Re: bad estimation together with large work_mem generates terrible slow hash joins

On Wed, Sep 10, 2014 at 2:25 PM, Heikki Linnakangas
<hlinnakangas@vmware.com> wrote:
> The dense-alloc-v5.patch looks good to me. I have committed that with minor
> cleanup (more comments below). I have not looked at the second patch.

Gah.  I was in the middle of doing this.  Sigh.

>> * the chunks size is 32kB (instead of 16kB), and we're using 1/4
>>    threshold for 'oversized' items
>>
>>    We need the threshold to be >=8kB, to trigger the special case
>>    within AllocSet. The 1/4 rule is consistent with ALLOC_CHUNK_FRACTION.
>
> Should we care about the fact that if there are only a few tuples, we will
> nevertheless waste 32kB of memory for the chunk? I guess not, but I thought
> I'd mention it. The smallest allowed value for work_mem is 64kB.

I think we should change the threshold here to 1/8th.  The worst case
memory wastage as-is ~32k/5 > 6k.

-- 
Robert Haas
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