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Subject: BUG #19441: Backend waits for serializable snapshot indefinitely on removing temp relations
To: pgsql-bugs@lists.postgresql.org
From: PG Bug reporting form <noreply@postgresql.org>
Cc: exclusion@gmail.com
Reply-To: exclusion@gmail.com, pgsql-bugs@lists.postgresql.org
Date: Sun, 29 Mar 2026 10:00:01 +0000
Message-ID: <19441-ec29f3b1363b4a68@postgresql.org>
Auto-Submitted: auto-generated
Archived-At: <https://www.postgresql.org/message-id/19441-ec29f3b1363b4a68%40postgresql.org>
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The following bug has been logged on the website:

Bug reference:      19441
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 18.3
Operating system:   Ubuntu 24.04
Description:       =20

The following script:
echo "
CREATE TEMPORARY TABLE tt (i int);
SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL SERIALIZABLE READ
ONLY DEFERRABLE;
SELECT pg_sleep(2);
" | psql &
sleep 1

echo "
CREATE TABLE t (i int);
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
INSERT INTO t VALUES (1);
PREPARE TRANSACTION 'pt';
" | psql

wait
psql -c "SELECT pid, pg_terminate_backend(pg_stat_activity.pid) FROM
pg_stat_activity WHERE backend_type=3D'client backend' AND query NOT LIKE
'%pg_stat_activity%'"
sleep 5

psql -c "SELECT * FROM pg_stat_activity WHERE backend_type=3D'client backen=
d'
AND query NOT LIKE '%pg_stat_activity%'"

(with max_prepared_transactions =3D 1 in postgresql.conf) instantiates a
backend hanging on exit, waiting for a snapshot:

   pid   | pg_terminate_backend
---------+----------------------
 2143677 | t

gdb -p 2143677

(gdb) bt
#0  0x000077c27fb2a007 in epoll_wait (epfd=3D5, events=3D0x5d3d166cd868,
maxevents=3D1, timeout=3Dtimeout@entry=3D-1)
    at ../sysdeps/unix/sysv/linux/epoll_wait.c:30
#1  0x00005d3ce11b11d2 in WaitEventSetWaitBlock
(set=3Dset@entry=3D0x5d3d166cd800, cur_timeout=3Dcur_timeout@entry=3D-1,
    occurred_events=3Doccurred_events@entry=3D0x7ffeb661f160,
nevents=3Dnevents@entry=3D1) at waiteventset.c:1193
#2  0x00005d3ce11b1bd4 in WaitEventSetWait (set=3D0x5d3d166cd800,
timeout=3Dtimeout@entry=3D-1,
    occurred_events=3Doccurred_events@entry=3D0x7ffeb661f160,
nevents=3Dnevents@entry=3D1,
    wait_event_info=3Dwait_event_info@entry=3D134217779) at waiteventset.c:=
1141
#3  0x00005d3ce11a4b78 in WaitLatch (latch=3D<optimized out>,
wakeEvents=3DwakeEvents@entry=3D33, timeout=3Dtimeout@entry=3D0,
    wait_event_info=3Dwait_event_info@entry=3D134217779) at latch.c:196
#4  0x00005d3ce11c8188 in ProcWaitForSignal
(wait_event_info=3Dwait_event_info@entry=3D134217779) at proc.c:2005
#5  0x00005d3ce11c42bf in GetSafeSnapshot
(origSnapshot=3DorigSnapshot@entry=3D0x5d3ce17753e0 <CurrentSnapshotData>)
    at predicate.c:1600
#6  0x00005d3ce11c4436 in GetSerializableTransactionSnapshot
(snapshot=3Dsnapshot@entry=3D0x5d3ce17753e0 <CurrentSnapshotData>)
    at predicate.c:1716
#7  0x00005d3ce137077d in GetTransactionSnapshot () at snapmgr.c:320
#8  0x00005d3ce0e67c65 in RemoveTempRelationsCallback (code=3D<optimized ou=
t>,
arg=3D<optimized out>) at namespace.c:4703
#9  0x00005d3ce11a3cad in shmem_exit (code=3Dcode@entry=3D0) at ipc.c:250
#10 0x00005d3ce11a3da6 in proc_exit_prepare (code=3Dcode@entry=3D0) at ipc.=
c:199
#11 0x00005d3ce11a3e3c in proc_exit (code=3Dcode@entry=3D0) at ipc.c:112
#12 0x00005d3ce11d7c07 in PostgresMain (dbname=3D<optimized out>,
username=3D<optimized out>) at postgres.c:5046
#13 0x00005d3ce11d0fac in BackendMain (startup_data=3D<optimized out>,
startup_data_len=3D<optimized out>)
    at backend_startup.c:124
...

Reproduced starting from 7c38ef2a5.