I feel like I'm missing something, but the article started by talking about SQL tables, and then in-memory representations, and then on-disk representation, but...isn't storing it on a disk already what a SQL database is doing? It sounds like data is being read from a disk into memory in one format and then written back to a disk (maybe a different one?) in another format, and the second format was not as efficient as the first. I'm not sure I understand why a third format was even introduced in the first place.