What levels of transaction isolation exist? Which one should be used for financial operations?

There are four main levels of transaction isolation:

1. Read Uncommitted — a transaction can see uncommitted changes made by other transactions (dirty read).
2. Read Committed — a transaction only sees committed changes, preventing dirty reads.
3. Repeatable Read — guarantees that data read during a transaction will not change until the transaction completes, preventing non-repeatable reads.
4. Serializable — the strictest level, transactions are executed as if they were sequential, preventing phantom reads.

For financial operations, it is recommended to use the Serializable level or, if performance is critical, Repeatable Read. This ensures maximum data integrity and prevents anomalies that could lead to errors in calculations or accounting.