Re: Wrong results from Parallel Hash Full Join

I noticed that BF animal conchuela has several times fallen over on the
test case added by 558c9d75f:

diff -U3 /home/pgbf/buildroot/HEAD/pgsql.build/src/test/regress/expected/join_hash.out /home/pgbf/buildroot/HEAD/pgsql.build/src/test/regress/results/join_hash.out
--- /home/pgbf/buildroot/HEAD/pgsql.build/src/test/regress/expected/join_hash.out 2023-04-19 10:20:26.159840000 +0200
+++ /home/pgbf/buildroot/HEAD/pgsql.build/src/test/regress/results/join_hash.out 2023-04-19 10:21:47.971900000 +0200
@@ -974,8 +974,8 @@
SELECT * FROM hjtest_matchbits_t1 t1 FULL JOIN hjtest_matchbits_t2 t2 ON t1.id = t2.id;
id | id
----+----
- 1 |
| 2
+ 1 |
(2 rows)

-- Test serial full hash join.

Considering that this is a parallel plan, I don't think there's any
mystery about why an ORDER-BY-less query might have unstable output
order; the only mystery is why more of the buildfarm hasn't failed.
Can we just add "ORDER BY t1.id" to this query? It looks like you
get the same PHJ plan, although now underneath Sort/Gather Merge.

regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=conchuela&dt=2023-04-19%2008%3A20%3A56
[2] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=conchuela&dt=2023-05-03%2006%3A21%3A03
[3] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=conchuela&dt=2023-05-19%2022%3A21%3A04