Is it possible to query the database without opening a transaction , i.e is it enough if I have an open Session ?

there is ALWAYS a transaction and it is a best practice to ALWAYS explicitly manage the transactions even for read only queries

I use Hibernate sessions for SELECTs and use Transactions for INSERTs, UPDATEs and DELETEs. I have not come across any documentation that suggests a necessity for Transactions in SELECT situations. But I could be wrong.

you'll find many posts discussing about this.
I've made the error of not using transaction for select because i thought it was unecessary, i was wrong...
You must know that even a select create in fact

begin transaction
select...
commit


because in most of the cases, db is configured with autocommit

But if you don't have anything to commit/rollback, why even have a transaction. The 4 methods in Transaction are commit(), rollback(), wasCommitted() and wasRolledBack(). If you don't need to do any of these, why create a transaction. If you call transaction.commit() for a SELECT statement, don't you end up opening yourself up to the possibility of updating data by some fluke chance. While the example in the javadocs for Session talks of using transaction, it seems to pertain more to modification of persistent objects, not retrieval of data.

There always is a transaction when doing database access, JDBC just uses autocommit by default, meaning it will commit and start a new transaction for every statement. Guys, this has been discussed a hundred times already, please search the forum.