Query evaluation for distributed heterogeneous relational databases

In this paper, we consider the query evaluation problem in relational multidatabases and develop a method for generating optimal plans for queries submitted to such a system. Two aspects will be discussed: join tree balance and node allocation. For the first problem, we extend the approach for balancing a join tree proposed by Du et al, so that more balanced join trees can be obtained, when, we present the concepts of dynamic time tables and constrained topological order to do the node allocation so that both the current load states of local database systems and load changes during the join operations can be handled. In this way, the deficiency of Evrendilek's method can be removed.