What are the transaction isolation levels in Oracle? Which is the default? What is the difference between READ COMMITTED and SERIALIZABLE?

In Oracle, the following transaction isolation levels exist:

• READ COMMITTED (default)
• SERIALIZABLE

READ COMMITTED is a level where a transaction only sees data that has been committed at the time of reading. This prevents dirty reads but allows non-repeatable reads, where data can change between two reads within the same transaction.

SERIALIZABLE is a stricter level where a transaction sees data as if all transactions were executed sequentially one after another. This prevents non-repeatable reads and phantom reads, ensuring full isolation, but may lead to locking and reduced concurrency.

Thus, the main difference is that READ COMMITTED allows data changes by other transactions during the current transaction, while SERIALIZABLE provides full data consistency throughout the transaction.