Re: Tuplesort merge pre-reading

On 09/22/2016 03:40 AM, Claudio Freire wrote:
> On Tue, Sep 20, 2016 at 3:34 PM, Claudio Freire <klaussfreire@gmail.com> wrote:
>> The results seem all over the map. Some regressions seem significant
>> (both in the amount of performance lost and their significance, since
>> all 4 runs show a similar regression). The worst being "CREATE INDEX
>> ix_lotsofitext_zz2ijw ON lotsofitext (z, z2, i, j, w);" with 4GB
>> work_mem, which should be an in-memory sort, which makes it odd.
>>
>> I will re-run it overnight just in case to confirm the outcome.
>
> A new run for "patched" gives better results, it seems it was some
> kind of glitch in the run (maybe some cron decided to do something
> while running those queries).
>
> Attached
>
> In essence, it doesn't look like it's harmfully affecting CPU
> efficiency. Results seem neutral on the CPU front.

Looking at the spreadsheet, there is a 40% slowdown in the "slow" 
"CREATE INDEX ix_lotsofitext_zz2ijw ON lotsofitext (z, z2, i, j, w);" 
test with 4GB of work_mem. I can't reproduce that on my laptop, though. 
Got any clue what's going on there?

- Heikki