This paper considers the efficient evaluation of recursive queries expressed using Horn clauses. We define sideways information passing formally and show how a query evaluation algorithm may be defined in terms of sideways information passing and control. We then consider a class of information-passing strategies that suffices to describe most query evaluation algorithms in the database literature, and show that these strategies may always be implemented by rewriting a given program and

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... ng the rewritten program bottom-up. We describe in detail several algorithms for rewriting a program. These algorithms generalize the counting and magic-sets algorithms to work with arbitrary programs. Safety and optimality of the algorithms are also considered. a anc(X,Y):-par(X,Y) anc(X, Y 1 :-pa&X, 23, anc(Z, Y 1 and let the query be Query : anc( john , Y ) ? Assume that a database contains a parenthood relation par. Then the program defines a derived relation describing ancestors, and the query asks for the ances-