Slow fulltext query plan

Hi,

I would like to understand why the following query execution don't use
any fulltext indexes
and takes more than 300s (using lot of temporary files):

EXPLAIN ANALYZE SELECT hierarchy.id
FROM hierarchy
JOIN fulltext ON fulltext.id = hierarchy.id,
TO_TSQUERY('whatever') query1,
TO_TSQUERY('whatever') query2
WHERE (query1 @@ nx_to_tsvector(fulltext.fulltext)) OR (query2 @@
nx_to_tsvector(fulltext.fulltext_title));

The query plan is here:
http://explain.depesz.com/s/YgP

While if I replace the query2 by query1 in the second clause:

EXPLAIN ANALYZE SELECT hierarchy.id
FROM hierarchy
JOIN fulltext ON fulltext.id = hierarchy.id,
TO_TSQUERY('whatever') query1,
TO_TSQUERY('whatever') query2
WHERE (query1 @@ nx_to_tsvector(fulltext.fulltext)) OR (query1 @@
nx_to_tsvector(fulltext.fulltext_title));

It is 5 order of magniude faster (15ms) using the gin indexes:
http://explain.depesz.com/s/RLa

The nx_to_tsvector is an immutable function with the following code:
SELECT TO_TSVECTOR('english', SUBSTR($1, 1, 250000))

Here is the list of indexes:
hierarchy: "hierarchy_pk" PRIMARY KEY, btree (id)
fulltext: "fulltext_fulltext_idx" gin
(nx_to_tsvector(fulltext::character varying))
fulltext: "fulltext_fulltext_title_idx" gin
(nx_to_tsvector(fulltext_title::character varying))

fulltext and fulltext_title are text type.

And some PostgreSQL configuration:
PostgreSQL 9.1.2 on x86_64-unknown-linux-gnu
shared_buffers: 4GB
effective_cache_size: 10GB
work_mem: 20MB

Thanks for your work and enlightenment

ben