From 177037341a4414f1089748686af9ce8bd9f8d383 Mon Sep 17 00:00:00 2001 From: Alexander Korotkov Date: Thu, 5 Mar 2026 19:47:20 +0200 Subject: [PATCH] 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 Discussion: https://postgr.es/m/CAPpHfdvzZSaNYdj5ac-tYRi6MuuZnYHiUkZ3D-AoY-ny8v%2BS%2Bw%40mail.gmail.com Author: Tender Wang Reviewed-by: Dean Rasheed Backpatch-through: 15 --- src/backend/executor/nodeModifyTable.c | 5 +++ src/test/isolation/expected/merge-update.out | 33 ++++++++++++++++++++ src/test/isolation/specs/merge-update.spec | 2 ++ 3 files changed, 40 insertions(+) diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c index 793c76d4f82..327c27abff9 100644 --- a/src/backend/executor/nodeModifyTable.c +++ b/src/backend/executor/nodeModifyTable.c @@ -3547,6 +3547,11 @@ 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 feceacf4818..821565b4303 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(), old, new, t.*; + +merge_action|old |new |key|val +------------+----------+-------------------------------+---+------------------------- +UPDATE |(1,setup1)|(2,"setup1 updated by merge2a")| 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; + +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 771ee5b70cf..b902779edd6 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"