A client has an ETL process that checks for success by ensuring –

Source Rows = Rows inserted + Rows in DML error logging table.

This is normally fine, however one needs to be aware that rows inserted by DML error logging are done so autonomously, outside the parent transaction. Thus you need to be careful when combining [Rows Inserted] (which is atomic) and [Rows in DML error logging table] (which is not).

As an example, if you combine all four of the below conditions you’ll get a scenario that’ll cause the above check to fail unnecessary.

1. DML Error Logging
2. 11.2.0.2
3. Deferred Segment Creation
4. Partitioning

Create demonstration table with indexes:

SQL> ALTER SESSION SET deferred_segment_creation = true
Session altered.
SQL> CREATE TABLE DML_DFC (ID     VARCHAR2(30)
                       ,NAME VARCHAR2(20)
                       ,ACTIVE varchar2(1 byte))
PARTITION BY LIST (ACTIVE)
(  
  PARTITION P_ACTIVE VALUES ('Y'),  
  PARTITION P_INACTIVE VALUES (NULL)
)
Table created.
SQL> BEGIN
dbms_errlog.create_error_log(dml_table_name     => 'DML_DFC',
                                err_log_table_name => 'DML_DFC_ERR');
END;
PL/SQL procedure successfully completed.
SQL> CREATE UNIQUE INDEX DFX_PK ON DML_DFC(id)
Index created.
SQL> CREATE UNIQUE INDEX DFC_IX01 ON DML_DFC(NAME)
Index created.

Confirm Deferred Segment Creation:

SQL> SELECT count(*) 
FROM user_extents
WHERE segment_name = 'DML_DFC'

  COUNT(*)
----------
         0
1 row selected.

Capture a few useful statistics before running the test case:

SQL> select name, value
FROM         v$sesstat a
INNER JOIN
  v$statname b
ON     a.statistic# = b.statistic#
  AND b.name like '%rollbacks%'
  AND a.sid = (select sid from v$mystat where rownum = 1)
order by 1

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
IMU CR rollbacks                                                          0
cleanouts and rollbacks - consistent read gets                            0
rollbacks only - consistent read gets                                     0
transaction rollbacks                                                     0
transaction tables consistent read rollbacks                              0
user rollbacks                                                            0

Simulate an ETL load with four source rows:

SQL> MERGE
INTO DML_DFC dst
USING (
      SELECT *
      FROM
      (
         select  '1'         	AS id
                  ,null    	AS name
                  ,null    	as ACTIVE
         FROM DUAL
         UNION ALL
         select  '1'         	AS id
                  ,null    	AS name
                  ,null    	as ACTIVE                  
         FROM DUAL
         UNION ALL
         select  '2'        	AS id
                  ,'hello'    	AS name
                  ,null    	as ACTIVE                  
         FROM DUAL
         UNION ALL
         select  '3'         	AS id
                  ,'hello'    	AS name
                  ,null    	as ACTIVE                  
         FROM DUAL                  
      )
      ORDER BY 1,2
      ) src
ON (dst.id = src.id)
WHEN NOT MATCHED THEN INSERT
VALUES (src.id
       ,src.name
       ,src.active)
LOG ERRORS INTO DML_DFC_err('tdh')
      REJECT LIMIT UNLIMITED

Monitor the changes to statistics captured earlier:

SQL> select name, value
FROM         v$sesstat a
INNER JOIN
  v$statname b
ON     a.statistic# = b.statistic#
  AND b.name like '%rollbacks%'
  AND a.sid = (select sid from v$mystat where rownum = 1)
order by 1

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
IMU CR rollbacks                                                          0
cleanouts and rollbacks - consistent read gets                            0
rollbacks only - consistent read gets                                     0
transaction rollbacks                                                     1
transaction tables consistent read rollbacks                              0
user rollbacks                                                            0

Interesting – At some point during the statement executing, Oracle has performed an implicit rollback and restart. If you weren’t aware that Oracle could do this, see Tom Kyte’s series on Transaction Restarts – Part i is here.
That series, from 2005, predate Deferred Segment Creation so it’s not a scenario that’ll be mentioned there.

Check the inserted and rejected row counts:

SQL> SELECT count(*) FROM DML_DFC

  COUNT(*)
----------
         2
1 row selected.
SQL> SELECT count(*) FROM DML_DFC_err

  COUNT(*)
----------
         3
1 row selected.

Oh dear – We had 4 source rows, we’ve inserted 2 but we’ve managed to reject 3 rows!

For the moment, the application in question has disabled deferred segment creation but the strategic fix is to find an implementation that doesn’t have an implicit expectation that DML error logging is transactional.