performance issue after upgrade to 10.2.0.5

Hi, 

I've got problem with merge statement and it has appeared exacly after 
upgrade to 10.2.0.5 
Executiom time before upgrade was about 1-1.5h and now it takes >4h. 
Execution plan didn't change (full scan of small table and nested loops 
using index on large table). Table has a mview log. 

Some stats from awr comparing to the same merge before upgrade: 
cpu time - 49,410 vs 3,253,170 
physical reads/exec 29,623 vs 1,447,152 
and it generates MUCH more undo now. 

some stats from actual trace: 
call            count cpu        elapsed       disk  query    current        rows 
--- ------ --     --------- ---------- ----------    ----------  --------- 
Parse            0     0.00       0.00          0      0                0                  0 
Execute        1      4259.95 16036.0 417980 257456884 262050477   1973401 
Fetch            0      0.00       0.00         0      0                 0                 0 
------- ------  -------- ----------     ---------- -   --------- ---------- 


Event waited on               Times   Max.Wait TotalWaitedWaited 
---------- 
db file sequential read           331046   1.87     7066.03 
latch: cache buffers lru chain    12837    1.40     36.09 
PX Deq: Execute Reply             13612    1.24     592.62 
log buffer space                  1978     0.98     214.82 
log file switch completion        196      0.98     24.14 
buffer busy waits                 239      1.01     79.76 

Only change of db parameters was compatible 10.2.0.4 to 10.2.0.5 
any idead what could be a reason ? 

thanks, hs

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