PostgreSQL challenge: unblock a session that freezes the till

Saturday, 10 a.m. The shop is full, and Nadia's till is spinning in the void. It does not crash, it does not display anything: it waits. Ludovic is not answering his phone. Mama Ngo is waiting with a customer, a pack of sardines in her hand. This challenge has no step-by-step guide. It has a mission, rules, and the official PostgreSQL documentation. For the first time you are on the other side of the server: superuser, with a view of every open session, what each one is doing, and what each one is waiting for. It is up to you to trace the blocked till back to the session blocking it, to understand what it holds and since when, then to cut it off, and only it, without restarting the server or stopping the till. You will also have to make sure it does not happen again: configure the server so that a forgotten transaction can no longer block the shop for more than thirty seconds, and leave Ludovic a note so he understands what happened. This is exactly the work of a database administrator: observing, understanding, stepping in without interrupting the service, and relying on the documentation. The natural next step after this path.