What are the transaction isolation levels in PostgreSQL? What is the default level in PostgreSQL and MySQL?

PostgreSQL supports four standard transaction isolation levels:

1. Read Uncommitted — the lowest level, allows reading uncommitted changes (dirty read). In PostgreSQL, it effectively behaves as Read Committed.
2. Read Committed — the default in PostgreSQL. A transaction sees only data that was committed at the moment the query started. Changes made by other transactions after the start of the query are not visible.
3. Repeatable Read — the transaction sees data as it was at the start of the transaction, preventing non-repeatable reads.
4. Serializable — the strictest level, provides full transaction isolation as if transactions are executed sequentially.

The default isolation level in PostgreSQL is Read Committed.

In MySQL (InnoDB), the default isolation level is Repeatable Read.

Example of setting the isolation level in PostgreSQL:

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
BEGIN;
-- operations
COMMIT;