RE: SMJ, NL or HJ

The CBO, presently does quite a good job. It chooses a sort merge join on the tables. Given my understanding of the data distribution in the tables, I agree its the best execution plan. But this kills my temporary tablespace, ORA-1652. To accomodate this query, I altered the sort area for the session to a high value, and then, I took a hit on my temporary tablespace utilization, not withstanding the rather small values for the extent sizes. And changing it would require me go thru a lot of bureaucracy, change management controls, approvals, the works.

So, I was actually looking for a way to get around using sort merge joins, and not compromise on performance. I would tend to use hash joins, when a join happens between a smaller row source, and a large one. But that, I know, is not the case here. I would like to drive this query via a full table access, since I expect the query to return me about 90% of the rows from each table. So, a nested loop is also not feasible.

Left without an option, I guess. Headed now to put in a change management request :(

Raj

                                                                                                                      
                    "Stephane                                                                                         
                    Faroult"               To:     Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>        
                    <sfaroult_at_oriol        cc:                                                                        
                    ecorp.com>             Subject:     RE: SMJ, NL or HJ                                             
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                    root_at_fatcity.co                                                                                   
                    m                                                                                                 
                                                                                                                      
                                                                                                                      
                    May 03, 2002                                                                                      
                    01:18 PM                                                                                          
                    Please respond                                                                                    
                    to ORACLE-L                                                                                       
                                                                                                                      
                                                                                                                      

Depends. The number of rows matching a given FK may vary widely. Collect stats, and let the CBO decide, it should not have it too wrong in such a case.

>----- Original Message -----
>From: Rajesh.Rao_at_jpmchase.com
>To: Multiple recipients of list ORACLE-L
><ORACLE-L_at_fatcity.com>
>Sent: Fri, 03 May 2002 08:13:27
>
>Hello Gurus,
>
>A SQL tuning question. Given three large tables
>with the same millions of
>rows, and all three are referenced in a query,
>without any filter, as
>under:
>
>Select ..............................
>from largetableA a, largeTableB b, largeTableC c
>where a.empnum = b.empnum
>and a.empnum = c.empnum;
>
>What would be the prefered way of joining these
>tables, Merge Join, Nested
>Loops or Hash Joins?
>
>Thanks
>Raj
>
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> INET: Rajesh.Rao_at_jpmchase.com
>
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Regards,

Stephane Faroult
Oriole

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Author: Stephane Faroul
  INET: sfaroult_at_oriolecorp.com

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  INET: Rajesh.Rao_at_jpmchase.com

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