Orion 2.0 implemented the uncertain model proposed in . It has two major kinds of attributes uncertain (or pdf attributes) and certain (or precise attributes). A database table is defined by a probabilistic schema consisting of a schema and dependency information. The schema is similar to the regular relational schema and specifies the names and data types of the table attributes (both certain and uncertain). The dependency information identifies the attributes in that are jointly distributed (i.e. correlated). For each dependent set of attributes in , the model maintains a history.