Re: generic plans and "initial" pruning

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From: Ilmar Yunusov <[email protected]>
To: [email protected]
Cc: Amit Langote <[email protected]>
Subject: Re: generic plans and "initial" pruning
Date: Tue, 02 Jun 2026 17:54:09 +0000
Message-ID: <178042284979.1017.17926732655528064040.pgcf@coridan.postgresql.org> (raw)
In-Reply-To: <CAA-aLv6NiPNMzeOv4g_bZdyzSvBHZLqabKv1vgrf8CmJHRqJ-w@mail.gmail.com>
References: <CA+HiwqFGkMSge6TgC9KQzde0ohpAycLQuV7ooitEEpbKB0O_mg@mail.gmail.com>
	<CAA-aLv6NiPNMzeOv4g_bZdyzSvBHZLqabKv1vgrf8CmJHRqJ-w@mail.gmail.com>

The following review has been posted through the commitfest application:
make installcheck-world:  not tested
Implements feature:       tested, failed
Spec compliant:           not tested
Documentation:            not tested

Hi,

I looked at v13, focusing on apply/build status and relation-lock behavior for
reused generic plans after initial partition pruning.

I used the v13 series from Amit's 2026-05-29 message, on origin/master at
4b0bf0788b066a4ca1d4f959566678e44ec93422.

The series applies cleanly with git am, and git diff --check reports no
issues.

I first built with:

./configure --prefix="$PWD/pg-install" --without-readline --without-zlib --without-icu
make -s -j8
make -s install

make -C src/test/regress check

passed; all 245 tests passed, including plancache and partition_prune.

I also built a cassert/debug tree with:

./configure --prefix="$PWD/pg-install" --without-readline --without-zlib --without-icu --enable-cassert --enable-debug 'CFLAGS=-O0 -g'
make -s -j8
make -s install

and ran:

make -C src/test/regress check

which also passed; all 245 tests passed.

For the lock behavior, I used a list-partitioned table with force_generic_plan.
After the generic plan had been built and then reused, EXECUTE held only the
matching child partition lock. For example, EXECUTE q(1) held only the
following child lock:

manual_prunelock_p1

EXPLAIN EXECUTE behaved the same way on a reused generic plan; EXPLAIN EXECUTE
q(2) removed the other subplans and held only the following child lock:

manual_prunelock_p2

With enable_partition_pruning = off and a newly prepared statement, executing
the same SELECT held all child partition locks:

manual_prunelock_p1, manual_prunelock_p2, manual_prunelock_p3

I also ran a bounded cassert/debug stress check around plan invalidation. It
did 20 cycles where a child index was created and dropped before EXECUTE, and
20 similar cycles before EXPLAIN EXECUTE. In each cycle, the first execution
after invalidation/replanning held all child partition locks, and the next
execution reusing the generic plan held only the matching child partition lock.
That matches my reading that the patch is reducing locks for reused generic
plans, not for the execution that has to rebuild the plan.

One behavior I wanted to confirm: prepared UPDATE execution still held all
child partition locks in my manual check, including on the second execution
where the generic plan was being reused.

The test was:

prepare upd(int, text) as
  update stress_prunelock_p set b = $2 where a = $1;

Then both:

execute upd(3, 'updated-row-3');

and an all-pruned value:

execute upd(99, 'no-row');

held:

stress_prunelock_p1, stress_prunelock_p2, stress_prunelock_p3,
stress_prunelock_p4

pg_prepared_statements showed generic_plans increasing for this prepared
statement, so this was not a custom-plan case.

Is this expected for ModifyTable/result relations in v13, or did I miss an
eligibility condition that prevents pruning-aware locking from being used for
this prepared UPDATE case? I saw the recent firstResultRels discussion, but I
was not sure whether those changes are intended only to make pruned
result-relation initialization safe, or whether actual prepared DML execution
is expected to see reduced child partition locking as well.

I did not review the broader plancache refactoring design, did not run
installcheck-world, and did not test concurrent DDL from a separate session.

Regards,
Ilmar Yunusov

The new status of this patch is: Waiting on Author

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