Re: Big performance slowdown from 11.2 to 13.3

I wrote:
> "ldh(at)laurent-hasson(dot)com" <ldh(at)laurent-hasson(dot)com> writes:
>> It's still spilling to disk and seems to cap at 2.5GB of memory usage in spite of configuration.

> That is ... weird.

Oh: see get_hash_mem:

hash_mem = (double) work_mem * hash_mem_multiplier;

/*
* guc.c enforces a MAX_KILOBYTES limitation on work_mem in order to
* support the assumption that raw derived byte values can be stored in
* 'long' variables. The returned hash_mem value must also meet this
* assumption.
*
* We clamp the final value rather than throw an error because it should
* be possible to set work_mem and hash_mem_multiplier independently.
*/
if (hash_mem < MAX_KILOBYTES)
return (int) hash_mem;

return MAX_KILOBYTES;

So basically, we now have a hard restriction that hashaggs can't use
more than INT_MAX kilobytes, or approximately 2.5GB, and this use case
is getting eaten alive by that restriction. Seems like we need to
do something about that.

regards, tom lane