v1-0001-Test-vacuum-setting-VM-for-unmodified-heap-buffer.patch

From fa16946d3ab986542436efa57248abaf11f77fc0 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplageman@gmail.com>
Date: Wed, 10 Dec 2025 12:34:58 -0500
Subject: [PATCH v1] Test vacuum setting VM for unmodified heap buffer

When setting the VM during phase I vacuum, it is possible that the heap
buffer requires no modification. This can happen if the VM was truncated
or destroyed. Because we still add the heap buffer to the WAL chain (we
use it to update the freespace map in recovery), currently, we require
that the heap buffer be marked dirty anyway. We could pass
REGBUF_NO_CHANGES when registering the buffer, but that adds fiddly
extra logic for an uncommon case.

Either way, we had no test coverage for this, so it's best to add it.
---
 .../pg_visibility/expected/pg_visibility.out    | 17 +++++++++++++++++
 contrib/pg_visibility/sql/pg_visibility.sql     | 13 +++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/contrib/pg_visibility/expected/pg_visibility.out b/contrib/pg_visibility/expected/pg_visibility.out
index 09fa5933a35..3608f801eee 100644
--- a/contrib/pg_visibility/expected/pg_visibility.out
+++ b/contrib/pg_visibility/expected/pg_visibility.out
@@ -204,6 +204,23 @@ select pg_truncate_visibility_map('test_partition');
  
 (1 row)
 
+-- test the case where vacuum phase I does not need to modify the heap buffer
+-- and only needs to set the VM
+create table test_vac_unmodified_heap(a int);
+insert into test_vac_unmodified_heap values (1);
+vacuum (freeze) test_vac_unmodified_heap;
+-- the checkpoint cleans the buffer dirtied by freezing the sole tuple
+checkpoint;
+-- truncating the VM ensures that the next vacuum will need to set it
+select pg_truncate_visibility_map('test_vac_unmodified_heap');
+ pg_truncate_visibility_map 
+----------------------------
+ 
+(1 row)
+
+-- vacuum sets the VM but does not need to set PD_ALL_VISIBLE so no heap page
+-- modification
+vacuum test_vac_unmodified_heap;
 -- test copy freeze
 create table copyfreeze (a int, b char(1500));
 -- load all rows via COPY FREEZE and ensure that all pages are set all-visible
diff --git a/contrib/pg_visibility/sql/pg_visibility.sql b/contrib/pg_visibility/sql/pg_visibility.sql
index 5af06ec5b76..6af7c179df0 100644
--- a/contrib/pg_visibility/sql/pg_visibility.sql
+++ b/contrib/pg_visibility/sql/pg_visibility.sql
@@ -94,6 +94,19 @@ select count(*) > 0 from pg_visibility_map_summary('test_partition');
 select * from pg_check_frozen('test_partition'); -- hopefully none
 select pg_truncate_visibility_map('test_partition');
 
+-- test the case where vacuum phase I does not need to modify the heap buffer
+-- and only needs to set the VM
+create table test_vac_unmodified_heap(a int);
+insert into test_vac_unmodified_heap values (1);
+vacuum (freeze) test_vac_unmodified_heap;
+-- the checkpoint cleans the buffer dirtied by freezing the sole tuple
+checkpoint;
+-- truncating the VM ensures that the next vacuum will need to set it
+select pg_truncate_visibility_map('test_vac_unmodified_heap');
+-- vacuum sets the VM but does not need to set PD_ALL_VISIBLE so no heap page
+-- modification
+vacuum test_vac_unmodified_heap;
+
 -- test copy freeze
 create table copyfreeze (a int, b char(1500));
 
-- 
2.43.0