Re: BUG #16104: Invalid DSA Memory Alloc Request in Parallel Hash

On Sun, Nov 10, 2019 at 7:27 AM Tomas Vondra
<tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> Hmmm, but the expected row width is only 16B, and with 6M rows that's
> only about 90GB. So how come this needs 1TB temporary files? I'm sure
> there's a bit of overhead, but 10X seems a bit much.

(s/6M/6B/) Yeah, that comes out to only ~90GB but ... PHJ doesn't
immediately unlink files from the previous generation when it
repartitions. You need at two generations' worth of files (old and
new) while repartitioning, but you don't need the grand-parent
generation. I didn't think this was a problem because I didn't expect
to have to repartition many times (and there is a similar but
different kind of amplification in the non-parallel code). If this
problem is due to the 100% extreme skew threshold causing us to go
berserk, then that 10X multiplier is of the right order, if you
imagine this thing started out with ~512 batches and got up to ~1M
batches before it blew a gasket.