From: Antonin Houska <ah@cybertec.at>
To: Andres Freund <andres@anarazel.de>
cc: Kirill Reshke <reshkekirill@gmail.com>,
    Heikki Linnakangas <hlinnaka@iki.fi>,
    Melanie Plageman <melanieplageman@gmail.com>,
    Matthias van de Meent <boekewurm+postgres@gmail.com>,
    pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>,
    Noah Misch <noah@leadboat.com>, Robert Haas <robertmhaas@gmail.com>,
    Michael Paquier <michael.paquier@gmail.com>
Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
In-reply-to: <aYtg622xasbSju7_@alap3.anarazel.de>
References: <1108f18d-cf7c-4f17-b29c-a119fe42f7e5@iki.fi> <5dwlfu2jyzkyf3nrlzxxblxctb6xio5es73ptgsahjnmfu5miu@772rc764hfhi> <ossv2eistssmubfsir6xjll76tynvxv5lup4zkrfzjkud7fycw@rf5vii6l6cha> <4csodkvvfbfloxxjlkgsnl2lgfv2mtzdl7phqzd4jxjadxm4o5@usw7feyb5bzf> <CALdSSPgyc7VuMLUZ8J7v2G-PebBa_vEi+mp-cLkcOwNycB56Hw@mail.gmail.com> <kjdgvws34gpnz4y6xu5aaeul5mspgy3ahvyiw4lndb3ecsacdb@b2ccyowotpqh> <jtg5cu4n6h5lib3kzx66ju4yhh6kmviaud7oq6dtut6c4q4rdi@xwsfoagt3c2b> <61812.1770637345@localhost> <aYpbjeiN2lKEBBn1@alap3.anarazel.de> <19720.1770709587@localhost> <aYtg622xasbSju7_@alap3.anarazel.de>
Comments: In-reply-to Andres Freund <andres@anarazel.de>
   message dated "Tue, 10 Feb 2026 11:49:36 -0500."
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Date: Thu, 12 Feb 2026 11:36:08 +0100
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Andres Freund <andres@anarazel.de> wrote:

> For something committable, I think we should probably split IsMVCCSnapshot
> into IsMVCCSnapshot(), just accepting SNAPSHOT_MVCC, and IsMVCCLikeSnapshot()
> accepting both SNAPSHOT_MVCC and SNAPSHOT_HISTORIC_MVCC. And then go through
> all the existing callers of IsMVCCSnapshot() - only about half should stay
> as-is, I think.

The attached patch tries to do that.

-- 
Antonin Houska
Web: https://www.cybertec-postgresql.com


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Content-Type: text/x-diff
Content-Disposition: attachment;
 filename=0001-Refine-checking-of-snapshot-type.patch

From dcdbaf3095e632a1f7f65f3abc43eccff0249d4c Mon Sep 17 00:00:00 2001
From: Antonin Houska <ah@cybertec.at>
Date: Thu, 12 Feb 2026 11:14:00 +0100
Subject: [PATCH] Refine checking of snapshot type.

It appears to be confusing if IsMVCCSnapshot() evaluates to true for both
"regular" and "historic" MVCC snapshot. This patch restricts the meaning of
the macro to the "regular" MVCC snapshot, and introduces a new macro
IsMVCCLikeSnapshot() to recognize both types.

IsMVCCLikeSnapshot() is only used in functions that can (supposedly) be called
during logical decoding.
---
 src/backend/access/heap/heapam_handler.c | 2 +-
 src/backend/access/index/indexam.c       | 2 +-
 src/backend/access/nbtree/nbtree.c       | 2 +-
 src/include/utils/snapmgr.h              | 6 ++++--
 4 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index cbef73e5d4b..332c788bab2 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -159,7 +159,7 @@ heapam_index_fetch_tuple(struct IndexFetchTableData *scan,
 		 * Only in a non-MVCC snapshot can more than one member of the HOT
 		 * chain be visible.
 		 */
-		*call_again = !IsMVCCSnapshot(snapshot);
+		*call_again = !IsMVCCLikeSnapshot(snapshot);
 
 		slot->tts_tableOid = RelationGetRelid(scan->rel);
 		ExecStoreBufferHeapTuple(&bslot->base.tupdata, slot, hscan->xs_cbuf);
diff --git a/src/backend/access/index/indexam.c b/src/backend/access/index/indexam.c
index 4ed0508c605..80623350d6f 100644
--- a/src/backend/access/index/indexam.c
+++ b/src/backend/access/index/indexam.c
@@ -445,7 +445,7 @@ index_markpos(IndexScanDesc scan)
 void
 index_restrpos(IndexScanDesc scan)
 {
-	Assert(IsMVCCSnapshot(scan->xs_snapshot));
+	Assert(IsMVCCLikeSnapshot(scan->xs_snapshot));
 
 	SCAN_CHECKS;
 	CHECK_SCAN_PROCEDURE(amrestrpos);
diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c
index 3dec1ee657d..07ba5997fbc 100644
--- a/src/backend/access/nbtree/nbtree.c
+++ b/src/backend/access/nbtree/nbtree.c
@@ -422,7 +422,7 @@ btrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 	 * Note: so->dropPin should never change across rescans.
 	 */
 	so->dropPin = (!scan->xs_want_itup &&
-				   IsMVCCSnapshot(scan->xs_snapshot) &&
+				   IsMVCCLikeSnapshot(scan->xs_snapshot) &&
 				   RelationNeedsWAL(scan->indexRelation) &&
 				   scan->heapRelation != NULL);
 
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index b8c01a291a1..dd5aaae6953 100644
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -53,12 +53,14 @@ extern PGDLLIMPORT SnapshotData SnapshotToastData;
 
 /* This macro encodes the knowledge of which snapshots are MVCC-safe */
 #define IsMVCCSnapshot(snapshot)  \
-	((snapshot)->snapshot_type == SNAPSHOT_MVCC || \
-	 (snapshot)->snapshot_type == SNAPSHOT_HISTORIC_MVCC)
+	((snapshot)->snapshot_type == SNAPSHOT_MVCC)
 
 #define IsHistoricMVCCSnapshot(snapshot)  \
 	((snapshot)->snapshot_type == SNAPSHOT_HISTORIC_MVCC)
 
+#define IsMVCCLikeSnapshot(snapshot)  \
+	(IsMVCCSnapshot(snapshot) || IsHistoricMVCCSnapshot(snapshot))
+
 extern Snapshot GetTransactionSnapshot(void);
 extern Snapshot GetLatestSnapshot(void);
 extern void SnapshotSetCommandId(CommandId curcid);
-- 
2.47.3


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