On 08/31/2017 02:56 AM, Peter Geoghegan wrote:
> On Wed, Aug 30, 2017 at 5:38 PM, Robert Haas <robertmh...@gmail.com> wrote:
>> Wow.  Just to be clear, I am looking for the BEST case for replacement
>> selection, not the worst case.  But I would have expected that case to
>> be a win for replacement selection, and it clearly isn't.  I can
>> reproduce your results here.
> But I *was* trying to get a best case. That's why it isn't even worse.
> That's what the docs say the best case is, after all.
> This is the kind of thing that replacement selection actually did do
> better with on 9.6. I clearly remember Tomas Vondra doing lots of
> benchmarking, showing some benefit with RS with a work_mem of 8MB or
> less. As I said in my introduction on this thread, we weren't wrong to
> add replacement_sort_tuples to 9.6, given where things were with
> merging at the time. But, it does very much appear to create less than
> zero benefit these days. The picture changed.

Do we need/want to repeat some of that benchmarking on these patches? I
don't recall how much this code changed since those benchmarks were done
in the 9.6 cycle.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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