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Re: pg_plan_advice (now with transparent SQL plan performance overrides - pg_stash_advice)

David G. Johnston <david.g.johnston@gmail.com>

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From: "David G. Johnston" <david.g.johnston@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Jakub Wartak <jakub.wartak@enterprisedb.com>, Alexandra Wang <alexandra.wang.oss@gmail.com>, Richard Guo <guofenglinux@gmail.com>, Lukas Fittl <lukas@fittl.com>, Tom Lane <tgl@sss.pgh.pa.us>, Jacob Champion <jacob.champion@enterprisedb.com>, Dian Fay <di@nmfay.com>, Matheus Alcantara <matheusssilv97@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-03-03T19:28:16Z
Lists: pgsql-hackers 
On Tuesday, March 3, 2026, Robert Haas <robertmhaas@gmail.com> wrote:

> >
> > However with below SEQ_SCAN is applied/matched, but marked as failed
> > (so bug?):
> >
> > postgres=# set pg_plan_advice.advice to 'SEQ_SCAN(t1@minmax_1)';
> > SET
> > postgres=# explain (plan_advice, costs off) select max(id) from t1;
> >                   QUERY PLAN
> > -----------------------------------------------
> >  Aggregate
> >    ->  Seq Scan on t1
> >  Supplied Plan Advice:
> >    SEQ_SCAN(t1@minmax_1) /* matched, failed */
> >  Generated Plan Advice:
> >    SEQ_SCAN(t1)
> >    NO_GATHER(t1)
>
> I think this is just out of scope for now. It's documented that we
> don't have a way of controlling aggregation behavior at present. Here
> again, we can do more things in future releases, but it's too late to
> add more to the scope for this release. There's still plenty of time
> to fix bugs, but this isn't a bug. We'd need a whole lot of new
> machinery to prevent this sort of thing, including new core hooks and
> new syntax. Here again, we have the freedom to decide that I chose the
> scope wrongly and that this is therefore not shippable as is, but I do
> not think there is time to significantly expand the scope at this
> point.
>
>
I mostly get why specifying an index that doesn't exist as part of advice,
alongside a target relation, produces "matched" along with "inapplicable"
and "failed".

INDEX_SCAN(f no_such_index) /* matched, inapplicable, failed */

But less understandable is why a failure to match a subplan-qualified
target produces "matched" when the subplan doesn't appear.

SEQ_SCAN(t1@minmax_1) /* matched, failed */

Maybe we need to do something like:

relname - matches anywhere in the plan tree
relname@somewhere - only looks at "somewhere" for matches; absence of
somewhere results in "not matched" (the expected feedback for the
advice/query combination above).

This would necessitate needing some way to specify "top-level" after the
"@" symbol - leaving it blank would suffice.  I haven't worked through
"directly at the named subplan" versus "the named subplan and any
descendants"...

If keeping the status quo the existing behavior should be documented. The
existing wording for not matched; "or it may occur if the relevant portion
of the query was not planned," seems to be the one that covers this case.

David 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.