I am experiencing the same symptoms as noted in this question which was closed and does not have a satisfactory answer.
Original Question We've recently upgraded from Oracle 10gR2 (10.2.0.4 ) to 11gR2 (18.104.22.168) and we are noticing a significant hit in performance although execution plans are the same for the offended queries before and after the upgrade. Allocation of more memory did improve the performance but just slightly. We also tried to set optimizer mode to 10204 at session level but queries produced no better results. The only difference we've noticed is when enabling autotrace the physical reads on 10gR2 were 0 whereas on 11gR2 it was of the magnitude of tens of thousands.
Including stats demonstrating the physical reads discrepancies:
10gR2 Stats ~~~~~~~~~~
0 recursive calls 0 db block gets 21189 consistent gets 0 physical reads 0 redo size 1014 bytes sent via SQL*Net to client 492 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed
0 recursive calls 0 db block gets 11243 consistent gets 11232 physical reads 0 redo size 1049 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed
Any ideas/suggestions/right-directions would be greatly appreciated.