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Re: pg_plan_advice

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Lukas Fittl <lukas@fittl.com>
Cc: 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>, Michael Paquier <michael@paquier.xyz>
Date: 2026-01-08T16:07:31Z
Lists: pgsql-hackers 
On Wed, Jan 7, 2026 at 2:04 AM Lukas Fittl <lukas@fittl.com> wrote:
> That said, good news: After a bunch of iterations, I get a clean pass on the pg_hint_plan regression tests,
> whilst completely dropping its copying of core code and hackish re-run of set_plain_rel_pathlist. See [0]
> for a draft PR (on my own fork of pg_hint_plan) with individual patches that explain some regression test
> differences.

That sounds AWESOME.

> The biggest change in the regression test output was due to how the "Parallel" hint worked in pg_hint_plan
> (basically it was setting parallel_*_cost to zero, and then messed with the gucs that factor into
> compute_parallel_worker) -- I think the only sensible thing to do is to change that in pg_hint_plan, and
> instead rely on rejecting non-partial paths with PGS_CONSIDER_NONPARTIAL if "hard" enforcement of
> parallelism is requested. That caused some minor plan changes, but I think they can still be argued to be
> matching the user's intent of "make a scan involving this relation parallel".

Cool. I'm sort of curious what changed, but maybe it's not important
enough to spend time discussing right now.

> There were two bugs in 0004 that I had to fix to make this work:
>
> In cost_index, we are checking "path->path.parallel_workers == 0", but parallel_workers only gets
> set later in the function, causing the PGS_CONSIDER_NONPARTIAL mask to not be applied. Replacing
> this with checking the "partial_path" argument instead makes it work.

I agree that this is a bug. I'm thinking this might be the appropriate fix:

     enable_mask = (indexonly ? PGS_INDEXONLYSCAN : PGS_INDEXSCAN)
-        | (path->path.parallel_workers == 0 ? PGS_CONSIDER_NONPARTIAL : 0);
+        | (partial_path ? 0 : PGS_CONSIDER_NONPARTIAL);

> In cost_samplescan, we set the PGS_CONSIDER_NONPARTIAL mask if its a non-partial path, but that
> causes Sample Scans to always be disabled when setting PGS_CONSIDER_NONPARTIAL on the
> relation. I think we could simply drop that check, since we never generate partial sample scan paths.

This one is less obvious to me. I mean, if PGS_CONSIDER_NONPARTIAL
lets us consider non-partial plans, and a sample scan is a non-partial
plan, then shouldn't the flag need to be set in order for us to
consider it? If not, maybe we need to rethink the name or the
semantics of that bit in some way.

-- 
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.