Re: Default setting for enable_hashagg_disk

On Thu, 2020-07-09 at 19:18 -0500, Justin Pryzby wrote:
> Maybe pretend that Jeff implemented something called CashAgg, which
> does
> everything HashAgg does but implemented from scratch.  Users would be
> able to
> tune it or disable it, and we could talk about removing HashAgg for
> the next 3
> years.

That's kind of what we'd have if we had the two escape-hatch GUCs.
Default gives new behavior, changing the GUCs would give the v12
behavior.

In principle, Stephen is right: the v12 behavior is a bug, lots of
people are unhappy about it, it causes real problems, and it would not
be acceptable if proposed today. Otherwise I wouldn't have spent the
time to fix it. 

Similarly, potential regressions are not the "fault" of my feature --
they are the fault of the limitations of work_mem, the limitations of
the planner, the wrong expectations from customers, or just
happenstance. 

But at a certain point, I have to weigh the potential anger of
customers hitting regressions versus the potential anger of hackers
seeing a couple extra GUCs. I have to say that I am more worried about
the former.

If there is some more serious consequence of adding a GUC that I missed
in this thread, please let me know. Otherwise, I intend to commit a new
GUC shortly that will enable users to bypass work_mem for HashAgg, just
as in v12.

Regards,
    Jeff Davis