Context

Query performance matters in production systems because slow SQL can increase latency, raise infrastructure costs, and block downstream workloads. Interviewers often ask this to assess whether you understand both query-writing habits and database behavior.

Core Question

Explain how you optimize SQL queries for performance in PostgreSQL. Your answer should cover:

1. How you identify a slow query
2. Common causes of poor performance
3. Practical techniques to improve performance
4. How indexes help and when they do not
5. Why execution plans matter

Scope Guidance

Keep the discussion at an easy, practical level. Focus on foundational techniques such as selecting only needed columns, filtering early, avoiding unnecessary work, using appropriate indexes, and validating changes with EXPLAIN or EXPLAIN ANALYZE. You do not need to cover advanced internals, but you should show a clear, structured approach to diagnosing and improving query speed.