Function mz_sql::plan::lowering::derive_equijoin_cols

pub(crate) fn derive_equijoin_cols(
    oa: usize,
    la: usize,
    ra: usize,
    on: Vec<MirScalarExpr>
) -> Option<(Vec<usize>, Vec<usize>)>

If the on clause of an outer join is an equijoin, figure out the join keys.

oa, la, and ra are the arities of outer, the lhs, and the rhs respectively.