-----Original Message-----
From: Tom Lane <t...@sss.pgh.pa.us>
Sent: Wednesday, July 21, 2021 19:43
To: l...@laurent-hasson.com
Cc: Peter Geoghegan <p...@bowt.ie>; Justin Pryzby <pry...@telsasoft.com>;
pgsql-performa...@postgresql.org
Subject: Re: Big performance slowdown from 11.2 to 13.3
"l...@laurent-hasson.com" <l...@laurent-hasson.com> writes:
> From: Peter Geoghegan <p...@bowt.ie>
>> I imagine that this has something to do with the fact that the hash
>> aggregate spills to disk in Postgres 13.
> So how is this happening? I mean, it's the exact same query, looks like the
> same plan to me, it's the same data on the exact same VM etc... Why is that
> behavior so different?
What Peter's pointing out is that v11 never spilled hashagg hash tables to
disk period, no matter how big they got (possibly leading to out-of-memory
situations or swapping, but evidently you have enough RAM to have avoided
that sort of trouble). I'd momentarily forgotten that, but I think he's
dead on about that explaining the difference. As he says, messing with
hash_mem_multiplier would be a more targeted fix than increasing work_mem
across the board.
regards, tom lane
OK, got it! That sounds and smells good. Will try later tonight or tomorrow and
report back.
Thank you!
Laurent.
