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Re: pg_plan_advice

Lukas Fittl <lukas@fittl.com>

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From: Lukas Fittl <lukas@fittl.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-03-26T19:51:29Z
Lists: pgsql-hackers 
Hi Robert,

On Thu, Mar 26, 2026 at 10:20 AM Robert Haas <robertmhaas@gmail.com> wrote:
> > The dangling pointers are a good point; I agree that's bad. However,
> > I'd be more inclined to fix it by nulling out the alternative_root
> > pointers at the end of set_plan_references. I think that would just be
> > the case where root->isAltSubplan[ndx] && root->isUsedSubplan[ndx].
> > The reason I'm reluctant to just store the name is that there's not an
> > easy way to find a PlannerInfo by name. I originally proposed an
> > "allroots" list in PlannerGlobal, but we went with subplanNames on
> > Tom's suggestion. I subsequently realized that this kind of stinks for
> > code that is trying to use this infrastructure for anything, for
> > exactly this reason, but Tom never responded and I never pressed the
> > issue. But I think we're boxing ourselves into a corner if we just
> > keep storing names that can't be looked up everywhere. It doesn't
> > matter for the issue before us, so maybe doing as you say here is the
> > right idea just so we can move forward, but I think we're probably
> > kidding ourselves a little bit.
>
> Here's a new version, where I've replaced alternative_root by
> alternative_plan_name, serving the same function.

Great, I think that's better for now, and if we have a broader use
case in the future we can always adjust this to be the full
PlannerInfo.

That said, reflecting on the change, I wonder if its odd that we're
copying a string pointer instead of making an actual string copy. I
think that's probably okay in practice?

I'm still 50/50 on the naming here, since we have the alternative sub
plan that has an "alternative plan name" that's not that of the
alternative itself, but rather the base plan that was utilized. But I
see your concern regarding the naming being confusing in terms of what
the "original" or "base" would actually refer to. I've also considered
whether something like "alternative_plan_group" could make sense
(since all alternative sub plans will have the same value), but maybe
that conveys too much intent on what this is used for.

That said, I think for now, to get the buildfarm happy again, v23/0001
seems good.

v23/0002 also looks good.

Thanks,
Lukas

--
Lukas Fittl

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.