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 <br...@momjian.us> https://momjian.us EnterpriseDB https://enterprisedb.com The usefulness of a cup is in its emptiness, Bruce Lee
