Can you explain the mechanisms of transaction isolation levels in PostgreSQL and their impact on concurrency and data integrity?

In PostgreSQL, there are four main transaction isolation levels defined by the SQL standard: Read Uncommitted, Read Committed, Repeatable Read, and Serializable.

• Read Uncommitted — the lowest level, in PostgreSQL it effectively behaves like Read Committed, as it does not allow reading uncommitted data (dirty reads).

• Read Committed (default) — a transaction sees only data that was committed at the start of each individual query. This prevents dirty reads but allows non-repeatable reads and phantom reads.

• Repeatable Read — a transaction sees data in the state it was at the start of the transaction. This prevents non-repeatable reads, but phantom reads can occur in some cases.

• Serializable — the strictest level, providing full transaction isolation as if transactions were executed sequentially. This prevents all types of read anomalies but can lead to locking and conflicts, reducing concurrency.

Impact on concurrency and integrity:

• Lower isolation levels (Read Committed) provide better performance and concurrency but allow some read anomalies.
• Higher levels (Repeatable Read, Serializable) increase data integrity but may reduce performance due to locking and conflicts.

PostgreSQL uses the MVCC (Multi-Version Concurrency Control) mechanism, which allows efficient implementation of these levels by minimizing read locks.