Issues when upgrading from Oracle 9i :
1. Hash Group by aggregation which allows a hash algorithm to process group by statements. You have to use for sorts ORDER BY.
2. 10g CBO has been added costed subquery unnesting and view merging functionality.
a. Symptoms might range from high parse times to wrong results
3.The CONNECT Role has been changed since 10g R2
4. Parse times may increase after upgrade/migration
Hope it helps....