Function columns_to_fetch 

pub(crate) fn columns_to_fetch<'a>(
    projection: &'a ProjectionMask,
    num_columns: usize,
    is_read: impl Fn(usize) -> bool + 'a,
) -> impl Iterator<Item = usize> + 'a

The leaf columns in projection that are not yet read, in column order.

A decoding stage of a row group does not fetch a column that an earlier stage of the same row group read.