Re: Default setting for enable_hashagg_disk

Greetings,

* Jeff Davis (pgsql(at)j-davis(dot)com) wrote:
> On Wed, 2020-07-08 at 10:00 -0400, Stephen Frost wrote:
> > That HashAgg previously didn't care that it was going wayyyyy over
> > work_mem was, if anything, a bug.
>
> I think we all agree about that, but some people may be depending on
> that bug.

That's why we don't make these kinds of changes in a minor release and
instead have major releases.

> > Inventing new GUCs late in the
> > cycle like this under duress seems like a *really* bad idea.
>
> Are you OK with escape-hatch GUCs that allow the user to opt for v12
> behavior in the event that they experience a regression?

The enable_* options aren't great, and the one added for this is even
stranger since it's an 'enable' option for a particular capability of a
node rather than just a costing change for a node, but I feel like
people generally understand that they shouldn't be messing with the
enable_* options and that they're not really intended for end users.

> The one for the planner is already there, and it looks like we need one
> for the executor as well (to tell HashAgg to ignore the memory limit
> just like v12).

No, ignoring the limit set was, as agreed above, a bug, and I don't
think it makes sense to add some new user tunable for this. If folks
want to let HashAgg use more memory then they can set work_mem higher,
just the same as if they want a Sort node to use more memory or a
HashJoin. Yes, that comes with potential knock-on effects about other
nodes (possibly) using more memory but that's pretty well understood for
all the other cases and I don't think that it makes sense to have a
special case for HashAgg when the only justification is that "well, you
see, it used to have this bug, so...".

Thanks,

Stephen