Re: Inconsistent Behavior of GROUP BY ROLLUP in v17 vs master

On Thu, 25 Sept 2025 at 13:01, Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
> I plan to push this patch soon, unless there are any objections.

What's your confidence levels on the logic now being correct? 100%?
90%? Hopeful?

I've not personally had the time to process the patch and figure out
that this is now safe. It sounds like you have, but (with respect) I
expect you also thought that before the initial commit too. I realise
that you now have more of the picture, but how do we know the picture
is now complete? I think we all know this stuff is hard and we also
know that even the most seasoned of planner hackers don't always get
it right first time.

What I'm now wondering is the risk to reward ratio of fixing this in
18.1. If it happens that it's still not right for 18.1, then we need
to wait until 18.2, which is currently due Feb-26. I don't quite have
the same confidence levels as you do, but maybe I would if I took the
time to more carefully think about this. For now, my thoughts are
that it's safer to revert and try again for v19. Probably it would
make more sense to try harder for an 18.1 fix if this optimisation was
a more critical and people had been waiting on it and there was no
other way to obtain the benefits of it. But that's not quite the case
here as, in theory, someone could rewrite their query if it's safe for
their use case and end up with the same performance benefits.

Just so it's clear, I'm not objecting to fixing for 18.1. I just want
to ensure your judgment is for the project and not for
self-preservation. I think most people will respect the decision if
you decide that you need more time to consider this and opt to revert
in v18 and fix only in master. Based on my current understanding of
the optimisation, I'd certainly be more at ease with that.

On the other hand, if you're 100% confident, I'll step aside.

David