ABSTRACT
The query processes of heterogeneous database are divided into four steps: grammar analysis, conversion decomposition, global optimization and results merge [2]. Global optimization is the major step of the whole
query processing, the query trees may be obtained which is optimized after the operations of the first two steps. Generally speaking, a query statement can have several different query trees, then do an assessment of the execution cost plan in which these query trees correspond, and the execution engine will execute the execution plan of the least cost. A good query execution plan needs to select a suitable replica site to find the optimal execution sequence of database and select the best execution way of join operation.
