I worked at a company that developed its own proprietary database for a financial application. The entire database, several gigabytes, not large by today's standards, was mmap'd and read at startup to warm the page cache. We also built in-memory "indexes" at startup.

This was back in the early 2000's, when having 4 gigabytes of RAM would be considered large. The "database server" was single threaded, and all changes were logged to a WAL-ish file before updating the mmap'd database. It was fun stuff to work on. It worked well, but it wasn't a general purpose DB.