BUG #5263: Query execution fails with "ERROR: FULL JOIN is only supported with merge-joinable join conditions"

The following bug has been logged online:

Bug reference: 5263
Logged by: Jozsef Szalay
Email address: jszalay(at)storediq(dot)com
PostgreSQL version: 8.4.2
Operating system: Windows, Linux (Fedora/CentOS)
Description: Query execution fails with "ERROR: FULL JOIN is only
supported with merge-joinable join conditions"
Details:

Create a table:

CREATE TABLE test
(
id integer,
description character varying,
CONSTRAINT test_pkey PRIMARY KEY (id)
);

Execute the following query:

SELECT *
FROM (SELECT id, 0 AS value
FROM test
WHERE description = 'abc'
) t1
FULL OUTER JOIN
(SELECT id, 1 AS value
FROM test
WHERE description = 'def'
) t2 USING (id, value);

You'll get the "ERROR: FULL JOIN is only supported with merge-joinable join
conditions"

This used to work with 8.3.7 (and before).

I can make the query work by adding an "ORDER BY id, value" clause to each
sub-query or if I use the same constant (e.g. "0") as value in both
sub-queries.