Re: pg_plan_advice

On Thu, Jan 08, 2026 at 11:07:31AM -0500, Robert Haas wrote:
> On Wed, Jan 7, 2026 at 2:04 AM Lukas Fittl <lukas(at)fittl(dot)com> wrote:
>> That said, good news: After a bunch of iterations, I get a clean
>> pass on the pg_hint_plan regression tests, whilst completely
>> dropping its copying of core code and hackish re-run of
>> set_plain_rel_pathlist. See [0] for a draft PR (on my own fork of
>> pg_hint_plan) with individual patches that explain some regression
>> test differences.
>
> That sounds AWESOME.

So you are telling me that I can commit code that deletes code. Count
me in. The project has some merge requests that I've been holding on
a bit due to what's happening here and because I did not really look
at the internals that have changed. It's great to see that you have
begun an investigation, Lukas.

>> The biggest change in the regression test output was due to how the
>> "Parallel" hint worked in pg_hint_plan (basically it was setting
>> parallel_*_cost to zero, and then messed with the gucs that factor
>> into compute_parallel_worker) -- I think the only sensible thing to
>> do is to change that in pg_hint_plan, and instead rely on rejecting
>> non-partial paths with PGS_CONSIDER_NONPARTIAL if "hard"
>> enforcement of parallelism is requested. That caused some minor
>> plan changes, but I think they can still be argued to be matching
>> the user's intent of "make a scan involving this relation
>> parallel".
>
> Cool. I'm sort of curious what changed, but maybe it's not important
> enough to spend time discussing right now.

I suspect that this is going to be an incremental integration process,
and it smells to me that it is going to require more than one major
release before being able to remove the whole set of hacks that
pg_hint_plan has been using, particularly with the GUCs, the costing
and the forced update of the backend routines which is a ugly
historical hack. Saying that, I would need to look at the plan
outputs to be sure, perhaps we would be OK even with slight changes.
These happen every year, because the plans tested are complex enough
that some of the sub-paths are changed, but the hints still work
properly. This year for v19 we have at least the changes in the
expression names.
--
Michael