Abstract
Outerjoin is used in distributed relational multidatabase systems for integrating local schemas to a global schema. Queries against the global schema need to be modified, optimized, and decomposed into subqueries at local sites for processing. Since outerjoin combines local relations in different databases to form a global relation, it is expensive to process. Based on the structure of the query and the definition of the schemas, queries with outerjoin, join, select, and project operations are optimized. Conditions where outerjoin can be avoided or transformed into a one-side outerjoin are identified. Considering these conditions makes it possible to reduce the response time for query processing.