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An abstract interface for representing federated data sources.
This interface abstracts away the specifics of working with various types of data sources.
Things one can do with a data source:
Determine the type of the data supplied by this data source by inspecting the
federated_typeproperty. The type returned should be a federated type. Note that depending on whether this data source contains one or a number of federated datasets, the type may or may not be a struct (with individual datasets appearing as elements of this struct).
Construct a new iterator for this data source by invoking
iteratoron it. Each iterator represents an independent pass over the data from this data source.
The type of the data returned by calling
select on an iterator.
Returns a new iterator for retrieving data from this data source.