This paper explains the rules subsystem that is being implemented in the POSTGRES DBMS. It is novel in several ways. First, it gives to users the capability of defining rules as well as data to a DBMS. Moreover, depending on the scope of each rule defined, optimization is handled differently. This leads to good performance both in the case that there are many rules each of small scope and a few rules each of large scope. In addition, rules provide either a forward chaining control flow or a backward chaining one, and the system will choose the control mechanism that optimizes performance in the cases that it is possible. Furthermore, priority rules can be defined, thereby allowing a user to specify rules systems that have conflicts. This use of exceptions seems necessary in many applications. Lastly, our rule system can support an implementation of views, protection and integrity control, simply by applying the rules system in a particular way. Consequently, no special purpose code need be included to handle these tasks.