Do existing R packages work with SparkR data frames

Hello, Is it possible for existing R Machine Learning packages (which work with R data frames) such as bnlearn, to work with SparkR data frames? Or do I need to convert SparkR data frames to R data frames? Is "collect" the function to do the conversion, or how else to do that? Many Thanks, Lan

Why is Columnar Parquet used as default for saving Row-based DataFrames/RDD?

Hello, I have the above naive question if anyone could help. Why not using a Row-based File format to save Row-based DataFrames/RDD? Thanks, Lan