Function stream_column_selection 

pub fn stream_column_selection(
    data: &dyn SparseIo<IndexIter = Vec<usize>>,
    selected_columns: &[usize],
    row_filter: Option<&[usize]>,
    out_row_names: &[Box<str>],
    out_col_names: &[Box<str>],
    file_out: &str,
    backend_out: &SparseIoBackend,
) -> Result<(usize, usize, usize)>

Stream a column selection (with an optional row filter) into a fresh backend, without ever materialising the survivors.

The shape split and subsample share: pick columns, optionally keep a subset of rows, write a new file. This is stream_mapped_selection over the rows row_filter keeps, in its order.