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Re: pg_plan_advice

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Jakub Wartak <jakub.wartak@enterprisedb.com>
Cc: Lukas Fittl <lukas@fittl.com>, Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-03-27T13:08:30Z
Lists: pgsql-hackers 
On Fri, Mar 27, 2026 at 4:00 AM Jakub Wartak
<jakub.wartak@enterprisedb.com> wrote:
> 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?)

Yeah ... I mean, I don't know what you want here.  If you fetch very
large quantities of data under a shared lock while concurrent activity
is trying to add data under an exclusive lock, that's going to be
slow. Now, as you say, there are ways to improve this. However, I
don't feel like running pg_get_collected_shared_advice() in a tight
loop is a normal use case. Normally you would turn it on, run a bunch
of queries, and then run that once at the end. Even that could hit
some issues because every session will be fighting to insert into the
hash table, but here you've made it much worse in a way that I would
say is artificial.

> 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?

Good question. Actually, couldn't both of those loops use a shared lock only?

-- 
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.