs01.dbf 2028370
3 TEMP
4 USERS /u01/app/oracle/oradata/PROD1/users01.dbf 2028370
6 EXAMPLE /u01/app/oracle/oradata/PROD1/example01.dbf 2028370
7 TEST /u01/app/oracle/oradata/PROD1/test.tbs 2028370
8 DEV_MDS /u01/app/oracle/oradata/PROD1/DEV_mds.dbf 2028370
9 DEV_IAS_TEMP
10 DEV_BIPLATFORM /u01/app/oracle/oradata/PROD1/DEV_biplatform.dbf 2028370
11 DEV1_ODI_TEMP
TS# NAME NAME CHECKPOINT_CHANGE# LAST_CHANGE#
---------- -------------------------------------------------- -------------------------------------------------- ------------------ ------------
13 DEV2_ODI_TEMP
14 DEV2_ODI_USER /u01/app/oracle/oradata/PROD1/DEV2_odi_user.dbf 2028370
13 rows selected.
SQL> alter tablespace test read only;
Tablespace altered.
SQL> select a.TS#,a.name,b.name,b.checkpoint_change#,b.LAST_CHANGE# from v$tablespace a, v$datafile b where a.ts#=b.ts#(+) order by ts#;
TS# NAME NAME CHECKPOINT_CHANGE# LAST_CHANGE#
---------- -------------------------------------------------- -------------------------------------------------- ------------------ ------------
0 SYSTEM /u01/app/oracle/oradata/PROD1/system01.dbf 2028370
1 SYSAUX /u01/app/oracle/oradata/PROD1/sysaux01.dbf 2028370
2 UNDOTBS1 /u01/app/oracle/oradata/PROD1/undotbs01.dbf 2028370
3 TEMP
4 USERS /u01/app/oracle/oradata/PROD1/users01.dbf 2028370
6 EXAMPLE /u01/app/oracle/oradata/PROD1/example01.dbf 2028370
7 TEST /u01/app/oracle/oradata/PROD1/test.tbs 2029996 2029996
8 DEV_MDS /u01/app/oracle/oradata/PROD1/DEV_mds.dbf 2028370
9 DEV_IAS_TEMP
10 DEV_BIPLATFORM /u01/app/oracle/oradata/PROD1/DEV_biplatform.dbf 2028370
11 DEV1_ODI_TEMP
TS# NAME NAME CHECKPOINT_CHANGE# LAST_CHANGE#
---------- -------------------------------------------------- -------------------------------------------------- ------------------ ------------
13 DEV2_ODI_TEMP
14 DEV2_ODI_USER /u01/app/oracle/oradata/PROD1/DEV2_odi_user.dbf 2028370
13 rows selected.
可见test表空间的数据文件结束SCN不为空，而其他数据文件结束SCN均为空。
如果将数据库处于mount状态下，所有数据文件均不可写，所以都具有结束SCN。
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.
Total System Global Area 722366464 bytes
Fixed Size 2216864 bytes
Variable Size 251661408 bytes
Database Buffers 461373440 bytes
Redo Buffers 7114752 bytes
Database mounted.
SQL> select a.TS#,a.name,b.name,b.checkpoint_change#,b.LAST_CHANGE# from v$tablespace a, v$datafile b where a.ts#=b.ts#(+) order by ts#;
TS# NAME NAME CHECKPOINT_CHANGE# LAST_CHANGE#
---------- -------------------------------------------------- -------------------------------------------------- ------------------ ------------
0 SYSTEM /u01/app/oracle/oradata/PROD1/system01.dbf 2030818 2030818
1 SYSAUX /u01/app/oracle/oradata/PROD1/sysaux01.dbf 2030818 2030818
2 UNDOTBS1 /u01/app/oracle/oradata/PROD1/undotbs01.dbf 2030818 2030818
3 TEMP
4 USERS /u01/app/oracle/oradata/PROD1/users01.dbf 2030818 2030818
6 EXAMPLE /u01/app/oracle/oradata/PROD1/example01.dbf 2030818 2030818
7 TEST /u01/app/oracle/oradata/PROD1/test.tbs 2029996 2029996
8 DEV_MDS /u01/app/oracle/oradata/PROD1/DEV_mds.dbf 2030818 2030818
9 DEV_IAS_TEMP
10 DEV_BIPLATFORM /u01/app/oracle/oradata/PROD1/DEV_biplatform.dbf 2030818 2030818
11 DEV1_ODI_TEMP
TS# NAME NAME CHECKPOINT_CHANGE# LAST_CHANGE#
---------- -------------------------------------------------- -------------------------------------------------- ------------------ ------------
13 DEV2_ODI_TEMP
14 DEV2_ODI_USER /u01/app/oracle/oradata/PROD1/DEV2_odi_user.dbf 2030818 2030818
13 rows selected.
数据文件头SCN：
不同于上述的SCN数据文件开始scn记录在每个数据文件中。当发生系统及文件级别的检查点后，不仅将这时的SCN号记录在控制文件中，同样也记录在数据文件中。
查询数据文件头SCN的命令如下SQL> select tablespace_name,file#,CHECKPOINT_CHANGE# from v$datafile_header; TABLESPACE_NAME FILE# CHECKPOINT_CHANGE# ------------------------------ ---------- ------------------ SYSTEM 1 2030818 SYSAUX 2 2030818 UNDOTBS1 3 2030818 USERS 4 2030818 EXAMPLE 5 2030818 TEST 6 2029996 DEV_MDS 7 2030818 DEV_BIPLATFORM 8 2030818 DEV2_ODI_USER 9 2030818 9 rows selected.