OLTP databases like PostgreSQL are optimized for fast reads and writes on specific records - orders, users, payments. OLAP databases like ClickHouse are built for a different job: aggregating and analyzing millions of rows at once. The same SQL query that takes minutes in PostgreSQL runs in seconds in ClickHouse, thanks to columnar storage. The two systems complement each other - PostgreSQL stores your application data, ClickHouse handles analytics on top of it.