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Re: pg_plan_advice

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Richard Guo <guofenglinux@gmail.com>
Cc: 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>, Jakub Wartak <jakub.wartak@enterprisedb.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-02-07T16:44:39Z
Lists: pgsql-hackers 
On Thu, Jan 29, 2026 at 9:10 AM Robert Haas <robertmhaas@gmail.com> wrote:
> I think cost_material() got an enabled argument because of
> materialize_finished_plan(). Most of the time, plan nodes are created
> from paths, and we want to use the parent's pgs_mask to determine
> whether the chosen strategy (e.g. materialization) is enabled.
> However, materialize_finished_plan() creates a plan directly, so
> there's no RelOptInfo or JoinPathExtraData whose pgs_mask we can
> consult, and so we have to fall back on consulting enable_material
> directly. Once that parameter got added to cost_material(), it made
> sense to me to add it to create_material_path() as well.

If this explanation seems a little weak, it's because it was. Here's a
better explanation: the patch added a per-rel flag
PGS_NESTLOOP_MATERIALIZE that enables the use of a Nested Loop join
with an inner Materialize node. When we construct such a path, the
"parent" point for the materialize path points to the rel that is on
the inner side of the join, not the joinrel itself. But the pgs_mask
flags of the joinrel and its inner side could be different. Therefore,
the Materialize node can't just look at path->parent->pgs_mask to
decide whether to mark the node as disabled. So, when I wrote the
patch originally, I added an enabled flag here to make sure that we
pass down the information about whether the join method was enabled at
the joinrel level, since the joinrel's pgs_mask is not otherwise
available to cost_material(). Later on, I discovered the need for
PGS_CONSIDER_NONPARTIAL, but by then I had forgotten why that
"enabled" argument existed and pushed the logic to handle
PGS_CONSIDER_NONPARTIAL into cost_material(). So the logic as
currently committed is buggy in the case where the joinrel and the
innerrel have differing pgs_mask values. match_unsorted_outer() and
consider_parallel_nestloop() have the intention of generating
Materialize nodes only when materialization is enabled, so that the
Materialize nodes are never marked disabled. But with the patch as
committed, a non-partial Nested Loop with inner Materialize can
disable the Materialize node if the inner rel's pgs_mask has
PGS_CONSIDER_NONPARTIAL unset.

The right fix is to make all the decisions about whether the
Materialize nod should be created, and whether it should be enabled,
in the caller, and have none of that logic in cost_material(). I will
post a new patch set shortly, which will include a patch to rectify
this issue.

-- 
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.