Tell about transaction isolation levels in PostgreSQL.

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

1. Read Uncommitted — the lowest isolation level. In PostgreSQL, it effectively behaves like Read Committed, as it does not allow reading uncommitted data (dirty reads).

2. Read Committed — the default level. A transaction sees only data that was committed at the moment of each individual query. Changes made by other transactions can appear between queries within the same transaction.

3. Repeatable Read — a transaction sees data as it was at the start of the transaction. This prevents non-repeatable reads and dirty reads but allows phantom reads.

4. Serializable — the strictest level. Guarantees full isolation as if transactions were executed sequentially. Prevents phantom reads and ensures data consistency.

Example of setting the isolation level in SQL:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- operations
COMMIT;

In PostgreSQL, isolation levels are implemented using MVCC (Multi-Version Concurrency Control), which allows efficient management of concurrent data access.