Need help understanding the behaviour of SELECT ... FOR UPDATE causing a deadlock - oracle

Question

I have two concurrent transactions executing this bit of code (simplified for illustration purposes):
#Transactional
public void deleteAccounts() {
List<User> users = em.createQuery("select u from User", User.class)
.setLockMode(LockModeType.PESSIMISTIC_WRITE)
.getResultList();
for (User user : users) {
em.remove(user);
}
}
My understanding is that one of the transactions, say transaction A, should execute the SELECT first, lock all the rows it needs and then go on with the DELETEs while the other transaction should wait for A's commit before performing the SELECT. However, this code is deadlocking. Where am I wrong?

Answer 1

The USER table probably has a lot of foreign keys referring to it. If any of them are un-indexed Oracle will lock the entire child table while it deletes the row from the parent table. If multiple statements run at the same time, even for a different user, the same child tables will be locked. Since the order of those recursive operations cannot be controlled it is possible that multiple sessions will lock the same resources in a different order, causing a deadlock.
See this section in the Concepts manual for more information.
To resolve this, add indexes to any un-indexed foreign keys. If the column names are standard a script like this could help you find potential candidates:
--Find un-indexed foreign keys.
--
--Foreign keys.
select owner, table_name
from dba_constraints
where r_constraint_name = 'USER_ID_PK'
and r_owner = 'THE_SCHEMA_NAME'
minus
--Tables with an index on the relevant column.
select table_owner, table_name
from dba_ind_columns
where column_name = 'USER_ID';

Answer 2

When you use a PESSIMISTIC_WRITE JPA generally traslate it to SELECT FOR UPDATE this make a lock in the database, not necessary for a row it depends of the database and how you configure the lock, by default the lock is by page or block not for row, so check your database documentation to confirm the how your database make the lock, also you can change it so you can apply the lock for a row.
When you call the method deleteAccounts it starts a new transaction and the lock will be active until the transaction commit (or rollback) in this case when the method has finished, if other transaction want to acquire the same lock it can't and I think this is why you have the dead lock, I suggest you to try annother mechanism maybe an optimistic lock, or a lock by entity.
You can try given a timeout to the acquire the lock so:
em.createQuery("select u from User", User.class)
.setLockMode(LockModeType.PESSIMISTIC_WRITE)
.setHint("javax.persistence.lock.timeout", 5000)
.getResultList();
I found a good article that explains better this error, it is cause by the database:
Oracle automatically detects deadlocks and resolves them by rolling
back one of the transactions/statements involved in the deadlock, thus
releasing one set of resources/data locked by that transaction. The
session that is rolled back will observe Oracle error: ORA-00060:
deadlock detected while waiting for resource. Oracle will also produce
detailed information in a trace file under database's UDUMP directory.
Most commonly these deadlocks are caused by the applications that
involve multi table updates in the same transaction and multiple
applications/transactions are acting on the same table at the same
time. These multi-table deadlocks can be avoided by locking tables in
same order in all applications/transactions, thus preventing a
deadlock condition.