Re: Big performance slowdown from 11.2 to 13.3

On Fri, 23 Jul 2021 at 04:14, Peter Geoghegan <pg@bowt.ie> wrote:
>
> On Thu, Jul 22, 2021 at 8:45 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > That is ... weird.  Maybe you have found a bug in the spill-to-disk logic;
> > it's quite new after all.  Can you extract a self-contained test case that
> > behaves this way?
>
> I wonder if this has something to do with the way that the input data
> is clustered. I recall noticing that that could significantly alter
> the behavior of HashAggs as of Postgres 13.

Isn't it more likely to be reaching the group limit rather than the
memory limit?

if (input_groups * hashentrysize < hash_mem * 1024L)
{
if (num_partitions != NULL)
*num_partitions = 0;
*mem_limit = hash_mem * 1024L;
*ngroups_limit = *mem_limit / hashentrysize;
return;
}

There are 55 aggregates on a varchar(255). I think hashentrysize is
pretty big. If it was 255*55 then only 765591 groups fit in the 10GB
of memory.

David