In a Rac environment, when you execute a parallel query, it is very likely that the parallel execution servers, get executed on all nodes in a Rac cluster. In releases prior to Oracle 11g, if you wanted to ensure that all the parallel execution servers for a single query get executed on the same node (or a group of nodes) you had to set the initialization parameter parallel_instance_groups
Starting with Oracle 11g, you can just create services which are only active on certain nodes in the Rac cluster . So when you login to the database using such a service name, your parallel query will only spawn parallel execution servers on the nodes where the service is active. Oracle automatically adjusts the values for parallel_instance_groups (Without you having to explicitly set it) based on your service name you used to connect. Simplifies our life.
