Hi!

> Indeed, the server log seems to indicate relationship to
> VACUUM:
> 2026-02-01 16:44:58.878 UTC autovacuum worker[22589] LOG:  automatic
vacuum of table "postgres.pg_catalog.pg_class": index scans: 1

O, it's a good clue!

I have added some vacuum calls for pg_class in a stress test - and now it
fails much more often (check attachment).

It is "ERROR:  cache lookup failed for relation" - but I think it may share
the cause with "attempted to overwrite invisible tuple.

See:
https://cirrus-ci.com/build/4852126532239360 - with "Use multiple snapshots
to copy the data."
https://cirrus-ci.com/build/6429084491710464 - with "Use background worker
to do logical decoding."

But I am unable to reproduce the issue with only "Add CONCURRENTLY option
to REPACK command."
https://cirrus-ci.com/build/6467070524653568


Best regards,
Mikhail.
From cbb3fc402e8a0ce14abee46f329b15df6157b7f2 Mon Sep 17 00:00:00 2001
From: Mikhail Nikalayeu <[email protected]>
Date: Tue, 27 Jan 2026 21:41:56 +0100
Subject: [PATCH vX 1/3] stress tests for repack concurrently

---
 contrib/amcheck/meson.build                  |   2 +
 contrib/amcheck/t/007_repack_concurrently.pl | 116 +++++++++++++++++++
 contrib/amcheck/t/008_repack_concurrently.pl | 106 +++++++++++++++++
 3 files changed, 224 insertions(+)
 create mode 100644 contrib/amcheck/t/007_repack_concurrently.pl
 create mode 100644 contrib/amcheck/t/008_repack_concurrently.pl

diff --git a/contrib/amcheck/meson.build b/contrib/amcheck/meson.build
index d5137ef691d..f726db2ffe0 100644
--- a/contrib/amcheck/meson.build
+++ b/contrib/amcheck/meson.build
@@ -50,6 +50,8 @@ tests += {
       't/004_verify_nbtree_unique.pl',
       't/005_pitr.pl',
       't/006_verify_gin.pl',
+      't/007_repack_concurrently.pl',
+      't/008_repack_concurrently.pl',
     ],
   },
 }
diff --git a/contrib/amcheck/t/007_repack_concurrently.pl b/contrib/amcheck/t/007_repack_concurrently.pl
new file mode 100644
index 00000000000..fb110b0b57d
--- /dev/null
+++ b/contrib/amcheck/t/007_repack_concurrently.pl
@@ -0,0 +1,116 @@
+
+# Copyright (c) 2021-2025, PostgreSQL Global Development Group
+
+# Test REPACK CONCURRENTLY with concurrent modifications
+use strict;
+use warnings FATAL => 'all';
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+
+use Test::More;
+
+my $node;
+
+#
+# Test set-up
+#
+$node = PostgreSQL::Test::Cluster->new('CIC_test');
+$node->init;
+$node->append_conf('postgresql.conf',
+	'lock_timeout = ' . (1000 * $PostgreSQL::Test::Utils::timeout_default));
+$node->append_conf(
+	'postgresql.conf', qq(
+wal_level = logical
+max_worker_processes = 32
+));
+
+my $n=1000;
+my $no_hot = int(rand(2));
+
+$node->start;
+$node->safe_psql('postgres', q(CREATE TABLE tbl(i int PRIMARY KEY, j int)));
+
+if ($no_hot)
+{
+	$node->safe_psql('postgres', q(CREATE INDEX test_idx ON tbl(j);));
+}
+else
+{
+	$node->safe_psql('postgres', q(CREATE INDEX test_idx ON tbl(i);));
+}
+
+
+# Load amcheck
+$node->safe_psql('postgres', q(CREATE EXTENSION amcheck));
+
+# Insert $n rows into tbl
+$node->safe_psql('postgres', qq(
+	INSERT INTO tbl SELECT i, i FROM generate_series(1,$n) i
+));
+
+my $sum = $node->safe_psql('postgres', q(
+	SELECT SUM(j) AS sum FROM tbl
+));
+
+
+$node->pgbench(
+'--no-vacuum --client=15 --jobs=4 --exit-on-abort --transactions=5000',
+0,
+[qr{actually processed}],
+[qr{^$}],
+'concurrent operations with REPACK CONCURRENTLY',
+{
+	'concurrent_ops' => qq(
+		SELECT pg_try_advisory_lock(42)::integer AS gotlock \\gset
+		\\if :gotlock
+			REPACK (CONCURRENTLY) tbl USING INDEX tbl_pkey;
+			SELECT bt_index_parent_check('tbl_pkey', heapallindexed => true);
+			SELECT bt_index_parent_check('test_idx', heapallindexed => true);
+			\\sleep 10 ms
+
+			REPACK (CONCURRENTLY) tbl USING INDEX test_idx;
+			SELECT bt_index_parent_check('tbl_pkey', heapallindexed => true);
+			SELECT bt_index_parent_check('test_idx', heapallindexed => true);
+			\\sleep 10 ms
+
+			REPACK (CONCURRENTLY) tbl;
+			SELECT bt_index_parent_check('tbl_pkey', heapallindexed => true);
+			SELECT bt_index_parent_check('test_idx', heapallindexed => true);
+			\\sleep 10 ms
+
+			SELECT pg_advisory_unlock(42);
+		\\else
+			\\set num_a random(1, $n)
+			\\set num_b random(1, $n)
+			\\set diff random(1, 10000)
+			BEGIN;
+			UPDATE tbl SET j = j + :diff WHERE i = :num_a;
+			\\sleep 1 ms
+			UPDATE tbl SET j = j - :diff WHERE i = :num_b;
+			\\sleep 1 ms
+			COMMIT;
+
+			\\set v random(1, 300)
+			\\ if :v = 1
+				VACUUM ANALYZE pg_catalog.pg_class;
+			\\ endif
+
+			BEGIN
+			--TRANSACTION ISOLATION LEVEL REPEATABLE READ
+			;
+			SELECT 1;
+			\\sleep 1 ms
+			SELECT COALESCE(SUM(j), 0) AS sum FROM tbl \\gset p_
+			\\if :p_sum != $sum
+				COMMIT;
+				SELECT (:p_sum) / 0;
+			\\endif
+
+			COMMIT;
+		\\endif
+	)
+});
+
+$node->stop;
+done_testing();
diff --git a/contrib/amcheck/t/008_repack_concurrently.pl b/contrib/amcheck/t/008_repack_concurrently.pl
new file mode 100644
index 00000000000..329e6d7ed47
--- /dev/null
+++ b/contrib/amcheck/t/008_repack_concurrently.pl
@@ -0,0 +1,106 @@
+
+# Copyright (c) 2021-2025, PostgreSQL Global Development Group
+
+# Test REPACK CONCURRENTLY with concurrent modifications
+use strict;
+use warnings FATAL => 'all';
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+
+use Test::More;
+
+my $node;
+
+#
+# Test set-up
+#
+$node = PostgreSQL::Test::Cluster->new('CIC_test');
+$node->init;
+$node->append_conf('postgresql.conf',
+	'lock_timeout = ' . (1000 * $PostgreSQL::Test::Utils::timeout_default));
+$node->append_conf(
+	'postgresql.conf', qq(
+wal_level = logical
+max_worker_processes = 32
+));
+
+my $no_hot = int(rand(2));
+
+$node->start;
+$node->safe_psql('postgres', q(CREATE TABLE tbl(i SERIAL PRIMARY KEY, j int)));
+if ($no_hot)
+{
+	$node->safe_psql('postgres', q(CREATE INDEX test_idx ON tbl(j);));
+}
+else
+{
+	$node->safe_psql('postgres', q(CREATE INDEX test_idx ON tbl(i);));
+}
+
+# Load amcheck
+$node->safe_psql('postgres', q(CREATE EXTENSION amcheck));
+
+my $sum = $node->safe_psql('postgres', q(
+	SELECT SUM(j) AS sum FROM tbl
+));
+
+$node->safe_psql('postgres', q(CREATE UNLOGGED SEQUENCE last_j START 1 INCREMENT 1;));
+
+
+$node->pgbench(
+'--no-vacuum --client=15 --jobs=4 --exit-on-abort --transactions=5000',
+0,
+[qr{actually processed}],
+[qr{^$}],
+'concurrent operations with REPACK CONCURRENTLY',
+{
+	'concurrent_ops' => qq(
+		SELECT pg_try_advisory_lock(42)::integer AS gotlock \\gset
+		\\if :gotlock
+			REPACK (CONCURRENTLY) tbl USING INDEX tbl_pkey;
+			SELECT bt_index_parent_check('tbl_pkey', heapallindexed => true);
+			SELECT bt_index_parent_check('test_idx', heapallindexed => true);
+			\\sleep 10 ms
+
+			REPACK (CONCURRENTLY) tbl USING INDEX test_idx;
+			SELECT bt_index_parent_check('tbl_pkey', heapallindexed => true);
+			SELECT bt_index_parent_check('test_idx', heapallindexed => true);
+			\\sleep 10 ms
+
+			REPACK (CONCURRENTLY) tbl;
+			SELECT bt_index_parent_check('tbl_pkey', heapallindexed => true);
+			SELECT bt_index_parent_check('test_idx', heapallindexed => true);
+			\\sleep 10 ms
+
+			SELECT pg_advisory_unlock(42);
+		\\else
+			SELECT pg_advisory_lock(43);
+				BEGIN;
+				INSERT INTO tbl(j) VALUES (nextval('last_j')) RETURNING j \\gset p_
+				COMMIT;
+			SELECT pg_advisory_unlock(43);
+			\\sleep 1 ms
+			\\set v random(1, 300)
+			\\ if :v = 1
+				VACUUM ANALYZE pg_catalog.pg_class;
+			\\ endif
+
+			BEGIN
+			--TRANSACTION ISOLATION LEVEL REPEATABLE READ
+			;
+			SELECT 1;
+			\\sleep 1 ms
+			SELECT COUNT(*) AS count FROM tbl WHERE j <= :p_j \\gset p_
+			\\if :p_count != :p_j
+				COMMIT;
+				SELECT (:p_count) / 0;
+			\\endif
+
+			COMMIT;
+		\\endif
+	)
+});
+
+$node->stop;
+done_testing();
-- 
2.43.0

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