What level of transaction isolation is provided in PostgreSQL and how does it affect data integrity?

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

1. Read Uncommitted — the lowest level, effectively behaves like Read Committed in PostgreSQL, as reading uncommitted data is not allowed.
2. Read Committed (default level) — a transaction sees only data that was committed at the moment the query started. This prevents dirty reads but allows non-repeatable reads and phantom reads.
3. Repeatable Read — a transaction sees a snapshot of data at the start of the transaction, preventing non-repeatable reads, but phantom reads can occur in some cases.
4. Serializable — the strictest level, provides full transaction isolation as if transactions were executed sequentially. It prevents all read anomalies.

Impact on data integrity:

• Higher isolation levels reduce the likelihood of conflicts and anomalies, increasing data integrity and consistency.
• However, increased isolation can reduce performance due to locking and waiting.

Example of setting the isolation level in SQL:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- operations
COMMIT;