Re: Default setting for enable_hashagg_disk

On Fri, Jul 24, 2020 at 12:55 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> Could that be caused by clustering in the data?
>
> If the input data is in totally random order then we have a good
> chance of never having to spill skewed "common" values. That is, we're
> bound to encounter common values before entering spill mode, and so
> those common values will continue to be usefully aggregated until
> we're done with the initial groups (i.e. until the in-memory hash
> table is cleared in order to process spilled input tuples). This is
> great because the common values get aggregated without ever spilling,
> and most of the work is done before we even begin with spilled tuples.
>
> If, on the other hand, the common values are concentrated together in
> the input...

I still don't know if that was a factor in your example, but I can
clearly demonstrate that the clustering of data can matter a lot to
hash aggs in Postgres 13. I attach a contrived example where it makes
a *huge* difference.

I find that the sorted version of the aggregate query takes
significantly longer to finish, and has the following spill
characteristics:

"Peak Memory Usage: 205086kB Disk Usage: 2353920kB HashAgg Batches: 2424"

Note that the planner doesn't expect any partitions here, but we still
get 2424 batches -- so the planner seems to get it totally wrong.
OTOH, the same query against a randomized version of the same data (no
longer in sorted order, no clustering) works perfectly with the same
work_mem (200MB):

"Peak Memory Usage: 1605334kB"

Hash agg avoids spilling entirely (so the planner gets it right this
time around). It even uses notably less memory.

--
Peter Geoghegan