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Re: pg_plan_advice

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Lukas Fittl <lukas@fittl.com>, Jacob Champion <jacob.champion@enterprisedb.com>, Dian Fay <di@nmfay.com>, Matheus Alcantara <matheusssilv97@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-01-12T14:50:03Z
Lists: 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

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. pg_plan_advice: Fix another unique-semijoin bug.

  2. pg_plan_advice: Export feedback-related definitions.

  3. pg_plan_advice: Fix a bug when a subquery is pruned away entirely.

  4. pg_plan_advice: Add alternatives test to Makefile.

  5. pg_plan_advice: Handle non-repeatable TABLESAMPLE scans.

  6. pg_stash_advice: Allow stashed advice to be persisted to disk.

  7. Add pg_stash_advice contrib module.

  8. pg_plan_advice: Avoid assertion failure with partitionwise aggregate.

  9. pg_plan_advice: Invent DO_NOT_SCAN(relation_identifier).

  10. Add an alternative_plan_name field to PlannerInfo.

  11. pg_plan_advice: Refactor to invent pgpa_planner_info

  12. Respect disabled_nodes in fix_alternative_subplan.

  13. get_memoize_path: Don't exit quickly when PGS_NESTLOOP_PLAIN is unset.

  14. test_plan_advice: Set TAP test priority 50 in meson.build.

  15. pg_plan_advice: Avoid a crash under GEQO.

  16. Test pg_plan_advice using a new test_plan_advice module.

  17. pg_plan_advice: Always install pg_plan_advice.h, and in the right place

  18. pg_plan_advice: Fix failures to accept identifier keywords.

  19. Add pg_plan_advice contrib module.

  20. Allow extensions to mark an individual index as disabled.

  21. Replace get_relation_info_hook with build_simple_rel_hook.

  22. Store information about Append node consolidation in the final plan.

  23. Store information about elided nodes in the final plan.

  24. Store information about range-table flattening in the final plan.

  25. Pass cursorOptions to planner_setup_hook.

  26. Fix PGS_CONSIDER_NONPARTIAL interaction with Materialize nodes.

  27. Fix mistakes in commit 4020b370f214315b8c10430301898ac21658143f

  28. Allow for plugin control over path generation strategies.

  29. Update some comments for fasthash

  30. Allow passing a pointer to GetNamedDSMSegment()'s init callback.

  31. Don't reset the pathlist of partitioned joinrels.

  32. Treat number of disabled nodes in a path as a separate cost metric.