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Re: pg_plan_advice

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Lukas Fittl <lukas@fittl.com>, Andrei Lepikhov <lepihov@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-04-06T20:15:21Z
Lists: pgsql-hackers 
On Sun, Apr 5, 2026 at 8:00 AM Alexander Lakhin <exclusion@gmail.com> wrote:
> And another error, which might be interesting to you:
> CREATE EXTENSION tsm_system_time;
> CREATE TABLE t(i int);
> SELECT 1 FROM (SELECT i FROM t TABLESAMPLE system_time (1000)), LATERAL (SELECT i LIMIT 1);
>
> ERROR:  XX000: plan node has no RTIs: 378
> LOCATION:  pgpa_build_scan, pgpa_scan.c:200

Thanks also for this report. The plan looks like this:

 Nested Loop  (cost=0.00..154.75 rows=2550 width=4)
   ->  Materialize  (cost=0.00..78.25 rows=2550 width=4)
         ->  Sample Scan on t  (cost=0.00..65.50 rows=2550 width=4)
               Sampling: system_time ('1000'::double precision)
   ->  Limit  (cost=0.00..0.01 rows=1 width=4)
         ->  Result  (cost=0.00..0.01 rows=1 width=4)

And it's unhappy because it's expecting the Materialize node to be the
RTI-bearing node. In a turn of events that will probably shock nobody
here, I also didn't quite realize that a Materialize node could get
inserted here. It's kind of a problem, too, because what if the sides
of the join were switched? Then we'd have a Nested Loop with an inner
Materialize node and would conclude that the strategy was
PGS_NESTLOOP_MATERIALIZE, when in reality it would be
PGS_NESTLOOP_PLAIN plus a Materialize node inserted at the scan level,
so the generated advice would be incorrect. I guess the fix is
probably to view a Materialize node on top of a Sample Scan for a
!repeatable_across_scans tsmhandler as part of the scan, which is kind
of annoying but probably doable. Not for the first time, I really wish
we stored an RTI set in every plan node, or (maybe more economically)
had some kind of enum in key plan nodes indicating why the node was
inserted. Right now, pg_plan_advice does a lot of reading the tea
leaves, which is great in that it avoids bloating Plan trees with
additional metadata, but a little scary in terms of being able to be
certain that one will get the right answer reliably.

I'll work on a fix.

-- 
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.