Is the Warning message a buglet?
julia> using DataFrames; df= DataFrame( n1=1:9, cat=[ 'A','B','A', 'B','C','B', 'C','A','C' ] );
julia> groupby(df, :cat)
GroupedDataFrame{DataFrame} with 3 groups based on key: :cat
First Group: 3 rows┌ Warning: iterating over value of eachcol on SubDataFrame will return a pair of column name and a view of column value
│ caller = ip:0x0
└ @ Core :-1
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