Have you worked with transactions in PostgreSQL? What isolation levels have you encountered, and when are they used?

Yes, I have worked with transactions in PostgreSQL. The main isolation levels I have encountered are:

• Read Uncommitted — the lowest level, allows reading uncommitted data (dirty read). Not actually supported in PostgreSQL as it uses MVCC.
• Read Committed — the default level in PostgreSQL. A transaction sees only data committed before the query began. It prevents dirty reads but allows non-repeatable reads.
• Repeatable Read — a transaction sees data as it was at the start of the transaction. It prevents non-repeatable reads but allows phantom reads.
• Serializable — the strictest level, provides full transaction isolation, preventing phantom reads and other anomalies, but may lead to locks and rollbacks.

I used Read Committed for most operations where data freshness is important but full isolation is not critical. Repeatable Read was used for complex analytical queries where a stable data state within a transaction is necessary. Serializable was used for critical financial operations requiring maximum consistency.