Constraint propagating for equal fields

Hello.

I've always thought that PostgreSQL would propagate constraint from field1
to field2 if condition says field1=field2, but this does not seem the case:
dict=# explain select * from domain_list,title.domains where processed_at is
not null and key=groupid and key < 1000000 and groupid < 1000000;
QUERY PLAN

--------------------------------------------------------------------------------------------------
Hash Join (cost=2179918.87..4529994.61 rows=4616 width=318)
Hash Cond: (domain_list.key = domains.groupid)
-> Bitmap Heap Scan on domain_list (cost=26253.02..2310541.55
rows=870759 width=123)
Recheck Cond: (key < 1000000)
-> Bitmap Index Scan on domain_list_new_pkey (cost=0.00..26035.33
rows=870759 width=0)
Index Cond: (key < 1000000)
-> Hash (cost=2119232.34..2119232.34 rows=864201 width=195)
-> Bitmap Heap Scan on domains (cost=16674.34..2119232.34
rows=864201 width=195)
Recheck Cond: (groupid < 1000000)
Filter: (processed_at IS NOT NULL)
-> Bitmap Index Scan on dgroup (cost=0.00..16458.29
rows=890154 width=0)
Index Cond: (groupid < 1000000)
(12 rows)

dict=# explain select * from domain_list,title.domains where processed_at is
not null and key=groupid and key < 1000000 ;
QUERY PLAN

--------------------------------------------------------------------------------------------------------
Hash Join (cost=2337583.04..18222634.81 rows=845372 width=318)
Hash Cond: (domains.groupid = domain_list.key)
-> Seq Scan on domains (cost=0.00..5423788.20 rows=158280964 width=195)
Filter: (processed_at IS NOT NULL)
-> Hash (cost=2310541.55..2310541.55 rows=870759 width=123)
-> Bitmap Heap Scan on domain_list (cost=26253.02..2310541.55
rows=870759 width=123)
Recheck Cond: (key < 1000000)
-> Bitmap Index Scan on domain_list_new_pkey
(cost=0.00..26035.33 rows=870759 width=0)
Index Cond: (key < 1000000)
(9 rows)

dict=# explain select * from domain_list,title.domains where processed_at is
not null and key=groupid and groupid < 1000000;
QUERY PLAN

--------------------------------------------------------------------------------------------
Hash Join (cost=2153665.85..16943819.35 rows=862710 width=318)
Hash Cond: (domain_list.key = domains.groupid)
-> Seq Scan on domain_list (cost=0.00..6887257.54 rows=162753054
width=123)
-> Hash (cost=2119232.34..2119232.34 rows=864201 width=195)
-> Bitmap Heap Scan on domains (cost=16674.34..2119232.34
rows=864201 width=195)
Recheck Cond: (groupid < 1000000)
Filter: (processed_at IS NOT NULL)
-> Bitmap Index Scan on dgroup (cost=0.00..16458.29
rows=890154 width=0)
Index Cond: (groupid < 1000000)
(9 rows)

The first query is the fastest one, but it is equal to both 2 and 3 and I
thought PostgreSQL can perform such propagation by itself.

Best regards, Vitalii Tymchyshyn.