I read that the Tandem NonStop OS from the 1980s had a built-in transaction manager (because the OS was designed to run databases) and they built their filesystem on top of the transaction manager which gave them filesystem transactions for "free".
I think the Pick OS, at least in its native form, had no file system but used a database.
It was very easy to program. I saw colleagues with no computer science education writing complex queries on "Pick Basic", what would have been impossible for them on classical computers of the time (or of today as well).
