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Re: pg_plan_advice

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Alexandra Wang <alexandra.wang.oss@gmail.com>
Cc: Richard Guo <guofenglinux@gmail.com>, Lukas Fittl <lukas@fittl.com>, Tom Lane <tgl@sss.pgh.pa.us>, 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>
Date: 2026-02-18T19:57:58Z
Lists: pgsql-hackers

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On Tue, Feb 10, 2026 at 6:03 PM Robert Haas <robertmhaas@gmail.com> wrote:
> I have committed those, as well as 0001 and 0002. Here's v15.

Here's v16. As a note, my biggest need right now is for some code
review. Testing help is, of course, also still very welcome.

What I have done in this version is run the test_plan_advice machinery
with the pg_plan_advice.feedback_warnings=on and try to fix as many of
the resulting failures as I could. The biggest revelation that I had
as a result of doing that is that it's really unfortunate for this
patch that get_relation_info_hook is only called for RTE_RELATION
baserels. I was somewhat aware of this problem before, but I thought
that it might be a non-critical thing that could be fixed at some
indefinite future point, maybe years in the future. However, this
testing convinced me that this could not really be postponed if I
wanted the patch set to actually work as intended. It's true that
there's nothing to control in terms of scan method for something like
a subquery scan, but you can want to control the placement of or
prevent the occurrence of Gather or Gather Merge nodes in join
problems that involve non-relation RTEs, and there's really no way to
do that with the existing hook placement.

But it seems like there's a pretty easy solution, which is to move the
hook up from get_relation_info to its sole caller, build_simple_rel.
Patch 0002 implements this and has a commit message explaining how
code that currently uses get_relation_info_hook can be modified to
work with the replacement, buld_simple_rel_hook.

The rest of the patch set is as before, except for the main patch, now
0003, which has the following additional changes:

- It has been updated to use build_simple_rel_hook rather than
get_relation_info_hook.

- I have corrected some order of operations problems in
pgpa_walk_recursively(). The previous coding code deliver incorrect
results when an elided SubqueryScan should have been considered the
only RTI-bearing plan node beneath a Gather or Gather Merge.

- I have adjusted pgpa_planner_apply_joinrel_advice() to fix a problem
where GATHER() or GATHER_MERGE() advice could be falsely reported as
conflicting with PARTITIONWISE() advice.

- I have adjusted pgpa_plan_walker() and pgpa_walk_recursively() to
fix a problem with Gather or Gather Merge nodes together with
partitionwise aggregation, where the generated plan advice was
sometimes incorrect.

- I have done some further work on the documentation. Specifically, I
added a list of known limitations, or at least the start of one; I
expanded the discussion of what "matched" and "partially matched"
mean; I expanded the discussion of how NO_GATHER works; and I and
changed some references from "table" to "relation".

- Comment improvements.

As you might or might not guess, a lot of these bug fixes and
documentation updates were indirect consequences of the replacement of
get_relation_info_hook with buld_simple_rel_hook. Once I made that
change, it fixed some problems, but revealed others which I then had
to fix or document. However, some of these fixes are just cleaning up
other things revealed by running test_plan_advice with
pg_plan_advice.feedback_warnings=on.

Even though I have been running test_plan_advice locally with
pg_plan_advice.feedback_warnings=on, I have not enabled that in this
version of the patch set. That's because that still causes 1 test
failure, due to self-join elimination doing something that perhaps it
shouldn't. More about that later, after I've further analyzed it.

-- 
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.