I find postgres source code very interesting, e.g https://github.com/postgres/postgres/tree/master/src/backend/optimizer https://github.com/postgres/postgres/t

I find postgres source code very interesting, e.g https://github.com/postgres/postgres/tree/master/src/backend/optimizer https://github.com/postgres/postgres/tree/master/src/backend... Sqlite is another good one to read, and play around with.