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Re: pg_plan_advice

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-03-18T20:44:34Z
Lists: pgsql-hackers 
Don't know if you noticed yet, but avocet has shown [1] that one
pg_plan_advice test case is unstable under debug_discard_caches = 1:

diff -U3 /home/buildfarm/avocet/buildroot/HEAD/pgsql.build/contrib/pg_plan_advice/expected/prepared.out /home/buildfarm/avocet/buildroot/HEAD/pgsql.build/contrib/pg_plan_advice/results/prepared.out
--- /home/buildfarm/avocet/buildroot/HEAD/pgsql.build/contrib/pg_plan_advice/expected/prepared.out	2026-03-16 11:04:23.453026641 +0100
+++ /home/buildfarm/avocet/buildroot/HEAD/pgsql.build/contrib/pg_plan_advice/results/prepared.out	2026-03-17 20:42:16.262657929 +0100
@@ -57,11 +57,13 @@
 -- Prepared, but always_store_advice_details = true, so should show feedback.
 PREPARE pt4 AS SELECT * FROM ptab;
 EXPLAIN (COSTS OFF, PLAN_ADVICE) EXECUTE pt2;
-       QUERY PLAN       
-------------------------
+           QUERY PLAN           
+--------------------------------
  Seq Scan on ptab
+ Supplied Plan Advice:
+   SEQ_SCAN(ptab) /* matched */
  Generated Plan Advice:
    SEQ_SCAN(ptab)
    NO_GATHER(ptab)
-(4 rows)
+(6 rows)

I can reproduce this pretty trivially by adding
SET debug_discard_caches = 1;
at the top of contrib/pg_plan_advice/sql/prepared.sql.

It looks like the appearance of "Supplied Plan Advice:" depends
on whether the prepared query's plan got regenerated or not.
I'm not sure if this represents a bug (ie undesirable behavior) or
it's just that the test is being insufficiently careful about
being reproducible.

			regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=avocet&dt=2026-03-16%2010%3A03%3A01

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.