Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an infinite loop

On 01/29/2018 02:02 AM, Andres Freund wrote:
> On 2018-01-26 18:48:35 -0500, Tom Lane wrote:
>> Andres Freund <andres@anarazel.de> writes:
>>> On 2017-12-10 23:09:42 +0100, Tomas Vondra wrote:
>>>> FWIW I do agree the data sets shared in this thread are pretty extreme
>>>> and it doesn't make much sense to slow the regular cases. I'll be
>>>> perfectly happy if we stop the OOM, making those cases fast is a bonus.
>>
>>> Yea, agreed on that. I'm kinda inclined to go for stop-growing in 10,
>>> and so something better in 11. And then later possibly backpatch if
>>> we've grown some confidence?
>>
>> +1.  I'd like to see some response to this included in 10.2, and time
>> grows short for that.
> 
> Here are two patches that I think we want for 10.2, and the start of one
> that I think we want for master.  0002 is needed because otherwise the
> lack of extra growth leads to noticeably worse performance when filling
> an underestimated a coordinator hash table from the workers - turns out
> our hash combine (and most hash combines) let a lot of clustering
> survive. By adding a final hashing round the bit perturbation is near
> perfect.  The commit messages need to be polished a bit, but other than
> that I think these are reasonable fixes. Plan to push by Monday evening
> at the latest.
> 
> The third patch is a version of the random IV discussed in this
> thread. I do think we want to add usage of the extended hash functions,
> as prototyped by Tomas, as that actually helps to fix issues with actual
> hash conflicts. But we additionally need a fallback path for types
> without extended hashtables, and the random IV is a good idea
> nonetheless.  There's no ABI difference in my patch, so I think this is
> actually something we could backpatch. But I don't think it's urgent, so
> I'm not planning to do that for 10.2.  The one thing that could confuse
> people is that it can lead to output order changes from run to run - I
> think that's actually good, nobody should rely on hashagg etc output
> being stable, but it might be a bit much in a stable release?
> 
> 
> In my tests this solves the worst performance issues in Todd's case,
> Tomas's, Thomas's and still does ok performancwith with a TPC-H Q18
> (which showcases the underestimated worker hashtable into leader
> hashtable issue).
> 

Sounds reasonable.

Do you think repeating the performance testing is necessary? Not sure
when do you plan to push the changes, I may not have time for the tests
until after FOSDEM.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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