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Re: pg_plan_advice

Jakub Wartak <jakub.wartak@enterprisedb.com>

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From: Jakub Wartak <jakub.wartak@enterprisedb.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Lukas Fittl <lukas@fittl.com>, Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-03-27T08:00:03Z
Lists: pgsql-hackers 
On Thu, Mar 26, 2026 at 6:20 PM Robert Haas <robertmhaas@gmail.com> wrote:
>[..v23]

0003: please be the judge here, as I'm not sure. Isn't there some too high
concurrency hit in pg_get_collected_shared_advice? If I do
pgbench -M extended -c 12 -j 12 -P 1 -S:

    progress: 59.0 s, 191008.4 tps, lat 0.063 ms stddev 0.200, 0 failed
    progress: 60.0 s, 197571.2 tps, lat 0.061 ms stddev 0.026, 0 failed
    progress: 61.0 s, 189825.5 tps, lat 0.063 ms stddev 0.208, 0 failed
    progress: 62.0 s, 197082.4 tps, lat 0.061 ms stddev 0.027, 0 failed
    progress: 63.0 s, 69345.9 tps, lat 0.173 ms stddev 1.651, 0 failed
    progress: 64.0 s, 47243.6 tps, lat 0.251 ms stddev 2.128, 0 failed
    progress: 65.0 s, 48211.6 tps, lat 0.247 ms stddev 2.156, 0 failed

there is visible collapse from 190k to 48k tps was due to constant flood
of artificial calls of: select count(*) from pg_get_collected_shared_advice();

The code does LW_SHARED there over potentially lots of of tuplestore_putvalues()
calls. However any other backend does pgca_planner_shutdown()->
pg_collect_advice_save()->store_shared_advice() which is trying to grab
LW_EXCLUSIVE lock, so everything might be be blocked across whole cluster? (I
mean for the duration of tuplestore entry and that seems to even talk about
"tape"/"disk", so to me it looks like prolonged I/O operations for temp might
impact CPU-only planning stuff?)

Maybe it is possible to buffer those reads under LW_SHARED into
backend-only (private)
memory and later just fill tuplestore later to avoid such hazard? (but the
obvious problem is how much memory we can have and how big shared area can
become). Or maybe after some time simply release it and sleep and re-take it?

0004: question, why in the pg_get_advice_stashes() the second call to
dshash_seq_init() nearby "Emit results" is done with exclusive=true , but
apparently only reads it?

-J.

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.