Horton
Menu

Re: pg_plan_advice

Lukas Fittl <lukas@fittl.com>

View thread
From: Lukas Fittl <lukas@fittl.com>
To: Robert Haas <robertmhaas@gmail.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: 2026-01-07T07:04:19Z
Lists: pgsql-hackers 
On Mon, Dec 29, 2025 at 5:15 PM Lukas Fittl <lukas@fittl.com> wrote:

> On Mon, Dec 15, 2025 at 12:06 PM Robert Haas <robertmhaas@gmail.com>
> wrote:
> > Here's v7.
>
> I'm excited about this patch series, and in an effort to help land the
> infrastructure, here is a review of 0001 - 0003 to start:
>

For the review of 0004, I decided to spend a few days to test if the plan
generation strategy logic will
work for pg_hint_plan, as an existing extension in the ecosystem that is
widely used, but functions today
by virtue of modifying planner GUCs whilst the planner is running.

First of all, as is expected, the extension completely stops working with
0004 in place - because we now
only read the GUCs at planner start, the mechanism of modifying them in the
middle doesn't work. I don't
think we can avoid this.

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.

Adding Michael in CC, since he's been thankfully maintaining pg_hint_plan
over the years, and I think if
0004 gets merged that should significantly reduce the maintenance burden,
independently of what happens
with pg_plan_advice - so his input would be useful here.

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".

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.

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.

Otherwise 0004 looks good to me, and the mechanism of working with mask
values felt natural to me,
especially in contrast with existing ways to achieve something similar. I
did not test partition wise joins,
since pg_hint_plan doesn't cover them today.

[0]: https://github.com/lfittl/pg_hint_plan/pull/1

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.