Transaction isolation levels in PostgreSQL: do you know them, have you changed them in your work, and why?

PostgreSQL supports four standard transaction isolation levels:

1. Read Uncommitted – the lowest level, allows reading uncommitted data (dirty reads). In PostgreSQL, it effectively behaves like Read Committed.
2. Read Committed (default level) – a transaction sees only data committed at the start of the query. Non-repeatable reads are possible.
3. Repeatable Read – a transaction sees data as it was at the start, preventing non-repeatable reads, but phantom reads can occur.
4. Serializable – the strictest level, provides full transaction isolation, preventing phantom reads and ensuring behavior equivalent to serial execution.

In practice, the isolation level is often changed to balance performance and data consistency. For example, Read Committed is commonly used for analytical queries or reports to avoid locking data. For critical operations where integrity is vital, Serializable or Repeatable Read are used.

The isolation level can be changed with the command:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Or at the session level:

SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL REPEATABLE READ;

Thus, the choice of isolation level depends on the consistency and performance requirements of the specific application.