Context

Production databases occasionally slow down because one user or application is running expensive queries, opening too many sessions, or holding locks. Interviewers want to know whether you can diagnose the issue quickly and respond safely.

Core question

Explain how you would identify a single user generating heavy load on a PostgreSQL database and how you would stop that activity. Your answer should cover:

1. Which PostgreSQL system views or tools you would check first
2. How you would determine whether the load is CPU, long-running queries, blocking locks, or too many connections
3. Which commands you would use to stop the user safely
4. The difference between cancelling a query and terminating a backend session
5. What follow-up steps you would take after mitigation

Scope guidance

Focus on practical PostgreSQL operations rather than generic database theory. A strong answer should mention specific views such as pg_stat_activity, lock inspection, and the operational trade-offs of pg_cancel_backend() versus pg_terminate_backend().