In this paper we study the use of networks of PCs to handle the parallel execution of relational database queries. This approach is based on a parallel extension, called parallel relational query evaluator, working in a coupled mode with a sequential DBMS. We present a detailed architecture of the parallel query evaluator and introduce Enkidu, the efficient Java-based prototype that has been build according to our concepts. We expose a set of measurements, conducted over Enkidu, and highlighting its performances. We finally discuss the interest and viability of the concept of parallel extension in the context of relational databases and in the wider context of high performance computing.