Re: Using quicksort for every external sort run

On Mon, Dec 14, 2015 at 7:22 PM, Peter Geoghegan <pg@heroku.com> wrote:
> On Mon, Dec 14, 2015 at 6:58 PM, Greg Stark <stark@mit.edu> wrote:
>> I ran sorts with various parameters on my small NAS server.
>
> ...
>
>> without the extra memory optimizations.
>
> Thanks for taking the time to benchmark the patch!
>
> While I think it's perfectly fair that you didn't apply the final
> on-the-fly merge "memory pool" patch, I also think that it's quite
> possible that the regression you see at the very low end would be
> significantly ameliorated or even eliminated by applying that patch,
> too. After all, Jeff Janes had a much harder time finding a
> regression, probably because he benchmarked all patches together.

The regression I found when building an index on a column of
400,000,000 md5(random()::text) with 64MB maintenance_work_mem was not
hard to find at all.  I still don't understand what is going on with
it, but it is reproducible.  Perhaps it is very unlikely and I just
got very lucky in finding it immediately after switching to that
data-type for my tests, but I wouldn't assume that on current
evidence.

If we do think it is important to almost never cause regressions at
the default maintenance_work_mem (I am agnostic on the importance of
that), then I think we have more work to do here.  I just don't know
what that work is.

Cheers,

Jeff