BUG #16965: Select query fails with ERROR: XX000: could not find pathkey item to sort

The following bug has been logged on the website:

Bug reference: 16965
Logged by: Jakub Zemanek
Email address: jakub(dot)zemanek(at)gmail(dot)com
PostgreSQL version: 13.2
Operating system: Ubuntu 20.04
Description:

Hi,
after upgrade of our zabbix database from v 12 to v 13, we have encountered
error in select queries. When we SET max_parallel_workers_per_gather=0; then
the query finishes without error. The query looks like this:
SELECT DISTINCT COUNT(DISTINCT t.triggerid) AS rowscount,i.hostid FROM
triggers t,functions f,items i WHERE i.hostid=10406 AND
f.triggerid=t.triggerid AND f.itemid=i.itemid AND t.flags IN (0,4) GROUP BY
i.hostid;'
ERROR: XX000: could not find pathkey item to sort
LOCATION: prepare_sort_from_pathkeys, createplan.c:6013

Environment info:
psql --version
psql (PostgreSQL) 13.2 (Ubuntu 13.2-1.pgdg20.04+1)

psql -c 'select version();'
PostgreSQL 13.2 (Ubuntu 13.2-1.pgdg20.04+1) on x86_64-pc-linux-gnu, compiled
by gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0, 64-bit

uname -a
Linux hostname 5.4.0-65-generic #73-Ubuntu SMP Mon Jan 18 17:25:17 UTC 2021
x86_64 x86_64 x86_64 GNU/Linux

pg_ctlcluster 13 main status
pg_ctl: server is running (PID: 595329)
/usr/lib/postgresql/13/bin/postgres "-D" "/var/lib/postgresql/13/main" "-c"
"config_file=/etc/postgresql/13/main/postgresql.conf"

How to reproduce:

createdb test
wget https://www.coreit.cz/download/test.sql.gz
zcat test-plain.sql.gz | psql test
vacuumdb -z test
psql test -X -v VERBOSITY=verbose -c 'SELECT DISTINCT COUNT(DISTINCT
t.triggerid) AS rowscount,i.hostid FROM triggers t,functions f,items i WHERE
i.hostid=10406 AND f.triggerid=t.triggerid AND f.itemid=i.itemid AND t.flags
IN (0,4) GROUP BY i.hostid;'