Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: quoted-printable
Subject: BUG #19684: Assertion in tuplesort_begin_heap() falsified by parallel
 plan with sort
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: Fri, 11 Sep 2026 20:00:01 +0000
Message-ID: <19684-fa0b7fdc7d235fd9@postgresql.org>
Auto-Submitted: auto-generated
Archived-At: 
 <https://www.postgresql.org/message-id/19684-fa0b7fdc7d235fd9%40postgresql.org>
Precedence: bulk

The following bug has been logged on the website:

Bug reference:      19684
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 19beta3
Operating system:   Ubuntu 24.04
Description:       =20

The following script:
SET cpu_tuple_cost =3D 1000;
SET min_parallel_table_scan_size =3D 1;

CREATE TABLE t(i int);
SELECT FROM t UNION SELECT FROM t;

triggers:
TRAP: failed Assert("nkeys > 0"), File: "tuplesortvariants.c", Line: 195,
PID: 1465852

EXPLAIN shows:
                                          QUERY PLAN
---------------------------------------------------------------------------=
--------------------
 Unique  (cost=3D3188844.07..3188856.82 rows=3D2 width=3D0)
   ->  Sort  (cost=3D3188844.07..3188856.82 rows=3D5100 width=3D0)
         ->  Gather  (cost=3D1000.00..3188530.00 rows=3D5100 width=3D0)
               Workers Planned: 2
               ->  Parallel Append  (cost=3D0.00..3187020.00 rows=3D2124
width=3D0)
                     ->  Parallel Seq Scan on t  (cost=3D0.00..1062510.00
rows=3D1062 width=3D0)
                     ->  Parallel Seq Scan on t t_1  (cost=3D0.00..1062510.=
00
rows=3D1062 width=3D0)

Without asserts enabled, SELECT succeeds and EXPLAIN (ANALYZE) shows the
same plan.

Reproduced starting from 66c0185a3/12933dc60.