FOR UPDATE lock problem ?

Hi all,

I had a lock problem on my database.
When I use a "select for update" request whitch uses an index, the
locking system is inconsistant.
Take this example:

test=# \d users
Table "public.users"
Column | Type | Modifiers
---------+---------+---------------------------------------------------------
id_user | integer | not null default
nextval('users_id_user_seq'::regclass)
name | text |
Indexes:
"users_pkey" PRIMARY KEY, btree (id_user)

test=# \d sessions
Table "public.sessions"
Column | Type | Modifiers
------------+-----------------------------+---------------------------------------------------------------
id_session | integer | not null default
nextval('sessions_id_session_seq'::regclass)
id_user | integer |
from_date | timestamp without time zone | default now()
to_date | timestamp without time zone |
Indexes:
"sessions_pkey" PRIMARY KEY, btree (id_session)
"idx_session_null" btree (id_session) WHERE to_date IS NULL
Foreign-key constraints:
"sessions_id_user_fkey" FOREIGN KEY (id_user) REFERENCES
users(id_user)

test =# INSERT INTO users (name) values ('bob');
test =# INSERT INTO users (name) values ('brad');

test =# INSERT INTO Sessions (id_user,to_date) select id_user,now() from
users;
x 200 times (for example)
test =# INSERT INTO Sessions (id_user) select id_user from users;

test =# ANALYSE Sessions;
test=# explain select s.id_session from users u, sessions s where
to_date IS NULL and u.id_user = s.id_user;
QUERY PLAN
-----------------------------------------------------------------------------------------
Nested Loop (cost=0.00..6.85 rows=1 width=4)
-> Index Scan using idx_session_null on sessions s (cost=0.00..1.01
rows=1 width=8)
-> Index Scan using users_pkey on users u (cost=0.00..5.82 rows=1
width=4)
Index Cond: (u.id_user = "outer".id_user)
(4 rows)

Then the problem with two backends:
bk1:
test=# begin;
test=# select s.id_session from users u, sessions s
where to_date IS NULL and u.id_user = s.id_user for update;
id_session
------------
403
404
(2 rows)

bk2:
test=# begin;
test=# select s.id_session from users u, sessions s
where to_date IS NULL and u.id_user = s.id_user for update;
=> ... Waiting

bk1:
test=# UPDATE sessions set to_date = now() where to_date is null;
UPDATE 2
test=# commit;

Then finaly on bk2:
id_session
------------
403
404
(2 rows)

=> But the rows were updated by the other backend so to_date field is
not null for these tuples...However these tuples are in the result
produced by the backend #2...

If I remove the idx_session_null index the problem disappears.

--
REYNAUD Jean-Samuel <reynaud(at)elma(dot)fr>
Elma