Major Description
Reading the code for jsonRDD, it appears that all fields of a JSON object are read into a ROW independent of the provided schema. I would expect it to be more efficient to only store in the ROW those fields that are explicitly included in the schema.
For example, assume that I only wish to extract the "id" field of a tweet. If I provided a schema that simply had one field within a map named "id", then the row object would only store that field within a map.
