Re: Primary Key seems to be harmful for performance

I was refering to the fact that i had multiple tables in my query and they all have PKs. I should have been clearer though and say that I have disabled a PK on one of the tables. I can't modify the SQL in any way because it is generated by
a reporting tool (Cognos) it won't add hints or +0 etc.

What I was mostly wondering is whether this situation (PK createing a performance degradation) is a symptom of some other problem which I need to look at. My first suspect was statistics, but I ruled that out by analyzing all tables involved.

thank you

Gene Gurevich
Oracle Engineering
224-405-4079

                                                                           
             Nuno Souto                                                    
             <dbvision_at_iinet.n                                             
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                                       Re: Primary Key seems to be harmful 
             03/08/2006 06:39          for performance                     
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             dbvision_at_iinet.ne                                             
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Quoting genegurevich_at_discoverfinancial.com:

> in over 2 hours. After looking at the results of the tkprof, I decided to
> disable one of the primary keys. After that the

"one of the primary keys"? That would be of one of the tables?

> same query completed in 10 seconds (!). I have reenabled the PK and query
> ran for much longer (I cancelled it after 10 minues). I have disabled
the
> PK and again the SQL finished in 10 sec.
>
> So it looks like PK is a problem performace-wise. How do I fix it? I
don't
> want to drop that PK; I think that it is needed to make
> sure there is no duplicates and from the DM perspective as well. I was
> wondering whether this is a symptom of some problem with the statistics?
> Does anyone have any suggestions?

Without any specific data on execution plans, all I can reasonably suggest is that you use a function in the query to disable use of the pk index by the optimizer. Something like the traditional use of a nvl() around the leading column(s) of the index, or concatenating a null to a string or adding 0 (zero) to a number column.

All of them classical ways of making the optimizer ignore a particular index. You could of course use a hint as well. That'd be a safer propposition than dropping a PK and losing uniqueness checks?

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Cheers
Nuno Souto
from sunny Sydney
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