"WHERE 1 = 2 OR ..." makes planner choose a very inefficient plan

Hello,

I recently stumbled upon on what could be a planner bug or a corner case.
If "<false condition> OR ..." is added to WHERE clause of SELECT query,
then the planner chooses a very inefficient plan. Consider a query:

SELECT count(k0.id)
FROM k0
WHERE 1 = 2
OR k0.id IN (
SELECT k1.k0_id
FROM k1
WHERE k1.k1k2_id IN (
SELECT k2.k1k2_id
FROM k2
WHERE k2.t = 2
AND (coalesce(k2.z, '')) LIKE '%12%'
)
);

EXPLAIN (ANALYZE, BUFFERS) for this query:
http://explain.depesz.com/s/tcn
Execution time: 2037872.420 ms (almost 34 minutes!!)

If I comment out "1=2 OR", then the plan changes dramatically:
http://explain.depesz.com/s/5rsW
Execution time: 617.778 ms

I know LEFT JOIN or EXISTS instead of NOT IN in this case will give better
plans. What bothers me is not performance of this particular query, but the
strange behavior of query planner. Is this behavior considered normal, or
should I file a bug?

database schema: http://pgsql.privatepaste.com/b297e685c5
version: PostgreSQL 9.1.9 on x86_64-unknown-linux-gnu, compiled by gcc (GCC)
4.4.7 20120313 (Red Hat 4.4.7-3), 64-bit
package: postgresql91-server.x86_64 9.1.9-1PGDG.rhel6
os: Scientific Linux 6.3
postgresql.conf: http://pgsql.privatepaste.com/e3e75bb789

--