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Re: pg_plan_advice

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Lukas Fittl <lukas@fittl.com>
Cc: Jakub Wartak <jakub.wartak@enterprisedb.com>, Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-04-02T16:44:32Z
Lists: pgsql-hackers 
On Wed, Apr 1, 2026 at 2:34 AM Lukas Fittl <lukas@fittl.com> wrote:
> Is there a reason you didn't use GetNamedDSA / GetNamedDSHash for the
> other allocations? (which we have as of fe07100e82b09)

I'm under the impression that GetNamedDSA and GetNamedDSHash exist for
the purposes of making it easy for extensions to coordinate with each
other across backends, rather than being something you're supposed to
use to improve observability. I think it's potentially good for there
to be a way to see the size of every DSA that exists in the system,
but this clearly isn't that, because none of the code in src/backend
uses it when creating DSAs. You might argue that DSAs for short-lived
things like parallel query or parallel VACUUM don't need to be tracked
like this (which seems arguable), but they are also used for
long-lived contexts in the logical replication launcher, by
LISTEN/NOTIFY, by the shared-memory statistics collector, and in
GetSessionDsmHandle(), and those places don't use GetNamedDSA()
either.

Architecturally, I don't like the idea of replacing "having a pointer
to an object" with "being able to look up that object by name". I
think it's good design that pg_stash_advice creates one structure that
serves as a sort of root and then hangs everything else off of that. I
admit that leaves me not knowing quite what to do about the problem of
knowing how much memory it's using, though. Adding a function just to
return size information seems a little clunky, but it might be the
right idea: it could for example return a count of stashes, a count of
entries, the total length of the entries, and the allocated size of
the DSA.

-- 
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.