Looks like merge join planning time is too big, 55 seconds

Hello, i have a problem with planning time, I do not understand why this can
happen.

PostgreSQL 9.2.2 on x86_64-unknown-linux-gnu, compiled by gcc-4.4.real
(Debian 4.4.5-8) 4.4.5, 64-bit

# explain
# select i.item_id, u.user_id from items i
# left join users u on u.user_id = i.user_id
# where item_id = 169946840;
QUERY PLAN

----------------------------------------------------------------------------------------------
Nested Loop Left Join (cost=0.00..397.14 rows=1 width=16)
-> Index Scan using items_item_ux on items i (cost=0.00..358.84 rows=1
width=16)
Index Cond: (item_id = 169946840)
-> Index Only Scan using users_user_id_pkey on users u
(cost=0.00..38.30 rows=1 width=8)
Index Cond: (user_id = i.user_id)

time: 55919.910 ms

# set enable_mergejoin to off;

# explain
select i.item_id, u.user_id from items i
left join users u on u.user_id = i.user_id
where item_id = 169946840;
QUERY PLAN

----------------------------------------------------------------------------------------------
Nested Loop Left Join (cost=0.00..397.14 rows=1 width=16)
-> Index Scan using items_item_ux on items i (cost=0.00..358.84 rows=1
width=16)
Index Cond: (item_id = 169946840)
-> Index Only Scan using users_user_id_pkey on users u
(cost=0.00..38.30 rows=1 width=8)
Index Cond: (user_id = i.user_id)

time: 28.874 ms

--
Sergey Burladyan