Foreign keys and locks.

Hi all,
Today I found this behavior on two tables.

I have 2 tables t2 and t3 , described as below:

testdb=# \d t2
Tabella "public.t2"
Colonna | Tipo | Ordinamento | Può essere null | Default
---------+---------+-------------+-----------------+---------
id | integer | | not null |
value | integer | | |
Indici:
"t2_id_p_key" PRIMARY KEY, btree (id)
"t2_rate_idx" btree (value)
Referenziato da:
TABLE "t3" CONSTRAINT "t3_roomrate_fkey" FOREIGN KEY (value) REFERENCES
t2(id) ON UPDATE CASCADE ON DELETE CASCADE

testdb=# \d t3
Tabella "public.t3"
Colonna | Tipo | Ordinamento | Può essere null | Default
---------+---------+-------------+-----------------+---------
id | integer | | not null |
value | integer | | |
Indici:
"t3_id_p_key" PRIMARY KEY, btree (id)
Vincoli di integrità referenziale
"t3_roomrate_fkey" FOREIGN KEY (value) REFERENCES t2(id) ON UPDATE
CASCADE ON DELETE CASCADE

the two tables are populated with records that satisfy the referential
integrity constraints.

1) If I execute

testdb=# delete from t2 where id=1978800 ;

it works;

2) but if I execute
testdb=# delete from t2 where value=20342;

postgresql applies locks and the query freezes.

testdb=# SELECT locktype, relation::regclass, mode, transactionid AS tid,
virtualtransaction AS vtid, l.pid, granted,a.query
FROM pg_catalog.pg_locks l
JOIN pg_stat_activity a on a.pid=l.pid
LEFT JOIN pg_catalog.pg_database db
ON db.oid = l.database WHERE (db.datname = 'testdb')
AND NOT l.pid = pg_backend_pid()
and a.state = 'active';
locktype | relation | mode | tid | vtid | pid | granted
| query
----------+-------------+------------------+-----+--------+------+---------+-----------------------------------
relation | t3_id_p_key | RowExclusiveLock | | 9/1475 | 8685 | t
| delete from t2 where value=20342;
relation | t3 | RowExclusiveLock | | 9/1475 | 8685 | t
| delete from t2 where value=20342;
relation | t2_rate_idx | RowExclusiveLock | | 9/1475 | 8685 | t
| delete from t2 where value=20342;
relation | t2_id_p_key | RowExclusiveLock | | 9/1475 | 8685 | t
| delete from t2 where value=20342;
relation | t2 | RowExclusiveLock | | 9/1475 | 8685 | t
| delete from t2 where value=20342;

I tried to check the problem on other tables but on other tables I did not
find the same behavior
and the second type of query works without problems.

Any Ideas?

Thanks in advance for your reply

Enrico