Re: pg_plan_advice

On Thu, Jan 8, 2026 at 8:07 AM Robert Haas <robertmhaas@gmail.com> wrote:
> > 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.

In my assessment there were roughly four kinds of changes on the
regression test outputs for pg_hint_plan:

1) The order of operations (e.g. whats the inner/outer rel in a
parallel plan) - I think that's probably caused by the previous zero
cost confusing the planner
2) The placement of the Gather node in the case of joins (its now on
top of the join in more cases, vs below it) - I think that's also
caused by the previous zero cost logic
3) Parallelism wasn't enforced before, but is enforced now (e.g.
sequential scans on empty relations didn't get a Gather node
previously)
4) Worker count changed because rel_parallel_workers is still limited
by max_parallel_workers_per_gather, and so my patched version now sets
that for the whole query to the highest of the workers requested in
Parallel hints (vs before it was able to keep that to just what a plan
node needed)

> > There were two bugs in 0004 that I had to fix to make this work:
> >
> > In cost_index, we are checking "path->path.parallel_workers == 0", but parallel_workers only gets
> > set later in the function, causing the PGS_CONSIDER_NONPARTIAL mask to not be applied. Replacing
> > this with checking the "partial_path" argument instead makes it work.
>
> I agree that this is a bug. I'm thinking this might be the appropriate fix:
>
>      enable_mask = (indexonly ? PGS_INDEXONLYSCAN : PGS_INDEXSCAN)
> -        | (path->path.parallel_workers == 0 ? PGS_CONSIDER_NONPARTIAL : 0);
> +        | (partial_path ? 0 : PGS_CONSIDER_NONPARTIAL);

Yup, that's exactly the change I had locally that worked as expected.

> > In cost_samplescan, we set the PGS_CONSIDER_NONPARTIAL mask if its a non-partial path, but that
> > causes Sample Scans to always be disabled when setting PGS_CONSIDER_NONPARTIAL on the
> > relation. I think we could simply drop that check, since we never generate partial sample scan paths.
>
> This one is less obvious to me. I mean, if PGS_CONSIDER_NONPARTIAL
> lets us consider non-partial plans, and a sample scan is a non-partial
> plan, then shouldn't the flag need to be set in order for us to
> consider it? If not, maybe we need to rethink the name or the
> semantics of that bit in some way.

Yeah, I would agree with you that is inconsistent with the flag's name
- but on the flip side, its difficult for the caller to conditionally
set the flag (which you'd have to do to avoid a "Disabled" showing in
the plan), since we're setting it on the RelOptInfo (do we know if the
scan is a sample scan at that point?).

I wonder if its worth considering to invert the flag, i.e.
"PGS_PREFER_PARTIAL" - that way its clear that in case of paths that
can't be partial, we use the non-partial version without a penalty.

PS: I realized my prior mails were not plain text and had bad word
wrap (thanks Gmail-based workflow!) - hopefully this one is better :)

Thanks,
Lukas

--
Lukas Fittl