More capable sources
EMERAC assumes sources can only do selection processing. Real sources tend to provide more capabilities
- Many sources can do union queries on attributes
- E.g. CNN Stock quote tracker allows upto 8 symbols at a time
- Some support constraints
- Give me all flights prices less than 300$
Theoretically, such sources can be modeled as supplying a (possibly infinite) number of views.
Query optimization is harder when the capabilities are neither full nor highly limited..