Integration of Semistructured Data Using Outer Joins

When we integrate information sources that are managed locally, the integrated objects are often semistructured, since di erent sets of information sources participate in forming each of the objects. Such integration requires outer join and conversion operators: outer join operators prevent information loss that would be caused by inner joins, and conversion operators are used to mediate the inconsistency among information sources. These operators impose restrictions on the order of query processing. This paper presents an algorithm for creating a processing plan to integrate information sources without information loss, in the presence of conversion operators. As a framework of our discussion, we use the TSIMMIS mediation system developed at Stanford University. Given a view speci cation and a query, we create an initial expression graph whose nodes represent query processing units, including outer join and conversion operators. Then, we convert the initial expression graph into an executable expression tree.