In practice, SERIALIZABLE is similar to REPEATABLE READ, but uses a different implementation for each database engine. In Oracle, the REPEATABLE READ level is not supported and SERIALIZABLE provides the highest isolation level.

What is the highest level of isolation?

The highest isolation level, serializable, guarantees that a transaction will retrieve exactly the same data every time it repeats a read operation. But it uses a level of locking that is likely to impact other users in multi-user systems.

Which isolation level provides the best concurrency?

Serializable. This is the highest isolation level. With a lock-based concurrency control DBMS implementation, serializability requires read and write locks (acquired on selected data) to be released at the end of the transaction.