Tuesday, September 03, 2013

How to accomplish on commit trigger in Oracle

If you have any use case for on commit trigger, it is almost always due to a given application using database improperly, fixing application design can take some time, especially if it is third party vendor application. In the spirit of knowing and keeping all possible lever's at DBA's disposal, one can use below approach to define on commit trigger for enforcing any data integrity checks at transaction level. This post is not to encourage anybody to use triggers, one should avoid them as much as possible.

The trick here is to leverage userenv('commitscn') feature.

You can read more about on commit trigger research at http://technology.amis.nl/2006/08/07/the-hunt-for-the-oracle-database-on-commit-trigger/

Jonathan Lewis has great information about commitscn at http://www.jlcomp.demon.co.uk/commit.html

Here is the test case:

01:38:59 SQL> create table tr1(a number not null, b number, primary key(a));

Table created.

01:38:59 SQL> create table tr2(a number not null, b number);

Table created.

01:38:59 SQL> create table tr3(a number not null, b number, primary key(a));

Table created.

01:38:59 SQL>
01:38:59 SQL> create or replace trigger tr1_tr before insert on tr1
01:38:59   2  for each row
01:38:59   3  begin
01:38:59   4  insert into tr2 values (1,userenv('commitscn'));
01:38:59   5  end;
01:38:59   6  /

Trigger created.

01:38:59 SQL>
01:38:59 SQL> create or replace trigger tr2_tr before update on tr2
01:38:59   2  for each row
01:38:59   3  begin
01:38:59   4  insert into tr3 values (0,0);
01:38:59   5  end;
01:38:59   6  /

Trigger created.

01:38:59 SQL>
01:38:59 SQL> insert into tr1 values (1,1);

1 row created.

01:38:59 SQL> --insert into tr2 values (1, userenv('commitscn'));
01:38:59 SQL> select * from tr1;

                   A                    B
-------------------- --------------------
                   1                    1

01:38:59 SQL> select * from tr2;

                   A                    B
-------------------- --------------------
                   1             40208814

01:38:59 SQL> select * from tr3;

no rows selected

01:38:59 SQL> commit;

Commit complete.

01:38:59 SQL> select * from tr1;

                   A                    B
-------------------- --------------------
                   1                    1

01:38:59 SQL> select * from tr2;

                   A                    B
-------------------- --------------------
                   1             40208816

01:38:59 SQL> select * from tr3;

                   A                    B
-------------------- --------------------
                   0                    0

01:38:59 SQL>
01:38:59 SQL>
01:38:59 SQL> insert into tr1 values (2,2);

1 row created.

01:38:59 SQL> --insert into tr2 values (2, userenv('commitscn'));
01:38:59 SQL> select * from tr1;

                   A                    B
-------------------- --------------------
                   1                    1
                   2                    2

01:38:59 SQL> select * from tr2;

                   A                    B
-------------------- --------------------
                   1             40208816
                   1             40208819

01:38:59 SQL> select * from tr3;

                   A                    B
-------------------- --------------------
                   0                    0

01:38:59 SQL> commit;
commit
*
ERROR at line 1:
ORA-00001: unique constraint (TEST123.SYS_C003852) violated
ORA-06512: at "TEST123.TR2_TR", line 2
ORA-04088: error during execution of trigger 'TEST123.TR2_TR'

01:38:59 SQL> select * from tr1;

                   A                    B
-------------------- --------------------
                   1                    1

01:38:59 SQL> select * from tr2;

                   A                    B
-------------------- --------------------
                   1             40208816

01:38:59 SQL> select * from tr3;

                   A                    B
-------------------- --------------------
                   0                    0

TR1 can be real application table, I introduced TR2 and TR3 tables to leverage commitscn feature, and all of this can be done without changing application code. By inserting one row into TR2 table with commitscn value, Oracle would automatically update commitscn value it inserted in TR2 table at commit time. Since we have before update trigger created on TR2 table, it will be fired at commit time. 

Any error during TR2 table based trigger execution will automatically cause the entire transaction to be rolled back (as seen above).

5 Comments:

At September 8, 2013 at 3:48:00 AM PDT, Blogger sam said...

Thanks Sai for the post. On-commit trigger can be useful in tracking age of the data as well as this workaround can be used to reject/accept transaction on base table depending on desired business logic. These can be also achieved by dml triggers. What can be a use case for on-commit trigger which cant be done using dml triggers ?

 
At September 8, 2013 at 5:44:00 PM PDT, Blogger Saibabu Devabhaktuni said...

Sam,

Yes, the need for on commit trigger is some what rare (only when data integrity rules at transaction level need to be enforced, i.e. in the lines of how deferred constraint work), it can also be used to work around mutating trigger problem

Thanks,
Sai.

 
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