Function mz_expr::join_permutations

pub fn join_permutations(
    key_arity: usize,
    stream_permutation: BTreeMap<usize, usize>,
    thinned_stream_arity: usize,
    lookup_permutation: BTreeMap<usize, usize>,
) -> BTreeMap<usize, usize>

Given the permutations (see permutation_for_arrangement and (dataflow::plan::AvailableCollections) corresponding to two collections with the same key arity, computes the permutation for the result of joining them.