If PostgreSQL is the "king," like everyone says, then why does MongoDB still serve millions of users daily? Here's where relational DBs actually break: Fixed schemas: - Your users table has 15 columns, but premium users need 5+ extra fields. - Now you're either adding nullable columns for everyone or creating awkward junction tables. Single-machine bottleneck: - PostgreSQL runs on one server. - When you hit CPU/memory limits, your only option is to buy bigger hardware. - Can't just add more servers. ACID overhead: - Every write needs disk persistence + locking for consistency. - Great for banking, terrible when you need 10k writes/second for real-time analytics. Relational impedance: - Storing a JSON document means either shoving it in a TEXT field (losing query ability) or - normalizing it across 5 tables (killing performance). NoSQL attacks these specific problems: - MongoDB: Flexible schemas, horizontal scaling - Redis: In-memory speed, simple data structures - Cassandra: Write-heavy workloads, multi-datacenter - Neo4j: Graph traversals without expensive JOINs Alright, SQL databases are optimized for ACID compliance and relational integrity. When you don't need that overhead, NoSQL gives you speed, flexibility, or scale. It's not about better/worse - it's about different engineering trade-offs.

I have used SQL server, MySQL, Postgres, mongodb, Cassandra, dynamodb, redis all in large production applications and maintained them over multiple years. Redis and Cassandra serves a specific use case so they are fine. But in all other generic use cases choose any relational db, mostly Postgres, period.