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Re: pg_plan_advice

Tender Wang <tndrwang@gmail.com>

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From: Tender Wang <tndrwang@gmail.com>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, Nathan Bossart <nathandbossart@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Lukas Fittl <lukas@fittl.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, "heikki.linnakangas" <heikki.linnakangas@iki.fi>
Date: 2026-04-15T10:30:01Z
Lists: pgsql-hackers 
Alexander Lakhin <exclusion@gmail.com> 于2026年4月15日周三 02:00写道：
>
> 13.04.2026 19:01, Robert Haas wrote:
>
> Sounds like we have a consensus. I have committed the three bug-fix
> patches (unrelated to the retry-loop stuff) ...
>
>
> Thanks again for committing these fixes, Robert! With all the fixes in
> place, I and SQLsmith have reached another error:
> CREATE TABLE t1(a int);
> CREATE TABLE t2(b int);
>
> SELECT 1 FROM t1 WHERE EXISTS
>   (SELECT 1 FROM
>     (SELECT 1 FROM
>       (SELECT 1) LEFT JOIN t2 ON true),
>     t2 WHERE a = b);
>
> ERROR:  XX000: unique semijoin found for relids (b 3 5 7) but not observed during planning
> LOCATION:  pgpa_plan_walker, pgpa_walker.c:153
>
> Could you please have a look?
>
I did some research, and  the sj_unique_rtis contains {3,5,6,7}.
You can see that 6 is in the set. How 6 is added into the uniquerel->relids.
After deconstruct_jointree(), the joinlist is as follow:
(gdb) call nodeToString(joinlist)
$1 = 0x1ef1238 "({RANGETBLREF :rtindex 1} {RANGETBLREF :rtindex 7}
{RANGETBLREF :rtindex 5} {RANGETBLREF :rtindex 3})"
You can see that no 6 in the list.
The 6 is added when processing (7, 5), in make_join_rel(), we have below logic:

   /*
    * Add outer join relid(s) to form the canonical relids.  Any added outer
    * joins besides sjinfo itself are appended to pushed_down_joins.
    */
   joinrelids = add_outer_joins_to_relids(root, joinrelids, sjinfo,
                                 &pushed_down_joins);

In this case, 6 was added to the joinrelids.
When processing {1}, {3,5,6,7}, the {3,5,6,7} is the uniquerel, so in
the pgpa_join_path_setup(),
the {3,5,6,7} was appended to proot->sj_unique_rels.

In the plan_showdown phase, in pgpa_qf_add_plan_rtis(), we can add 7,
5, and 3 to qf->relids.
It seems difficult to add "6" to qf->relids when walking through the
plan tree.(Maybe have an easy way, I don't know too much
pg_plan_advice related code).

-- 
Thanks,
Tender Wang

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.