not using my GIN index in JOIN expression

I am running the last version of PostgreSQL 9.3.3
I have two tables detm and corm and a lot of datas in the column
cormdata of corm table (1.4 GB).

I have a GIN index on cormdata:
CREATE INDEX ix_corm_fulltext_cormdata ON corm
USING gin (to_tsvector('french'::regconfig, cormdata))
WHERE cormishtml IS FALSE AND length(cormdata) < 20000;

select distinct b.detmmailid from corm b where
(to_tsvector('french',b.cormdata) @@ to_tsquery('mauritanie') and
b.cormishtml is false and length(b.cormdata) < 20000)
is very fast and use the GIN index.

"HashAggregate (cost=2027.72..2031.00 rows=328 width=52)"
" -> Bitmap Heap Scan on corm b (cost=24.25..2026.35 rows=548 width=52)"
" Recheck Cond: ((to_tsvector('french'::regconfig, cormdata) @@
to_tsquery('mauritanie'::text)) AND (cormishtml IS FALSE) AND
(length(cormdata) < 20000))"
" -> Bitmap Index Scan on ix_corm_fulltext_cormdata
(cost=0.00..24.11 rows=548 width=0)"
" Index Cond: (to_tsvector('french'::regconfig, cormdata)
@@ to_tsquery('mauritanie'::text))"

With a join an another table detm, GIN index is not used

explain select distinct a.detmmailid from detm a JOIN corm b on
a.detmmailid = b.detmmailid where ((to_tsvector('french',b.cormdata) @@
to_tsquery('mauritanie') and b.cormishtml is false and
length(b.cormdata) < 20000) OR ( detmobjet ~* 'mauritanie' ))

"HashAggregate (cost=172418.27..172423.98 rows=571 width=52)"
" -> Hash Join (cost=28514.92..172416.85 rows=571 width=52)"
" Hash Cond: (b.detmmailid = a.detmmailid)"
" Join Filter: (((to_tsvector('french'::regconfig, b.cormdata) @@
to_tsquery('mauritanie'::text)) AND (b.cormishtml IS FALSE) AND
(length(b.cormdata) < 20000)) OR (a.detmobjet ~* 'mauritanie'::text))"
" -> Seq Scan on corm b (cost=0.00..44755.07 rows=449507
width=689)"
" -> Hash (cost=19322.74..19322.74 rows=338574 width=94)"
" -> Seq Scan on detm a (cost=0.00..19322.74 rows=338574
width=94)"

If I remove OR ( detmobjet ~* 'mauritanie' ) in the select, the GIN
index is used
explain select distinct a.detmmailid from detm a JOIN corm b on
a.detmmailid = b.detmmailid where ((to_tsvector('french',b.cormdata) @@
to_tsquery('mauritanie') and b.cormishtml is false and
length(b.cormdata) < 20000))

"HashAggregate (cost=4295.69..4301.17 rows=548 width=52)"
" -> Nested Loop (cost=24.67..4294.32 rows=548 width=52)"
" -> Bitmap Heap Scan on corm b (cost=24.25..2026.35 rows=548
width=52)"
" Recheck Cond: ((to_tsvector('french'::regconfig,
cormdata) @@ to_tsquery('mauritanie'::text)) AND (cormishtml IS FALSE)
AND (length(cormdata) < 20000))"
" -> Bitmap Index Scan on ix_corm_fulltext_cormdata
(cost=0.00..24.11 rows=548 width=0)"
" Index Cond: (to_tsvector('french'::regconfig,
cormdata) @@ to_tsquery('mauritanie'::text))"
" -> Index Only Scan using pkey_detm on detm a (cost=0.42..4.13
rows=1 width=52)"
" Index Cond: (detmmailid = b.detmmailid)"

How can i force the use of the GIN index ?
thanks for your tips,

--
Jean-Max Reymond
CKR Solutions Open Source http://www.ckr-solutions.com