PostgreSQL: Playing a little bit with race conditions, SQL and isolation levels

buying_new_tickets.sql

--
-- Trying to buy a new ticket for an event
--
BEGIN;
    UPDATE events e
       SET updated_at = now()
     WHERE e.id = :event_id
       AND e.max_tickets > (SELECT count(*)           -- (does this logic work with READ_COMMITTED??)
                              FROM tickets t
                             WHERE t.event_id = e.id); 

    if ROW_COUNT == 0 then
        RAISE EXCEPTION 'sorry, there is no more tickets to sell'
    end if;

    INSERT INTO tickets 
         VALUES (:event_id :customer_id, :code, now());
COMMIT;

• Postgres Repeatable Read vs Serializable
• Postgres Oficial Doc: 13.2.2. Repeatable Read Isolation Level

⭐️ The answer to this question:

I've made some integration tests, and this code avoids race conditions when executed inside a transaction that uses the following isolation levels:

• SERIALIZABLE;
• REPEATABLE READ;

Any level less strict than those above, as READ_COMMITED, is susceptible to race conditions!