Query processing in domains like the Internet is much more difficult than query processing in traditional databases because the performance of the different operators is fundamentally unpredictable, due to network delays and the difficulty of doing size and cost estimation on remotely located objects. Adaptive query processing, of which Eddies are an example, deals with this unpredictability by allowing the query processor to change query plans during query execution. Eddies are an extreme version of this -- each tuple in an Eddy can be executed using a different plan. The first paper describes the basics of how this works, and the second shows that this can be made to be relatively efficient.
As you read the papers, consider the following questions: