BUG #19684: Assertion in tuplesort_begin_heap() falsified by parallel plan with sort

The following bug has been logged on the website:

Bug reference: 19684
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 19beta3
Operating system: Ubuntu 24.04
Description:

The following script:
SET cpu_tuple_cost = 1000;
SET min_parallel_table_scan_size = 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=3188844.07..3188856.82 rows=2 width=0)
-> Sort (cost=3188844.07..3188856.82 rows=5100 width=0)
-> Gather (cost=1000.00..3188530.00 rows=5100 width=0)
Workers Planned: 2
-> Parallel Append (cost=0.00..3187020.00 rows=2124
width=0)
-> Parallel Seq Scan on t (cost=0.00..1062510.00
rows=1062 width=0)
-> Parallel Seq Scan on t t_1 (cost=0.00..1062510.00
rows=1062 width=0)

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

Reproduced starting from 66c0185a3/12933dc60.