Query execution strategies for missing data in distributed heterogeneous object databases

The problem of missing data arises in the distributed heterogeneous object databases because of the missing attribute conflict and the existence of null values. A set of algorithms are provided in this paper for query processing when the predicates in global queries involve missing data. For providing more informative answers to the users, the maybe results due to missing data are presented in addition to the certain results. One algorithm is designed based on the centralized approach in which the data are sent to the same site for integration and then processing. On the other hand, for reducing the response time, the localized approaches evaluate the predicates in different component databases in parallel. The proposed algorithms are compared and discussed by the simulation result on the total execution time and response time.