Legacy databases — or relational databases, aka RDBMS — were built for well-mapped, stable, and predictable processes like finance and accounting. These databases use rigid rows, columns, and tables that don’t require frequent modifications, but when the database model does need to change, it’s a big and expensive undertaking.
Today, when building software applications, business and user requirements change often. Yet, most legacy database software fights against these changes rather than evolving with them.