Function compare_sort_expr

datafusion/expr/src/utils.rs:563–619

Compare the sort expr as PostgreSQL's common_prefix_cmp():

(
    sort_expr_a: &Sort,
    sort_expr_b: &Sort,
    schema: &DFSchemaRef,
)

/// Compare the sort expr as PostgreSQL's common_prefix_cmp():
/// <https://github.com/postgres/postgres/blob/master/src/backend/optimizer/plan/planner.c>
pub fn compare_sort_expr(
    sort_expr_a: &Sort,
    sort_expr_b: &Sort,
    schema: &DFSchemaRef,
) -> Ordering {
    let Sort {
        expr: expr_a,
        asc: asc_a,
        nulls_first: nulls_first_a,
    } = sort_expr_a;

    let Sort {
        expr: expr_b,
        asc: asc_b,
        nulls_first: nulls_first_b,
    } = sort_expr_b;

    let ref_indexes_a = find_column_indexes_referenced_by_expr(expr_a, schema);
    let ref_indexes_b = find_column_indexes_referenced_by_expr(expr_b, schema);
    for (idx_a, idx_b) in ref_indexes_a.iter().zip(ref_indexes_b.iter()) {
        match idx_a.cmp(idx_b) {
            Ordering::Less => {
                return Ordering::Less;
            }
            Ordering::Greater => {
                return Ordering::Greater;
            }
            Ordering::Equal => {}
        }
    }
    match ref_indexes_a.len().cmp(&ref_indexes_b.len()) {
        Ordering::Less => return Ordering::Greater,
        Ordering::Greater => {
            return Ordering::Less;
        }
        Ordering::Equal => {}
    }
    match (asc_a, asc_b) {
        (true, false) => {
            return Ordering::Greater;
        }
        (false, true) => {
            return Ordering::Less;
        }
        _ => {}
    }
    match (nulls_first_a, nulls_first_b) {
        (true, false) => {
            return Ordering::Less;
        }
        (false, true) => {
            return Ordering::Greater;
        }
        _ => {}
    }
    Ordering::Equal
}