Re: Big performance slowdown from 11.2 to 13.3 - Mailing list pgsql-performance

I wrote:
> "ldh@laurent-hasson.com" <ldh@laurent-hasson.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