From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Thu, 5 Mar 2026 17:47:20 +0000 (+0200)
Subject: Fix handling of updated tuples in the MERGE statement
X-Git-Tag: REL_17_10~98
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=2dcac93c00655cc62e8105dc12b6cd302d2c0625;p=thirdparty%2Fpostgresql.git

Fix handling of updated tuples in the MERGE statement

This branch missed the IsolationUsesXactSnapshot() check.  That led to EPQ on
repeatable read and serializable isolation levels.  This commit fixes the
issue and provides a simple isolation check for that.  Backpatch through v15
where MERGE statement was introduced.

Reported-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/CAPpHfdvzZSaNYdj5ac-tYRi6MuuZnYHiUkZ3D-AoY-ny8v%2BS%2Bw%40mail.gmail.com
Author: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Backpatch-through: 15
---

diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index 4719cc9e840..14795857a7a 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -3178,6 +3178,11 @@ lmerge_matched:
 							   *inputslot;
 					LockTupleMode lockmode;
 
+					if (IsolationUsesXactSnapshot())
+						ereport(ERROR,
+								(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
+								 errmsg("could not serialize access due to concurrent update")));
+
 					/*
 					 * The target tuple was concurrently updated by some other
 					 * transaction.  If we are currently processing a MATCHED
diff --git a/src/test/isolation/expected/merge-update.out b/src/test/isolation/expected/merge-update.out
index 22a32e93b04..dd6a9f6c6ca 100644
--- a/src/test/isolation/expected/merge-update.out
+++ b/src/test/isolation/expected/merge-update.out
@@ -549,3 +549,36 @@ step c1: COMMIT;
 step pa_merge2c_dup: <... completed>
 ERROR:  MERGE command cannot affect row a second time
 step a2: ABORT;
+
+starting permutation: merge2a c1 s1beginrr merge1 c2
+step merge2a: 
+  MERGE INTO target t
+  USING (SELECT 1 as key, 'merge2a' as val) s
+  ON s.key = t.key
+  WHEN NOT MATCHED THEN
+	INSERT VALUES (s.key, s.val)
+  WHEN MATCHED THEN
+	UPDATE set key = t.key + 1, val = t.val || ' updated by ' || s.val
+  WHEN NOT MATCHED BY SOURCE THEN
+	UPDATE set key = t.key + 1, val = t.val || ' source not matched by merge2a'
+  RETURNING merge_action(), t.*;
+
+merge_action|key|val                      
+------------+---+-------------------------
+UPDATE      |  2|setup1 updated by merge2a
+(1 row)
+
+step c1: COMMIT;
+step s1beginrr: BEGIN ISOLATION LEVEL REPEATABLE READ;
+step merge1: 
+  MERGE INTO target t
+  USING (SELECT 1 as key, 'merge1' as val) s
+  ON s.key = t.key
+  WHEN NOT MATCHED THEN
+	INSERT VALUES (s.key, s.val)
+  WHEN MATCHED THEN
+    UPDATE set key = t.key + 1, val = t.val || ' updated by ' || s.val;
+ <waiting ...>
+step c2: COMMIT;
+step merge1: <... completed>
+ERROR:  could not serialize access due to concurrent update
diff --git a/src/test/isolation/specs/merge-update.spec b/src/test/isolation/specs/merge-update.spec
index d3f031b1574..36835ef345d 100644
--- a/src/test/isolation/specs/merge-update.spec
+++ b/src/test/isolation/specs/merge-update.spec
@@ -93,6 +93,7 @@ step "pa_merge3"
 }
 step "c1" { COMMIT; }
 step "a1" { ABORT; }
+step "s1beginrr" { BEGIN ISOLATION LEVEL REPEATABLE READ; }
 
 session "s2"
 setup
@@ -223,3 +224,4 @@ permutation "pa_merge2" "c1" "pa_merge2a" "pa_select2" "c2" # succeeds
 permutation "pa_merge3" "pa_merge2b_when" "c1" "pa_select2" "c2" # WHEN not satisfied by updated tuple
 permutation "pa_merge1" "pa_merge2b_when" "c1" "pa_select2" "c2" # WHEN satisfied by updated tuple
 permutation "pa_merge1" "pa_merge2c_dup" "c1" "a2"
+permutation "merge2a" "c1" "s1beginrr" "merge1" "c2"