What types of transactions (isolation levels) are there in PostgreSQL?

PostgreSQL supports the following transaction isolation levels:

1. Read Uncommitted — the lowest level, PostgreSQL behaves like Read Committed, not allowing uncommitted changes to be visible.
2. Read Committed (default level) — a transaction only sees data that was committed at the start of the query.
3. Repeatable Read — a transaction sees data in the state it was at the start of the transaction, preventing non-repeatable reads.
4. Serializable — the strictest level, providing full isolation, transactions execute as if sequential.

Example of setting the isolation level in SQL:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- operations
COMMIT;

The choice of level depends on the requirements for consistency and performance.