Re: Default setting for enable_hashagg_disk

On Thu, Jun 25, 2020 at 11:10:58AM -0700, Jeff Davis wrote:
> On Wed, 2020-06-24 at 13:29 -0400, Tom Lane wrote:
> > If we feel we need something to let people have the v12 behavior
> > back, let's have
> > (1) enable_hashagg on/off --- controls planner, same as it ever was
> > (2) enable_hashagg_spill on/off --- controls executor by disabling
> > spill
> >
> > But I'm not really convinced that we need (2).
>
> If we're not going to have a planner GUC, one alternative is to just
> penalize the disk costs of HashAgg for a release or two. It would only
> affect the cost of HashAgg paths that are expected to spill, which
> weren't even generated in previous releases.
>
> In other words, multiply the disk costs by enough that the planner will
> usually not choose HashAgg if expected to spill unless the average
> group size is quite large (i.e. there are a lot more tuples than
> groups, but still enough groups to spill).

Well, the big question is whether this costing is actually more accurate
than what we have now. What I am hearing is that spilling hash agg is
expensive, so whatever we can do to reflect the actual costs seems like
a win. If it can be done, it certainly seems better than a cost setting
few people will use.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

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