Sometimes queries keep running longer than you’d like, this creates a bit of a backup on your DB. As a result you can kill them, except when you can’t, here’s a great look at the exceptional case and how to handle it.
A relational database as the system of record is incredibly common, and Postgres is increasingly at the core of it. Yet, in today’s world you have more than one system and connecting them can be a good bit of work. Foreign Data Wrappers or FDWs can go a long way here.