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Re: pg_plan_advice

Jacob Champion <jacob.champion@enterprisedb.com>

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From: Jacob Champion <jacob.champion@enterprisedb.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Dian Fay <di@nmfay.com>, Matheus Alcantara <matheusssilv97@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-01-13T18:48:36Z
Lists: pgsql-hackers

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On Mon, Dec 8, 2025 at 5:18 PM Jacob Champion
<jacob.champion@enterprisedb.com> wrote:
> a) fuzz the parser first, because it's easy and we can get interesting inputs
> b) fuzz the AST utilities, seeded with "successful" corpus members from a)
> c) stare really hard at the corpus of b) and figure out how to
> usefully mutate a PlannedStmt with it

Got stuck a bit at (c). The first two fit very well with my preferred
fuzzer setup, where I mock the world and fuzz the heck out of a tiny
corner of it. But a "mutated plan" would 1) take a lot of time for me
to design and 2) probably be counterproductive if I start chasing
impossible plans.

So I've inverted it, so that the server calls libfuzzer midquery,
instead of libfuzzer driving the code under test. That gives me *real*
plans that I can then hit with a bunch of garbage advice -- but it's
more than an order of magnitude slower, unfortunately, so I have to
seed it with the output of a+b before it gets anywhere, and then I
cannot minimize the corpus (which fills up rapidly with unoptimized
inputs) because libfuzzer isn't driving. I feel like there is
considerable room for improvement here... but I could spend a bunch of
time finding it that is then not spent fuzzing.

--

The first thing found with the new architecture is this:

    -- note that f is not a partitioned table
    SET pg_plan_advice.advice = 'join_order(f/e (f d))';
    EXPLAIN (COSTS OFF, PLAN_ADVICE)
        SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id;
    ERROR: cannot determine RTI for advice target

Test, and a quick guess at expected output, attached.

--Jacob

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.