SQL is running SLOW in group by

Question

Dear Experts,
I have a query which is fetching around 2,603,675 records. When it is running without group clause it is running fine but as soon as Group by claused comes into picture it's dying. I have seent the plan which is correct and hitting to right indexs with right order. We have oracle 11g. Could some one please help me how to get rid of this? Below is the expaling plan for the same. Cost Bytes and Carfinality looks ok to me.
Plan
SELECT STATEMENT  HINT: ALL_ROWSCost: 1,363  Bytes: 304  Cardinality: 1                                                                   
     31 HASH GROUP BY  Cost: 1,363  Bytes: 304  Cardinality: 1                                                              
          30 NESTED LOOPS                                                         
               28 NESTED LOOPS  Cost: 1,362  Bytes: 304  Cardinality: 1                                                    
                    25 NESTED LOOPS OUTER  Cost: 1,360  Bytes: 183  Cardinality: 1                                               
                         22 NESTED LOOPS  Cost: 1,359  Bytes: 168  Cardinality: 1                                          
                              19 NESTED LOOPS  Cost: 1,357  Bytes: 121  Cardinality: 1                                     
                                   16 NESTED LOOPS  Cost: 1,356  Bytes: 107  Cardinality: 1                                
                                        13 NESTED LOOPS  Cost: 1,356  Bytes: 100  Cardinality: 1                           
                                             6 NESTED LOOPS  Cost: 54  Bytes: 2,209  Cardinality: 47                      
                                                  2 TABLE ACCESS BY INDEX ROWID TABLE FDM.WH_SOURCES_D Cost: 2  Bytes: 16  Cardinality: 1                 
                                                       1 INDEX UNIQUE SCAN INDEX (UNIQUE) FDM.WSRC_UK1 Cost: 1  Cardinality: 1            
                                                  5 INLIST ITERATOR                 
                                                       4 TABLE ACCESS BY INDEX ROWID TABLE FDM.WH_ACCOUNTS_D Cost: 52  Bytes: 1,457  Cardinality: 47            
                                                            3 INDEX UNIQUE SCAN INDEX (UNIQUE) FDM.WACC_UK1 Cost: 50  Cardinality: 47       
                                             12 PARTITION RANGE SINGLE  Cost: 1,356  Bytes: 53  Cardinality: 1  Partition #: 15  Partitions accessed #KEY(AP)                    
                                                  11 TABLE ACCESS BY LOCAL INDEX ROWID TABLE FDM.WH_ATM_BALANCES_F Cost: 1,356  Bytes: 53  Cardinality: 1  Partition #: 15  Partitions accessed #KEY(AP)               
                                                       10 BITMAP CONVERSION TO ROWIDS            
                                                            9 BITMAP AND       
                                                                 7 BITMAP INDEX SINGLE VALUE INDEX (BITMAP) FDM.WABF_BM6 Partition #: 15  Partitions accessed #KEY(AP)
                                                                 8 BITMAP INDEX SINGLE VALUE INDEX (BITMAP) FDM.WABF_BM10 Partition #: 15  Partitions accessed #KEY(AP)
                                        15 TABLE ACCESS BY INDEX ROWID TABLE FDM.T_SDM_GLPRODUCT Cost: 0  Bytes: 7  Cardinality: 1                           
                                             14 INDEX UNIQUE SCAN INDEX (UNIQUE) FDM.TSGP_PK Cost: 0  Cardinality: 1                      
                                   18 TABLE ACCESS BY INDEX ROWID TABLE FDM.WH_PRODUCTS_D Cost: 1  Bytes: 14  Cardinality: 1                                
                                        17 INDEX UNIQUE SCAN INDEX (UNIQUE) FDM.WPRD_PK Cost: 0  Cardinality: 1                           
                              21 TABLE ACCESS BY INDEX ROWID TABLE FDM.WH_COMMON_TRADES_D Cost: 2  Bytes: 47  Cardinality: 1                                     
                                   20 INDEX UNIQUE SCAN INDEX (UNIQUE) FDM.WCTD_PK Cost: 1  Cardinality: 1                                
                         24 TABLE ACCESS BY INDEX ROWID TABLE FDM.T_SDM_SECURITYINSTRUMENT Cost: 1  Bytes: 15  Cardinality: 1                                          
                              23 INDEX UNIQUE SCAN INDEX (UNIQUE) FDM.TSSI_PK Cost: 0  Cardinality: 1                                     
                    27 PARTITION RANGE SINGLE  Cost: 1  Cardinality: 1  Partition #: 29  Partitions accessed #KEY(AP)                                             
                         26 INDEX RANGE SCAN INDEX (UNIQUE) FDM.WBKS_UK2 Cost: 1  Cardinality: 1  Partition #: 29  Partitions accessed #KEY(AP)                                        
               29 TABLE ACCESS BY LOCAL INDEX ROWID TABLE FDM.WH_BOOKS_D Cost: 2  Bytes: 121  Cardinality: 1  Partition #: 29  Partitions accessed #1Edited by: BluShadow on 21-Oct-2011 08:57
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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            

Answer 1

user535789 wrote:
Dear Experts,
I have a query which is fetching around 2,603,675 records. When it is running without group clause it is running fine but as soon as Group by claused comes into picture it's dying. I have seent the plan which is correct and hitting to right indexs with right order. We have oracle 11g. Could some one please help me how to get rid of this? Below is the expaling plan for the same. Cost Bytes and Carfinality looks ok to me.I think you have a common misunderstanding there.
Your original query did run fine fetching 2.6 million records?
Usually such claims are only made when the query just returned the first few records (default 50 for tools like sql deeveloper or toad).
How long did it take till you saw the very last record?
If you add a group by then essentially you tell the database it has to do a sort operation over the whole dataset. Because it might be that the very last record has some information that would influence the first output row that is shown on the screen. Therefore the database has to read all the records including the very last one (no. 2,603,675) before it can show you one single output line. Same goes for all kind of sort operations, but the most typical are "Order by", "distinct", "group by", "union".
My guess is that both queries take approximately the same time. You just don't know whow slow the original query was, becuase you saw some output rows while the query was still running.