Re: Default setting for enable_hashagg_disk

On Sat, Jul 25, 2020 at 5:05 PM Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
> I'm not sure what you mean by "reported memory usage doesn't reflect the
> space used for transition state"? Surely it does include that, we've
> built the memory accounting stuff pretty much exactly to do that.
>
> I think it's pretty clear what's happening - in the sorted case there's
> only a single group getting new values at any moment, so when we decide
> to spill we'll only add rows to that group and everything else will be
> spilled to disk.

Right.

> In the unsorted case however we manage to initialize all groups in the
> hash table, but at that point the groups are tiny an fit into work_mem.
> As we process more and more data the groups grow, but we can't evict
> them - at the moment we don't have that capability. So we end up
> processing everything in memory, but significantly exceeding work_mem.

work_mem was set to 200MB, which is more than the reported "Peak
Memory Usage: 1605334kB". So either the random case significantly
exceeds work_mem and the "Peak Memory Usage" accounting is wrong
(because it doesn't report this excess), or the random case really
doesn't exceed work_mem but has a surprising advantage over the sorted
case.

-- 
Peter Geoghegan