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Re: pg_plan_advice

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: John Naylor <johncnaylorls@gmail.com>
Cc: Lukas Fittl <lukas@fittl.com>, 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: 2026-01-13T15:09:14Z
Lists: pgsql-hackers 
On Tue, Jan 13, 2026 at 6:39 AM John Naylor <johncnaylorls@gmail.com> wrote:
> It looks like it would be helpful if fasthash_accum_cstring just
> returned zero when given a NULL string, as in the attached. We could
> also do something like add a large number to the hash, but I'm not
> sure that's necessary.

I think that pg_plan_advice's requirement are unusual here, so I would
suggest not adding a branch to fasthash_accum_cstring. If this were a
requirement that almost every caller had, then it would make sense to
pay the cost in the common function. But there will probably be a
small performance cost to this, hash function are often called in hot
paths, and pg_plan_advice is, I think, unusual, so I don't really like
doing it given thoe facts.

> + /*
> + * hashfn_unstable.h recommends using string length as tweak. It's not
> + * clear to me what to do if there are multiple strings, so for now I'm
> + * just using the total of all of the lengths.
> + */
> + return fasthash_final32(&hs, sp_len);
>
> Sounds reasonable, so the patch also documents that.

Some kind of comment change here seems useful to me. I wonder whether
it should be generalized even more than this statement. I also wonder
if this is really the optimal strategy. But I definitely agree that
clarifying this in whatever way makes sense is a good idea.

-- 
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.