Consideration was given to the functional decomposition of the discrete systems which is reducible to the functional decomposition of the discrete functions, where by the decomposition is meant the representation of a function by a formula in the basis of unary and binary operations. The algebraic decomposition in an algebra consisting of two binary operations and functions of two variables was studied. A procedure of formula design on the basis of composition of repetition-free subformulas was substantiated. Both exact and asymptotic complexity estimates of the designed formulas were given.