PostgreSQL suddenly became slower. Where to start diagnostics?

When experiencing a sudden decrease in PostgreSQL performance, start diagnostics with the following steps:

1. Check server load: CPU, memory, disk, network. Resources might be exhausted.
2. Look at active queries: use pg_stat_activity to identify long-running or blocking queries.
3. Check for locks: pg_locks will show if there are mutual locks.
4. Analyze PostgreSQL logs: enable detailed logging of slow queries (log_min_duration_statement) to identify problematic queries.
5. Check planner statistics: pg_stat_statements helps understand which queries are most frequent and slow.
6. Assess index status: perhaps indexes are outdated or missing, slowing down data retrieval.
7. Review configuration settings: memory parameters (shared_buffers, work_mem), parallelism, etc.
8. Check external factors: such as network issues or disk subsystem problems.

This systematic approach will help localize the cause of slowdown and take appropriate measures.