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Re: pg_plan_advice

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Haibo Yan <tristan.yim@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-01-08T19:53:14Z
Lists: pgsql-hackers 
On Wed, Jan 7, 2026 at 5:47 PM Haibo Yan <tristan.yim@gmail.com> wrote:
> Instead, the planner seems to silently ignore the structural constraint of the advice and falls back to a path GEQO can actually find.

Does the plan end up disabled in that case?

> I believe this behavior is acceptable because pg_plan_advice is intended to stabilize plans that the optimizer can generate. Since GEQO cannot generate Bushy plans, users should not be supplying them.

Right, I agree. A core principal here is that you can only nudge the
planner towards a plan it would have considered anyway. In the case of
GEQO, there is some randomness to which plans are considered. Your
advice will only be reliably take into account if it applies to
elements that must be part of the final plan. For instance, if you
advise the use of a sequential scan or an index scan, that should
work, because that relation has to be scanned somehow. Advice on a
join method should almost always work, since it can apply to any
non-leading table. Of course, you also won't be able to advise an
infeasible join method, but that would be true without GEQO, too.
Advice on the join order is going to be iffy when using GEQO -- if a
compatible join order is highly likely to be considered, e.g. because
you specify something like JOIN_ORDER(just_one) that only sets the
driving table -- then it'll probably work, but if you give a complete
join order specification, it probably won't. If you want to avoid
that, you can adjust geqo_threshold.

Thanks for looking into this.

-- 
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.