Re: Sort performance cliff with small work_mem

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Wed, May 2, 2018 at 11:38 AM, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>> To fix, I propose that we change the above so that we always subtract
>> tapeSpace, but if there is less than e.g. 32 kB of memory left after that
>> (including, if it went below 0), then we bump availMem back up to 32 kB. So
>> we'd always reserve 32 kB to hold the tuples, even if that means that we
>> exceed 'work_mem' slightly.

> Sounds very reasonable.

Agreed. I think that was my code to start with, and the issue certainly
didn't occur to me at the time.

I don't like the idea of using hardwired "32kB" though; some multiple
of TAPE_BUFFER_OVERHEAD seems more plausible. Perhaps MINORDER times
TAPE_BUFFER_OVERHEAD would be good?

regards, tom lane