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From Katherine Marsden <kmarsdende...@sbcglobal.net>
Subject Re: How Much Memory for hash joins
Date Mon, 04 Mar 2013 14:19:00 GMT
On 3/4/2013 5:21 AM, Knut Anders Hatlen wrote:
> Mike Matrigali <mikem_app@sbcglobal.net> writes:
>
>> Also note that these are all estimates within the system.  As Knut
>> pointed out there are some known problems with the estimates.  And
>> even with fixes he has suggested, the code is probably just guessing
>> with things like blobs/clobs.
> I haven't verified, but I think HashJoinStrategy uses
> DataTypeDescriptor.estimatedMemoryUsage() to estimate how much memory
> the hash table will consume. That method has no case for BLOB or CLOB,
> so it looks as if it will return zero for LOB columns. If that's so, it
> will definitely overestimate how many rows fits in maxMemoryPerTable
> kilobytes if the rows contain LOBs.
>
Thanks Knut for finding that, I filed DERBY-6096 for the issue.


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