Interesting that you chose Reddit as an example. They have a fascinating origin story with respect to data mart. Early Reddit had just two tables: Thing and Data, where Thing was metadata about types and Data was a three column table with: type, id, and value.

Wrap your head around that. All of Reddit, two tables. A database couldn’t be less normalized (final boss of normal forms) and they did it in an rdb. So horrific it’s actually kind of cool.