First order logic can express precisely what relational algebra can express. And what Datalog represents is First order logic with least fixed point, in case if

First order logic can express precisely what relational algebra can express. And what Datalog represents is First order logic with least fixed point, in case if there is linear order on the data (cells), it could represent P-solvable problems. https://en.wikipedia.org/wiki/Descriptive_complexity_theory https://en.wikipedia.org/wiki/Descriptive_complexity_theory