Re: pg_plan_advice - Mailing list pgsql-hackers

On Thu, Jan 8, 2026 at 6:29 PM Michael Paquier <michael@paquier.xyz> wrote:
> 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.

:-)

> 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.

I think it's quite possible that you could do a full rip and replace
with some study of what needs to be added on top of 0004. The core
idea of 0004 is that it adds a field called pgs_mask in several of
places, which allows you to set a bitmask to control which operations
the planner will consider to be enabled. You can set this via some
existing hooks, and it also adds a  a new hook (joinrel_setup_hook)
which is called near the start of add_paths_to_joinrel(). So, you can
set pgs_mask in PlannerGlobal to control planning for the entire
operation, RelOptInfo to control it for a particular rel, or
JoinPathExtraData to set it for a particular joinrel and a particular
choice of outer and inner rel. I am pretty well convinced that this is
a good model: instead of duplicating a bunch of planner code, as
pg_hint_plan currently does, just have a way to tell the existing
planner code what you want it to do.

Now, one fly in the ointment is that pgs_mask is just a mask -- that
is, it gives us space to store 64 related Booleans, but nothing else.
So if we want to store an integer, like a number of parallel workers,
we need a separate field for that. But if that integer can reasonably
be set at the same levels that are possible for pgs_mask, it's really
easy: just look at the places where 0004 adds a pgs_mask field, and
add an integer field as well. There is obviously some limit to the
number of fields we can reasonably add to PlannerGlobal, RelOptInfo,
and JoinPathExtraData, but if it starts to become too much, we could
bundle some or all of them up in a struct. The patch has already
figured out how to get the mask to propagate down to the places where
low-level planning decisions are being made, so it seems worth trying
to piggy-back on that for anything else that we need.

Now, maybe that won't work out for some reason. But on the other hand,
maybe it will work out. I think it's possible that for the price of a
quite small patch adding another field or a couple of fields on top of
pgs_mask and in the same places, you could get rid of a lot more code.

--
Robert Haas
EDB: http://www.enterprisedb.com