From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
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To: | Justin Pryzby <pryzby(at)telsasoft(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net> |
Cc: | Peter Geoghegan <pg(at)bowt(dot)ie>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Bruce Momjian <bruce(at)momjian(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, David Rowley <dgrowleyml(at)gmail(dot)com>, Melanie Plageman <melanieplageman(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Default setting for enable_hashagg_disk |
Date: | 2020-07-10 08:29:18 |
Message-ID: | 6963442a2654bd56d386d5a253586815d0c3a17f.camel@j-davis.com |
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Lists: | pgsql-docs pgsql-hackers |
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
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