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From Harshad <harshad...@gmail.com>
Subject Re: Performance issue of derby using JDBC
Date Sat, 20 Dec 2008 09:04:25 GMT
Rick Hillegas wrote:
>> I would have thought that the DISTINCT is not redundant and actually
>> allows for the transformation into a normal join.
>>   
> I think that, for instance, a problem arises when there are M tuples in
> PROVIDES with name='Fred' but there are < M distinct ids in those
> tuples. In that case, you can imagine data sets on which the original
> query returns < M tuples but the transformed query returns M tuples.

You are right. I goofed up.

> It may be that Derby did apply the EXISTS transformation--and that
> degraded rather than improved the performance. If you are interested in
> seeing the plan which Derby selected, please consult the following wiki
> pages:
> 
>   http://wiki.apache.org/db-derby/PerformanceDiagnosisTips
>   http://wiki.apache.org/db-derby/StmtExecutionPlan

I tried looking at the optimiser query plan, but it is beyond my understanding right now.
Thanks for all your help and the links. I will use them as reference later.

cheers,
Harshad


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