On Thu, Jun 25, 2020 at 12:59 AM Robert Haas <robertmh...@gmail.com> wrote: > > So, I don't think we can wire in a constant like 10x. That's really > unprincipled and I think it's a bad idea. What we could do, though, is > replace the existing Boolean-valued GUC with a new GUC that controls > the size at which the aggregate spills. The default could be -1, > meaning work_mem, but a user could configure a larger value if desired > (presumably, we would just treat a value smaller than work_mem as > work_mem, and document the same). > > I think that's actually pretty appealing. Separating the memory we > plan to use from the memory we're willing to use before spilling seems > like a good idea in general, and I think we should probably also do it > in other places - like sorts. >
+1. I also think GUC on these lines could help not only the problem being discussed here but in other cases as well. However, I think the real question is do we want to design/implement it for PG13? It seems to me at this stage we don't have a clear understanding of what percentage of real-world cases will get impacted due to the new behavior of hash aggregates. We want to provide some mechanism as a safety net to avoid problems that users might face which is not a bad idea but what if we wait and see the real impact of this? Is it too bad to provide a GUC later in back-branch if we see users face such problems quite often? I think the advantage of delaying it is that we might see some real problems (like where hash aggregate is not a good choice) which can be fixed via the costing model. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
