Your application's database connection pool is exhausted. How do you diagnose and fix?

Diagnose: SELECT count(*), state, wait_event_type FROM pg_stat_activity GROUP BY state, wait_event_type — shows connection distribution. Check for idle-in-transaction connections (long-running transactions holding connections). Fix: 1) Connection pooler (PgBouncer in transaction mode): all app threads share a smaller pool of actual DB connections. pool_size=20 DB connections serve 500 app threads.