Tell me about the transaction isolation levels in PostgreSQL and what anomalies they address.

In PostgreSQL, there are four main transaction isolation levels that determine what anomalies can occur during concurrent transaction execution:

1. Read Uncommitted — the lowest level, allows seeing even uncommitted changes of other transactions (dirty read). In PostgreSQL, it effectively behaves like Read Committed.

2. Read Committed (default) — a transaction sees only data that was committed at the moment of each individual query. Prevents dirty reads but allows non-repeatable reads and phantom reads.

3. Repeatable Read — a transaction sees data as it was at the start of the transaction. Prevents dirty and non-repeatable reads, but phantom reads can occur in some cases.

4. Serializable — the strictest level, provides full isolation, preventing all anomalies including phantom reads, by simulating sequential transaction execution.

Anomalies addressed by isolation levels:

• Dirty Read: reading uncommitted data from another transaction.
• Non-repeatable Read: data changes by another transaction between two reads.
• Phantom Read: new rows added by another transaction appear upon re-execution of a query.

Example of setting the isolation level in PostgreSQL:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- operations
COMMIT;