Db Query Tuning

Hi
   
  I had faced an interesting scenario yesterday when I moved some code from Dev 
to QA and tried to execute the queries, they behaved very differently. QA was 
newly refreshed with the data using Export/Import. I had clalcuated the Stat 
using 9i DBMS_STATS and auto sample size. I had even copied the Stats from dev 
to QA to make sure that stats are same but it still failed. The only difference 
between Dev and QA is that QA might have few more record than Dev as Dev was 
refreshed from Prod one week BAck. Anyway Data will continue to change in 
Production and so I don't think that one week Data change shoudl be an issue. 
There is no Hints been used and all init.ora setting including sga size are 
same. 
   
  I had a script of around 15-20 query and each query is very big about 5-20 
pages. Half of them are working fine while others are hanging. Trace will also 
show that it is running  but I appreciate if you can provide any insight as 
what else can be done, Explain Plan is more or less the same except that their 
sequence are different and I don't understand as why it happen when even Stats 
are new or even copied using dbms_stats.export.. procedure. 
   
  TIA
  Sanjay

 
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