Re: [HACKERS] qsort again (was Re: Strange Create Index

Tom Lane wrote:
> I increased the size of the test case by 10x (basically s/100000/1000000/)
> which is enough to push it into the external-sort regime. I get
> amazingly stable runtimes now --- I didn't have the patience to run 100
> trials, but in 30 trials I have slowest 11538 msec and fastest 11144 msec.
> So this code path is definitely not very sensitive to this data
> distribution.
>
> While these numbers aren't glittering in comparison to the best-case
> qsort times (~450 msec to sort 10% as much data), they are sure a lot
> better than the worst-case times. So maybe a workaround for you is
> to decrease maintenance_work_mem, counterintuitive though that be.
> (Now, if you *weren't* using maintenance_work_mem of 100MB or more
> for your problem restore, then I'm not sure I know what's going on...)
>

Good call. I basically reversed your test by keeping the number of rows
the same (200000), but reducing maintenance_work_mem. Reducing to 8192
made no real difference. Reducing to 4096 flattened out all the times
nicely. Slower overall, but at least predictable. Hopefully only a
temporary solution until qsort is fixed.

My restore now takes 22 minutes :)

I think the reason I wasn't seeing performance issues with normal sort
operations is because they use work_mem not maintenance_work_mem which was
only set to 2048 anyway. Does that sound right?

Regards,
Gary.