10g:ora_rowscn伪列介绍和使用
10g里为表新增加了一个伪列ora_rowscn
下面一段英文是官方SQL Reference里的说明：
For each row,ORA_ROWSCNreturns the conservative upper bound system change number (SCN) of the most recent change to the row. This pseudocolumn is useful for determining approximately when a row was last updated. It is not absolutely precise, because Oracle tracks SCNs by transaction committed for the block in which the row resides. You can obtain a more fine-grained approximation of the SCN by creating your tables with row-level dependency tracking.
Please refer toCREATE TABLE ... NOROWDEPENDENCIES | ROWDEPENDENCIESfor more information on row-level dependency tracking.
You cannot use this pseudocolumn in a query to a view. However, you can use it to refer to the underlying table when creating a view. You can also use this pseudocolumn in the WHERE clause of anUPDATE or DELETEstatement.
ORA_ROWSCNis not supported for Flashback Query. Instead, use the version query pseudocolumns,
which are provided explicitly for Flashback Query.
Please refer to the SELECT ... flashback_query_clause for information on Flashback Query and
"Version Query Pseudocolumns" for additional information on those pseudocolumns.
主要是这么几点：
1.默认情况，ora_rowscn记录的scn并不准确，记录的是block的scn。
2.创建表的时候ROWDEPENDENCIES可以来使ora_rowscn真正记录行一级的scn。
3.不能用在查询view时。
4.不能用于flashback query。
可以用来：
1.确认行所在块最后一次修改的scn
2.确认行所在块最后一次修改的大概时间
3.大部分的时候可以确定出两行记录创建的先后顺序(没在同一个块上)
4.如果CREATE TABLE ... ROWDEPENDENCIES 可以精确到行
下面的实验涉及：
1.验证缺省情况Ora_rowscn只记录block级
2.通过scn_to_timestamp 来查询update/insert时间
3.CREATE TABLE ... ROWDEPENDENCIES让ora_rowscn精确到行一级
1.验证缺省情况Ora_rowscn只记录block级
1.1 创建表
SQL> create table emptt as select empno,ename from emp where empno>7700;
表已创建。
1.2 查询ora_rowscn：
SQL> select ora_rowscn,empno,ename from emptt;
ORA_ROWSCN EMPNO ENAME
---------------------- ---------- ----------
794054 7782 CLARK
794054 7788 SCOTT
794054 7839 KING
794054 7844 TURNER
794054 7876 ADAMS
794054 7900 JAMES
794054 7902 FORD
794054 7934 MILLER
已选择8行。
1.3 修改一行：
SQL> update emptt set ename='CLARKCLARK' where empno=7782;
已更新 1 行。
1.4 发现所有行的ora_rowscn都相同，并且rowid显示这些行都在同一个块：
SQL> select rowid,ora_rowscn,empno,ename from emptt;
ROWID ORA_ROWSCN EMPNO ENAME
--------------------------------- ---------- ---------- ----------
AAAM2dAAEAAAAG8AAA 794099 7782 CLARKCLARK
AAAM2dAAEAAAAG8AAB 794099 7788 SCOTT
AAAM2dAAEAAAAG8AAC 794099 7839 KING
AAAM2dAAEAAAAG8AAD 794099 7844 TURNER
AAAM2dAAEAAAAG8AAE 794099 7876 ADAMS
AAAM2dAAEAAAAG8AAF 794099 7900 JAMES
AAAM2dAAEAAAAG8AAG 794099 7902 FORD
AAAM2dAAEAAAAG8AAH 794099 7934 MILLER
已选择8行。
1.5 再向表里插入一些数据：
SQL> insert into emptt
2 select empno,ename from emp;
已创建14行。
SQL> commit;
提交完成。
1.6 现在表在两个块上，不同块上的行ora_rowscn分别不同：
SQL> select rowid,ora_rowscn,empno,ename from emptt;
ROWID ORA_ROWSCN EMPNO ENAME
--------------------------------- --------------- ---------- ----------
AAAM2dAAEAAAAG8AAA 794099 7782 CLARKCLARK
AAAM2dAAEAAAAG8AAB 794099 7788 SCOTT
AAAM2dAAEAAAAG8AAC 794099 7839 KING
AAAM2dAAEAAAAG8AAD 794099 7844 TURNER
AAAM2dAAEAAAAG8AAE 794099 7876 ADAMS
AAAM2dAAEAAAAG8AAF 794099 7900 JAMES
AAAM2dAAEAAAAG8AAG 794099 7902 FORD
AAAM2dAAEAAAAG8AAH 794099 7934 MILLER
AAAM2dAAEAAAAG9AAA 794170 7369 SMITH
AAAM2dAAEAAAAG9AAB 794170 7499 ALLEN
AAAM2dAAEAAAAG9AAC 794170 7521 WARD
AAAM2dAAEAAAAG9AAD 794170 7566 JONES
AAAM2dAAEAAAAG9AAE 794170 7654 MARTIN
AAAM2dAAEAAAAG9AAF 794170 7698 BLAKE
AAAM2dAAEAAAAG9AAG 794170 7782 CLARK
AAAM2dAAEAAAAG9AAH 794170 7788 SCOTT
AAAM2dAAEAAAAG9AAI 794170 7839 KING
AAAM2dAAEAAAAG9AAJ 794170 7844 TURNER
AAAM2dAAEAAAAG9AAK 794170 7876 ADAMS
AAAM2dAAEAAAAG9AAL 794170 7900 JAMES
AAAM2dAAEAAAAG9AAM 794170 7902 FORD
AAAM2dAAEAAAAG9AAN 794170 7934 MILLER
已选择22行。
1.7 更新一行再次验证：
SQL> update emptt set ename='CLARKtt' where ename='CLARKCLARK';
已更新 1 行。
SQL> commit;
提交完成。
SQL> select rowid,ora_rowscn,empno,ename from emptt;
ROWID ORA_ROWSCN EMPNO ENAME
---------------------------------- ---------- ---------- ----------
AAAM2dAAEAAAAG8AAA 794199 7782 CLARKtt
AAAM2dAAEAAAAG8AAB 794199 7788 SCOTT
AAAM2dAAEAAAAG8AAC 794199 7839 KING
AAAM2dAAEAAAAG8AAD 794199 7844 TURNER
AAAM2dAAEAAAAG8AAE 794199 7876 ADAMS
AAAM2dAAEAAAAG8AAF 794199 7900 JAMES
AAAM2dAAEAAAAG8AAG 794199 7902 FORD
AAAM2dAAEAAAAG8AAH 794199 7934 MILLER
AAAM2dAAEAAAAG9AAA 794170 7369 SMITH
AAAM2dAAEAAAAG9AAB 794170 7499 ALLEN
AAAM2dAAEAAAAG9AAC 794170 7521 WARD
AAAM2dAAEAAAAG9AAD 794170 7566 JONES
AAAM2dAAEAAAAG9AAE 794170 7654 MARTIN
AAAM2dAAEAAAAG9AAF 794170 7698 BLAKE
AAAM2dAAEAAAAG9AAG 794170 7782 CLARK
AAAM2dAAEAAAAG9AAH 794170 7788 SCOTT
AAAM2dAAEAAAAG9AAI 794170 7839 KING
AAAM2dAAEAAAAG9AAJ 794170 7844 TURNER
AAAM2dAAEAAAAG9AAK 794170 7876 ADAMS
AAAM2dAAEAAAAG9AAL 794170 7900 JAMES
AAAM2dAAEAAAAG9AAM 794170 7902 FORD
AAAM2dAAEAAAAG9AAN 794170 7934 MILLER
已选择22行。
2. 通过scn_to_timestamp 来查询update/insert时间
此功能很有用。
SQL> col update_time format a40
SQL> select scn_to_timestamp(ora_rowscn) update_time,empno,ename from emptt;
UPDATE_TIME EMPNO ENAME
------------------------------------------------ ---------- ----------
08-5月 -08 10.45.12.000000000 上午 7782 CLARKtt
08-5月 -08 10.45.12.000000000 上午 7788 SCOTT
08-5月 -08 10.45.12.000000000 上午 7839 KING
08-5月 -08 10.45.12.000000000 上午 7844 TURNER
08-5月 -08 10.45.12.000000000 上午 7876 ADAMS
08-5月 -08 10.45.12.000000000 上午 7900 JAMES
08-5月 -08 10.45.12.000000000 上午 7902 FORD
08-5月 -08 10.45.12.000000000 上午 7934 MILLER
08-5月 -08 10.44.09.000000000 上午 7369 SMITH
08-5月 -08 10.44.09.000000000 上午 7499 ALLEN
08-5月 -08 10.44.09.000000000 上午 7521 WARD
08-5月 -08 10.44.09.000000000 上午 7566 JONES
08-5月 -08 10.44.09.000000000 上午 7654 MARTIN
08-5月 -08 10.44.09.000000000 上午 7698 BLAKE
08-5月 -08 10.44.09.000000000 上午 7782 CLARK
08-5月 -08 10.44.09.000000000 上午 7788 SCOTT
08-5月 -08 10.44.09.000000000 上午 7839 KING
08-5月 -08 10.44.09.000000000 上午 7844 TURNER
08-5月 -08 10.44.09.000000000 上午 7876 ADAMS
08-5月 -08 10.44.09.000000000 上午 7900 JAMES
08-5月 -08 10.44.09.000000000 上午 7902 FORD
08-5月 -08 10.44.09.000000000 上午 7934 MILLER
已选择22行。
3.CREATE TABLE ... ROWDEPENDENCIES让ora_rowscn精确到行一级
3.1 创建表
SQL> create table empaa rowdependencies as select empno,ename from emp where empno>7700;
表已创建。
3.2 查询
SQL> select rowid,ora_rowscn,empno,ename from empaa;
ROWID ORA_ROWSCN EMPNO ENAME
------------------------------ ---------- ---------- ----------
AAAM3qAAEAAAAJFAAA 799250 7782 CLARK
AAAM3qAAEAAAAJFAAB 799250 7788 SCOTT
AAAM3qAAEAAAAJFAAC 799250 7839 KING
AAAM3qAAEAAAAJFAAD 799250 7844 TURNER
AAAM3qAAEAAAAJFAAE 799250 7876 ADAMS
AAAM3qAAEAAAAJFAAF 799250 7900 JAMES
AAAM3qAAEAAAAJFAAG 799250 7902 FORD
AAAM3qAAEAAAAJFAAH 799250 7934 MILLER
已选择8行。
3.3更新一行并查看结果
SQL> update empaa set ename='CLARKCLARK' where empno=7782;
已更新 1 行。
没有commit的时候ora_rowscn为空
SQL> select rowid,ora_rowscn,empno,ename from empaa;
ROWID ORA_ROWSCN EMPNO ENAME
------------------------------- ---------- ---------- ----------
AAAM3qAAEAAAAJFAAA 7782 CLARKCLARK
AAAM3qAAEAAAAJFAAB 799250 7788 SCOTT
AAAM3qAAEAAAAJFAAC 799250 7839 KING
AAAM3qAAEAAAAJFAAD 799250 7844 TURNER
AAAM3qAAEAAAAJFAAE 799250 7876 ADAMS
AAAM3qAAEAAAAJFAAF 799250 7900 JAMES
AAAM3qAAEAAAAJFAAG 799250 7902 FORD
AAAM3qAAEAAAAJFAAH 799250 7934 MILLER
已选择8行。
SQL> commit;
提交完成。
Commit之后同一块上的ora_rowscn也能对应到独立行:
SQL> select rowid,ora_rowscn,empno,ename from empaa;
ROWID ORA_ROWSCN EMPNO ENAME
--------------------------------- ---------- ---------- ----------
AAAM3qAAEAAAAJFAAA 799302 7782 CLARKCLARK
AAAM3qAAEAAAAJFAAB 799250 7788 SCOTT
AAAM3qAAEAAAAJFAAC 799250 7839 KING
AAAM3qAAEAAAAJFAAD 799250 7844 TURNER
AAAM3qAAEAAAAJFAAE 799250 7876 ADAMS
AAAM3qAAEAAAAJFAAF 799250 7900 JAMES
AAAM3qAAEAAAAJFAAG 799250 7902 FORD
AAAM3qAAEAAAAJFAAH 799250 7934 MILLER
已选择8行。
附：一个使用ora_rowscn辅助闪回的例子
--创建测试表
create table test (id int,name varchar2(20)) rowdependencies;
--插入测试数据
insert into test values(1,'a');
insert into test values(2,'b');
insert into test values(3,'c');
insert into test values(4,'d');
insert into test values(5,'e');
insert into test values(6,'f');
insert into test values(7,'g');
insert into test values(8,'h');
commit;
--查看数据
select * from test;
/*ID NAME
-- -------
1 a
2 b
3 c
4 d
5 e
6 f
7 g
8 h*/
--查看当前ora_rowscn
select id,name,ora_rowscn,dbms_rowid.rowid_block_number(rowid) blockno from test;
/*ID NAME ORA_ROWSCN BLOCKNO
1 a 8249759 288
2 b 8249759 288
3 c 8249759 288
4 d 8249759 288
5 e 8249759 288
6 f 8249759 288
7 g 8249759 288
8 h 8249759 288*/
--执行更新
update test set name='8' where id=8;
commit
--查看更新后的数据
select * from test;
/*ID NAME
-- -------
1 a
2 b
3 c
4 d
5 e
6 f
7 g
8 8*/
--查看更新后的ora_rowscn
select id,name,ora_rowscn,dbms_rowid.rowid_block_number(rowid) blockno from test;
/*ID NAME ORA_ROWSCN BLOCKNO
1 a 8249759 288
2 b 8249759 288
3 c 8249759 288
4 d 8249759 288
5 e 8249759 288
6 f 8249759 288
7 g 8249759 288
8 8 8249896 288*/
--现在可以通过闪回查询恢复这条记录
select id,name,ora_rowscn,to_char(scn_to_timestamp(ora_rowscn),'yyyy-mm-dd hh24:mi:ss') time,dbms_rowid.rowid_block_number(rowid) blockno from test;
/*ID NAME ORA_ROWSCN TIME BLOCKNO
1 a 8249759 2008-08-01 10:04:19 288
2 b 8249759 2008-08-01 10:04:19 288
3 c 8249759 2008-08-01 10:04:19 288
4 d 8249759 2008-08-01 10:04:19 288
5 e 8249759 2008-08-01 10:04:19 288
6 f 8249759 2008-08-01 10:04:19 288
7 g 8249759 2008-08-01 10:04:19 288
8 8 8249896 2008-08-01 10:08:22 288
*/
select * from test as of timestamp to_date('2008-08-01 10:08:22','yyyy-mm-dd hh24:mi:ss')
--然后删除这条错误记录
delete from test where id=8
--将闪回查询的数据插入原表
insert into test select * from test as of timestamp to_date('2008-08-01 10:08:22','yyyy-mm-dd hh24:mi:ss') where id=8
--查看数据
select * from test
--至此借助ora_rowscn恢复完毕
--如果这个这个表包含LONG数据类型，就采用EXP/IMP方式进行恢复
C:\>exp test/test@acf file=test.dmp tables=test flashback_scn=8249896 query=\"where id=8\"
--然后删除这条错误记录
delete from test where id=8;
--最后倒入这条记录完成恢复
C:\>imp test/test@acf file=test.dmp ignore=y