Re: Default setting for enable_hashagg_disk

On Sat, Jul 25, 2020 at 05:13:00PM -0700, Peter Geoghegan wrote:
>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

That's 1.6GB, if I read it right. Which is more than 200MB ;-)

>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

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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