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Date: Fri, 10 Oct 2025 19:09:01 +0200
From: "Matthijs van der Vleuten" <postgresql@zr40.nl>
To: "Peter Geoghegan" <pg@bowt.ie>
Cc: pgsql-bugs@lists.postgresql.org
Message-Id: <08a4226e-9886-439e-b745-3d65defd078e@app.fastmail.com>
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References: <19082-628e62160dbbc1c1@postgresql.org>
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Subject: Re: BUG #19082: Failing assert in index scan
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On Fri, Oct 10, 2025, at 18:28, Peter Geoghegan wrote:
> On Fri, Oct 10, 2025 at 12:13=E2=80=AFPM Matthijs van der Vleuten
> <postgresql@zr40.nl> wrote:
>> No, none of the tables and indexes in that database are unlogged or t=
emporary.
>
> Are these backwards scans? In other words, does EXPLAIN show "Index
> Scan Backwards" for these queries? I see that the first query "ORDER
> BY "mentions"."id" ASC", but perhaps that index column is declared as
> DESC (in which case it'd be a backwards scan)?

No, the index columns aren't declared as DESC, and, as far as I've been =
able to verify, the query plan doesn't use backward scans.

I've isolated one particular row that triggers this assert. It's a diffe=
rent table but it fails the assertion the same way.

zr40@[local]:5432 mastodon_production=3D# explain SELECT "preview_cards_=
statuses".* FROM "preview_cards_statuses" WHERE "preview_cards_statuses"=
."status_id" =3D 114337794133378371 LIMIT 1;
                                                   QUERY PLAN           =
                                        =20
------------------------------------------------------------------------=
-----------------------------------------
 Limit  (cost=3D0.42..2.84 rows=3D1 width=3D81)
   ->  Index Scan using preview_cards_statuses_pkey on preview_cards_sta=
tuses  (cost=3D0.42..2.84 rows=3D1 width=3D81)
         Index Cond: (status_id =3D '114337794133378371'::bigint)
(3 rows)

zr40@[local]:5432 mastodon_production=3D# select "preview_cards_statuses=
".* FROM "preview_cards_statuses" WHERE "preview_cards_statuses"."status=
_id" =3D 114337794133378371 LIMIT 1;
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
The connection to the server was lost. Attempting reset: Failed.

Server logs:

TRAP: failed Assert("!XLogRecPtrIsInvalid(so->currPos.lsn)"), File: "./b=
uild/../src/backend/access/nbtree/nbtutils.c", Line: 1782, PID: 50833
postgres: zr40 mastodon_production [local] SELECT(ExceptionalCondition+0=
x75) [0x5654f247eea5]
postgres: zr40 mastodon_production [local] SELECT(_bt_killitems+0x550) [=
0x5654f1ff4710]
postgres: zr40 mastodon_production [local] SELECT(+0x1eee95) [0x5654f1fe=
be95]
postgres: zr40 mastodon_production [local] SELECT(_bt_next+0x92) [0x5654=
f1fed472]
postgres: zr40 mastodon_production [local] SELECT(btgettuple+0x7c) [0x56=
54f1fe9ddc]
postgres: zr40 mastodon_production [local] SELECT(index_getnext_tid+0x56=
) [0x5654f1fdab06]
postgres: zr40 mastodon_production [local] SELECT(index_getnext_slot+0x7=
b) [0x5654f1fdad4b]
postgres: zr40 mastodon_production [local] SELECT(+0x37bfbd) [0x5654f217=
8fbd]
postgres: zr40 mastodon_production [local] SELECT(+0x37e3e9) [0x5654f217=
b3e9]
postgres: zr40 mastodon_production [local] SELECT(standard_ExecutorRun+0=
x19b) [0x5654f215013b]
postgres: zr40 mastodon_production [local] SELECT(+0x531141) [0x5654f232=
e141]
postgres: zr40 mastodon_production [local] SELECT(PortalRun+0x2b0) [0x56=
54f232fa10]
postgres: zr40 mastodon_production [local] SELECT(+0x52e651) [0x5654f232=
b651]
postgres: zr40 mastodon_production [local] SELECT(PostgresMain+0x1618) [=
0x5654f232d088]
postgres: zr40 mastodon_production [local] SELECT(+0x49036b) [0x5654f228=
d36b]
postgres: zr40 mastodon_production [local] SELECT(PostmasterMain+0xeee) =
[0x5654f228e50e]
postgres: zr40 mastodon_production [local] SELECT(main+0x21c) [0x5654f1f=
6806c]
/lib/x86_64-linux-gnu/libc.so.6(+0x29ca8) [0x7f0b6922eca8]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x85) [0x7f0b6922ed65]
postgres: zr40 mastodon_production [local] SELECT(_start+0x21) [0x5654f1=
f68631]
2025-10-10 19:03:26 CEST 68e8cff4.6ce1 | LOG:  server process (PID 50833=
) was terminated by signal 6: Aborted
2025-10-10 19:03:26 CEST 68e8cff4.6ce1 | DETAIL:  Failed process was run=
ning: select "preview_cards_statuses".* FROM "preview_cards_statuses" WH=
ERE "preview_cards_statuses"."status_id" =3D 114337794133378371 LIMIT 1;
2025-10-10 19:03:26 CEST 68e8cff4.6ce1 | LOG:  terminating any other act=
ive server processes