Slightly offtopic; I am quite well vetted in the relational database models (starting with the Date book in uni in the 80s; I worked with DBs before that, but d

Slightly offtopic; I am quite well vetted in the relational database models (starting with the Date book in uni in the 80s; I worked with DBs before that, but didn't know how they worked or relational theory), however are there similar theories for other types of databases; more specific graph databases? But any other type would suffice. For instance, I can find a lot of implementation detail about columnar databases, but not a lot of theory (probably my inability to search for the right phrases).