What levels of transaction isolation do you know in relation to PostgreSQL?

PostgreSQL supports four standard transaction isolation levels defined in the SQL standard:

1. Read Uncommitted — the lowest level, allows reading uncommitted data (dirty read). In PostgreSQL, it effectively behaves like Read Committed.

2. Read Committed — the default in PostgreSQL. A transaction sees only data committed before each individual query. It prevents dirty reads but allows non-repeatable reads.

3. Repeatable Read — a transaction sees data as it was at the start of the transaction. It guarantees that repeated reads of the same data within a transaction will be identical, preventing non-repeatable reads but allowing phantom reads.

4. Serializable — the strictest level, provides full transaction isolation as if transactions were executed sequentially. It prevents phantom reads and all types of anomalies.

Example of setting the isolation level in Java using JDBC:

connection.setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);