fn try_push_reduce_through_join(
    inputs: &Vec<MirRelationExpr>,
    equivalences: &Vec<Vec<MirScalarExpr>>,
    group_key: &Vec<MirScalarExpr>,
    aggregates: &Vec<AggregateExpr>,
    monotonic: bool,
    expected_group_size: Option<usize>
) -> Option<MirRelationExpr>