In this paper we describe the XSB system, an in-memory deductive database engine. XSB began from a Prolog foundation, and traditional Prolog systems are known to have a number of serious deficiencies when they are used as database systems. Accordingly, XSB has a fundamental bottom-up extension, introduced through tabling (or memoing) [4], which makes it appropriate as an underlying query engine for deductive database systems. Because it eliminates redundant computation, the tabling extension...