Because the world needs more PL/SQL blogs
I did some digging on this issue and found the problem was caused when Transaction Backout executed the following statement which failed: SQL> INSERT INTO json 2 VALUES(EMPTY_CLOB()); INSERT INTO json * ERROR at line 1: ORA-02290: check constraint (DRH.KEEP_IT_REAL) violated At this point I wondered where the EMPTY_CLOB insert was coming from so I … Read more
I was testing transaction backout with non-scalar datatypes. And since I’m doing JSON research I decided to try JSON columns. I discovered that transaction backout does not work with JSON. Not sure just why yet. Here is the test code I used. SQL> CREATE TABLE JSON 2 ( always_json CLOB 3 CONSTRAINT keep_it_real 4* CHECK … Read more
All of the examples I’ve shown so far on this blog rely on transaction names to specify what to backout. This is simpler than specifying transaction ID’s. But it does have potential drawback. It backs out ALL discovered. Here is an example that executes single INSERTs in 3 separate transactions – notice the transaction names … Read more
And here are the results. Two modes raised the error during backout and 2 succeeded. NOCASCADE Raised Error NOCASCADE_FORCE Raised Error CASCADE No Error 2 Records Afterwards DEMO_ID DEMO_NAME ———- ——— 2 Demo 2 1 Demo 1 SQL Executed delete from DRH.DEMO where DEMO_ID = ‘1’ and DEMO_NAME = ‘Demo 1’ insert into DRH.DEMO(DEMO_ID,DEMO_NAME) values … Read more
When backing out transactions from the past using undo SQL statement it is important to consider the current data – especially if the undo SQL violates database constraints. Consider this example scenario that inserts 2 records in one transaction and then deletes them both in a second transaction. Then in a third transaction it re-inserts … Read more
The SQL generated for a backout transaction is available in the USER_FLASHBACK_TXN_REPORT. It’s in XML format but here is a query I concocted to extract it. SQL> SELECT REPLACE(EXTRACTVALUE(column_value,’/USQL’),CHR(9),”) oh_yeah 2 FROM ( SELECT column_value 3 FROM TABLE( SELECT XMLSEQUENCE(XMLTYPE(xid_report).EXTRACT(‘/COMP_XID_REPORT/TRANSACTION/UNDO_SQL/USQL’)) xmlcol 4 FROM user_flashback_txn_report 5 WHERE compensating_xid = ( SELECT xid 6 FROM v$transaction ))) … Read more
In this post I’ll explain how values that get updated back to how the look at the originating transaction are handled. I’ll use three transactions – one inserts a record then a second transaction updates it and then – this is the important part – a third transaction updates the record back to the originally … Read more
Last but definitely not least is the NOCASCADE_FORCE mode. It implements the following rule when backing out transactions. Backout the desired transaction on an as-is, where-is basis. Here’s an example: — save the current SCN SQL> BEGIN 2 hold_stuff.v_scn := DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER; 3 END; 4 / PL/SQL procedure successfully completed. — Perform a single insert in … Read more
The third mode available for backing out transactions is NOCONFLICT_ONLY mode. It implements the following rule: Only backout transactions for records that were not touched by any subsequent transactions. Here’s an example that inserts 2 records in one transaction and then updates one of them in another. Then an attempt is made to backout the … Read more
In my previous blog entry I introduced the NOCASCADE mode. Thus you probably guessed there would be a CASCADE mode as well. And you would be correct. I won’t show all of the SQL for the test case because that is available in the CASCADE blog entry. I’ll just highlight the change and show the … Read more