Can you list the types of transaction isolation levels used in PostgreSQL?

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

1. Read Uncommitted — the lowest level, allows reading uncommitted data (dirty reads). In PostgreSQL, it effectively behaves as Read Committed.
2. Read Committed — a transaction sees only data that was committed at the moment the query started. This is the default level in PostgreSQL.
3. Repeatable Read — a transaction sees data as it was at the start of the transaction, preventing non-repeatable reads.
4. Serializable — the strictest level, provides full transaction isolation as if transactions were executed sequentially.

Example of setting the isolation level in SQL:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- your operations
COMMIT;