Oracle11新特性——撤銷事務（一）

打算寫一系列的文章介紹11g的新特性和變化。

Oracle11g提供了撤銷事務的功能，可以撤銷一個已經提交的事務。

這一篇簡單介紹撤銷事務的用法。

Oracle的發起一個事務後，可以選擇提交或者回滾。回滾的話，會恢復到這個事務發生之前的狀態，而一旦提交，這個事務的所有修改都被儲存到資料庫中。

從11g開始，Oracle提供了撤銷一個已經提交事務的能力。這個功能是透過DBMS_FLASHBACK.TRANSACTION_BACKOUT過程實現的。

首先建立一張測試表：

SQL> CREATE TABLE T (ID NUMBER);

Table created.

SQL> INSERT INTO T VALUES (1);

1 row created.

SQL> COMMIT;

Commit complete.

SQL> SELECT XID, TABLE_NAME, OPERATION, UNDO_SQL FROM FLASHBACK_TRANSACTION_QUERY WHERE TABLE_NAME = 'T';

XID TABLE_NAME OPERATION UNDO_SQL
---------------- ---------- ---------- ------------------------------------------------------------
0007001100000157 T INSERT delete from "YANGTK"."T" where ROWID = 'AAARNZAAFAAACYxAAA';

找到要撤銷的事務後，透過呼叫DBMS_FLASHBACK.TRANSACTION_BACKOUT過程：

SQL> DECLARE
2 V_XID SYS.XID_ARRAY;
3 BEGIN
4 V_XID := SYS.XID_ARRAY('0002003A00000157');
5 DBMS_FLASHBACK.TRANSACTION_BACKOUT(1, V_XID);
6 END;
7 /
DECLARE
*
ERROR at line 1:
ORA-01031: insufficient privileges
ORA-06512: at "SYS.DBMS_FLASHBACK", line 37
ORA-06512: at "SYS.DBMS_FLASHBACK", line 70
ORA-06512: at line 5

系統報錯沒有許可權，當前使用者擁有DBA角色，仍然許可權不足，看來必須要用SYS來執行：

SQL> SELECT * FROM SESSION_ROLES;

ROLE
------------------------------
CONNECT
RESOURCE
DBA
SELECT_CATALOG_ROLE
HS_ADMIN_ROLE
EXECUTE_CATALOG_ROLE
DELETE_CATALOG_ROLE
EXP_FULL_DATABASE
IMP_FULL_DATABASE
DATAPUMP_EXP_FULL_DATABASE
DATAPUMP_IMP_FULL_DATABASE
GATHER_SYSTEM_STATISTICS
SCHEDULER_ADMIN
WM_ADMIN_ROLE
JAVA_ADMIN
JAVA_DEPLOY
XDBADMIN
XDB_SET_INVOKER
OLAP_XS_ADMIN
OLAP_DBA

20 rows selected.

使用SYS執行報錯如下：

SQL> CONN / AS SYSDBA
Connected.
SQL> DECLARE
2 V_XID XID_ARRAY;
3 BEGIN
4 V_XID := SYS.XID_ARRAY('0002003A00000157');
5 DBMS_FLASHBACK.TRANSACTION_BACKOUT(1, V_XID);
6 END;
7 /
DECLARE
*
ERROR at line 1:
ORA-55510: Mining could not start
ORA-06512: at "SYS.DBMS_FLASHBACK", line 37
ORA-06512: at "SYS.DBMS_FLASHBACK", line 70
ORA-06512: at line 5

對於這個錯誤，Oracle的文件上的描述是：

ORA-55510: Mining could not start
Cause: Mining could not start for the following reasons.
1. A logminer session was processing
2. The database was not mounted or not opened for read and write
3. Minimum supplemental logging was not enabled
4. Archiving was not enabled
Action: Fix the mentioned problems and try again. Note that if you enable supplemental logging now, you will not be able to remove a transaction that has committed without supplemental logging.

檢查上面這幾項，似乎只是違反了第三項：

SQL> SELECT LOG_MODE, SUPPLEMENTAL_LOG_DATA_MIN FROM V$DATABASE;

LOG_MODE SUPPLEME
------------ --------
ARCHIVELOG NO

下面設定資料庫SUPPLEMENTAL LOG DATA：

SQL> ALTER DATABASE ADD SUPPLEMENTAL LOG DATA;

Database altered.

SQL> DECLARE
2 V_XID XID_ARRAY;
3 BEGIN
4 V_XID := SYS.XID_ARRAY('0002003A00000157');
5 DBMS_FLASHBACK.TRANSACTION_BACKOUT(1, V_XID);
6 END;
7 /
DECLARE
*
ERROR at line 1:
ORA-55515: Mining sees input transaction changes without seeing transaction start
ORA-06512: at "SYS.DBMS_FLASHBACK", line 37
ORA-06512: at "SYS.DBMS_FLASHBACK", line 70
ORA-06512: at line 5

這個錯誤造成的原因顯然是由於INSERT語句發生在設定SUPPLEMENTAL LOG DATA之後，Oracle在LOG中無法找到這個事務。

而且，如果執行與事務發生的間隔超過了UNDO_RETENTION的設定，那麼還可能出現下面的錯誤：

SQL> DECLARE
2 V_XID XID_ARRAY;
3 BEGIN
4 V_XID := SYS.XID_ARRAY('0002003A00000157');
5 DBMS_FLASHBACK.TRANSACTION_BACKOUT(1, V_XID);
6 END;
7 /
DECLARE
*
ERROR at line 1:
ORA-01555: snapshot too old: rollback segment number with name "" too small
ORA-06512: at "SYS.DBMS_FLASHBACK", line 37
ORA-06512: at "SYS.DBMS_FLASHBACK", line 70
ORA-06512: at line 5

從上面的錯誤已經可以看到，Oracle的撤銷事務是透過FLASHBACK查詢配合LOGMINER進行的。由於LOGMINER操作只能SYS來進行操作，因此撤銷事務也必須使用SYS使用者。

瞭解了使用撤銷事務可能碰到的問題，下面看一個真正的例子：

SQL> CONN YANGTK/yangtk
Connected.
SQL> CREATE TABLE T_FLASH_TRANS (ID NUMBER, NAME VARCHAR2(30));

Table created.

SQL> INSERT INTO T_FLASH_TRANS VALUES (1, 'A');

1 row created.

SQL> INSERT INTO T_FLASH_TRANS VALUES (2, 'B');

1 row created.

SQL> COMMIT;

Commit complete.

SQL> INSERT INTO T_FLASH_TRANS SELECT 3 + ROWNUM, 'C' FROM TAB;

4 rows created.

SQL> COMMIT;

Commit complete.

SQL> SELECT XID, TABLE_NAME, OPERATION, UNDO_SQL FROM FLASHBACK_TRANSACTION_QUERY
2 WHERE TABLE_NAME = 'T_FLASH_TRANS';

XID TABLE_NAME OPERATIO UNDO_SQL
---------------- ------------- -------- --------------------------------------------------------------------
0002001100000171 T_FLASH_TRANS INSERT delete from "YANGTK"."T_FLASH_TRANS" where ROWID = 'AAARO3AAFAAACY4AAB';
0002001100000171 T_FLASH_TRANS INSERT delete from "YANGTK"."T_FLASH_TRANS" where ROWID = 'AAARO3AAFAAACY4AAA';
000600180000016D T_FLASH_TRANS INSERT delete from "YANGTK"."T_FLASH_TRANS" where ROWID = 'AAARO3AAFAAACY4AAC';
000600180000016D T_FLASH_TRANS INSERT delete from "YANGTK"."T_FLASH_TRANS" where ROWID = 'AAARO3AAFAAACY4AAD';
000600180000016D T_FLASH_TRANS INSERT delete from "YANGTK"."T_FLASH_TRANS" where ROWID = 'AAARO3AAFAAACY4AAE';
000600180000016D T_FLASH_TRANS INSERT delete from "YANGTK"."T_FLASH_TRANS" where ROWID = 'AAARO3AAFAAACY4AAF';

6 rows selected.

對於T_FLASH_TRANS表存在兩個事務，分別插入了2條和4條記錄，下面利用SYS來撤銷掉第一個事務：

SQL> CONN / AS SYSDBA
Connected.
SQL> DECLARE
2 V_XID XID_ARRAY;
3 BEGIN
4 V_XID := SYS.XID_ARRAY('0002001100000171');
5 DBMS_FLASHBACK.TRANSACTION_BACKOUT(1, V_XID);
6 END;
7 /

PL/SQL procedure successfully completed.

下面看看T_FLASH_TRANS表中的資料：

SQL> SELECT * FROM YANGTK.T_FLASH_TRANS;

ID NAME
---------- ------------------------------
4 C
5 C
6 C
7 C