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Fix handling of updated tuples in the MERGE statement

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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
This commit is contained in:
Alexander Korotkov 2026-03-05 19:47:20 +02:00
parent bffd7130e9
commit 177037341a
3 changed files with 40 additions and 0 deletions
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5
src/backend/executor/nodeModifyTable.c
View file

@ -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

33
src/test/isolation/expected/merge-update.out
View file

@ -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;
<waiting ...>
step c2: COMMIT;
step merge1: <... completed>
ERROR: could not serialize access due to concurrent update

2
src/test/isolation/specs/merge-update.spec
View file

@ -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"