Re: Default setting for enable_hashagg_disk

On Tue, Jul 14, 2020 at 03:49:40PM -0700, Peter Geoghegan wrote:
> Maybe I missed your point here. The problem is not so much that we'll
> get HashAggs that spill -- there is nothing intrinsically wrong with
> that. While it's true that the I/O pattern is not as sequential as a
> similar group agg + sort, that doesn't seem like the really important
> factor here. The really important factor is that in-memory HashAggs
> can be blazingly fast relative to *any* alternative strategy -- be it
> a HashAgg that spills, or a group aggregate + sort that doesn't spill,
> whatever. We're mostly concerned about keeping the one available fast
> strategy than we are about getting a new, generally slow strategy.

Do we have any data that in-memory HashAggs are "blazingly fast relative
to *any* alternative strategy?"  The data I have tested myself and what
I saw from Tomas was that spilling sort or spilling hash are both 2.5x
slower.  Are we sure the quoted statement is true?

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee