Reasons for doing this:
- Simpler: Don't have to create, populate, and
then reclaim the set.
- Storage utilization: Space requirements for the
stream are O(1). An explicit set would be O(n).
- Faster: First output row is immediately
available. No set maintenance.
- The Iterator-based approach to operators follows from the
goal of using pipelining whenever possible.
- The query plan we just examined used pipelining for each operator.