Nothing in the specs mandate a relational DB.
One of the most performant relay implementations store the events in LMDB, a file based no-SQL database that's supposedly very efficient. https://github.com/hoytech/strfry
jb55 (npub1xts…kk5s) is also working on one such LMDB-backed database design. https://github.com/damus-io/nostrdb