Right a Wrong in 10g – Undo transactions using FLASHBACK_TRANSACTION_QUERY
Among the topics we discuss in the Oracle 7Up Workshop at AMIS is the Flashback functionality in 9i and 10g databases. We look in depth at Flashback Query and Flashback Session (dbms_flashback.enable_at_timestamp or .enable_at_scn), at Flashback Table and Flashback Table to before undrop. And this week we also took a brief look at how you can undo the results of a certain transaction, not just by using something like
update table set col = (select col from table as of timestampe sysdate - 10/60/24), which works but can be somewhat elaorate to write in full. In this case, we want to take another step and make use of the view FLASHBACK_TRANSACTION_QUERY that has an UNDO_SQL column! Note that erroneously in some publications based on the 10g Beta release this view is called DBA_TRANSACTION_QUERY (I spent quite some time looking for that darned thing!).
I will show in this article how to undo the following changes:
I make some ludicrous changes in the EMP table of the SCOTT schema:
insert into emp (empno, ename, deptno, job, sal) values (1010,'TOBIAS',10,'KLEUTER', 10) / update emp set sal = sal * 1.3 / delete from emp where job='MANAGER' /
and then I commit:
This commits the changes made in this transaction. The data in my table is changed. But not for good as we will see.
In 10g I can use the following query – with the pseudocolumns versions_startscn, versions_endscn, versions_operation and versions_xid – to show the history of the data in my table, in so far still available from the UNDO data. Note that if I wait too long, I may not find the previous state of my records as it may have been moved out of the UNDO area.
SELECT versions_xid , versions_startscn , versions_endscn , DECODE( versions_operation , 'I', 'Insert' , 'U', 'Update' , 'D', 'Delete' , 'Original' ) "Operation" , ename , sal FROM emp VERSIONS BETWEEN SCN MINVALUE AND MAXVALUE /
I have used the
VERSIONS BETWEEN SCN MINVALUE AND MAXVALUE clause to specify that I want to look at the history of EMP. With MINVALUE and MAXVALUE you can indicate that you want the maximum history still available from the UNDO_DATA. Instead of these keywords, you can also specify explicit values for SCN, such as 1937296253. Alternatively, you can use TIMESTAMP instead of SCN.
The result of this query is shown below:
VERSIONS_XID VERSIONS_STARTSCN VERSIONS_ENDSCN Operatio ENAME SAL ---------------- ----------------- --------------- -------- ---------- ---------- 040024006B4B0000 1937296253 Insert TOBIAS 13 040024006B4B0000 1937296253 Delete CLARK 9800 040024006B4B0000 1937296253 Delete BLAKE 11400 040024006B4B0000 1937296253 Delete JONES 12495 040024006B4B0000 1937296253 Update MILLER 1690 040024006B4B0000 1937296253 Update FORD 16380 040024006B4B0000 1937296253 Update JAMES 4940 040024006B4B0000 1937296253 Update TURNER 7800 040024006B4B0000 1937296253 Update ADAMS 6006 040024006B4B0000 1937296253 Update KING 4160 040024006B4B0000 1937296253 Update SCOTT 16380 040024006B4B0000 1937296253 Update MARTIN 6500 040024006B4B0000 1937296253 Update WARD 6500 040024006B4B0000 1937296253 Update ALLEN 8320 040024006B4B0000 1937296253 Update SMITH 4368 1937296253 Original SMITH 3360 1937296253 Original ALLEN 6400 1937296253 Original WARD 5000 1937296253 Original JONES 12495 1937296253 Original MARTIN 5000 1937296253 Original BLAKE 11400 1937296253 Original CLARK 9800 1937296253 Original SCOTT 12600 1937296253 Original KING 3200 1937296253 Original TURNER 6000 1937296253 Original ADAMS 4620 1937296253 Original JAMES 3800 1937296253 Original FORD 12600 1937296253 Original MILLER 1300 29 rows selected.
Note that the record for the new Employee, Tobias, shows up only once, with a salary of 13. His initial salary of 10 was never committed, so it does not exist in the transaction history.
We can now find the UNDO_SQL for transaction 040024006B4B0000 , the value found in the VERSIONS_XID column, from the FLASHBACK_TRANSACTION_QUERY view:
SET PAGESIZE 120 SET LINESIZE 100 TTITLE 'Current FLASHBACK_TRANSACTION_QUERY Contents For Selected Employees' COL xid FORMAT A16 HEADING 'XID#' COL commit_scn FORMAT 99999999 HEADING 'Commit|SCN' COL operation FORMAT A10 HEADING 'Operation' COL logon_user FORMAT A06 HEADING 'User|Logon' COL table_owner FORMAT A06 HEADING 'Table|Owner' COL table_name FORMAT A12 HEADING 'Table Name' COL undo_sql FORMAT A80 HEADING 'UNDO SQL' SELECT operation , logon_user , commit_scn , undo_sql FROM flashback_transaction_query WHERE xid = '040024006B4B0000'
The results look like this:
Fri Apr 22 Current FLASHBACK_TRANSACTION_QUERY Contents For Selected Employee User Table Commit XID# Operation Logon Owner Table Name SCN ---------------- ---------- ------ ------ ------------ --------- UNDO SQL -------------------------------------------------------------------------------- 040024006B4B0000 DELETE SCOTT SCOTT EMP ######### insert into "SCOTT"."EMP"("EMPNO","ENAME","JOB","MGR","HIREDATE","SAL","COMM","D EPTNO") values ('7782','CLARK','MANAGER','7839',TO_DATE('09-JUN-81', 'DD-MON-RR' ),'12740',NULL,'10'); 040024006B4B0000 UPDATE SCOTT SCOTT EMP ######### update "SCOTT"."EMP" set "SAL" = '1300' where ROWID = 'AAAUmPAAEAAABVXAAN'; 040024006B4B0000 INSERT SCOTT SCOTT EMP ######### delete from "SCOTT"."EMP" where ROWID = 'AAAUmPAAEAAABVWAAA';
Several things are clear: the original transaction contained only three statements that acted on multiple records. The UNDO_SQL is created per record. The reason is simple: it is derived from comparing values before and after the transaction, not by interpreting the statement history. Since a record can be changed many times by different statements in a single transaction, this is only logical. We see that an INSERT operation is undone by a delete – no surprise – as well as a DELETE by an INSERT. Here we see illustrated the fact that Bulk Deletes generate so much UNDO log data – and a truncate table is much faster!
We can apply individual UNDO_SQL statements to return to the situation before our somewhat frivolous transaction. We could also write a small piece of PL/SQL that will do that for us.
begin for r_undo in (SELECT operation , logon_user , commit_scn , substr(undo_sql, 1, length(undo_sql) -1) undo_sql -- made this change inspired by Wilfred's comment to strip off the semi-colon FROM flashback_transaction_query WHERE xid = '040024006B4B0000' ) loop execute immediate r_undo.undo_sql; end loop; end;
Note: although this code seems correct to me, when I run it, I get ORA-00911: invalid character errors. So I had to resort to application of indivual SQL statements. I am not sure where these errors come from.
An excellent article that I found when I had already completed most of this post: Oracle 10g Availability Enhancements, Part 3: FLASHBACK Enhancements by Jim Czuprynski