4. Query processing

4.1 Principles

In a distributed environment, queries formulated at a global level are broken down into sub-queries. These sub-queries are addressed to the systems available at the local sites where they are executed. The local responses are then grouped together to form the response to the global query.

We will describe this process by considering global queries initially formulated in SQL. They are rewritten in algebraic form to be reduced and optimized. The fragmentation scheme is used to determine the local queries addressed to each site. The phases of this process are shown in figure 6 .