Function assert_valid_optimization 

fn assert_valid_optimization(
    plan: &LogicalPlan,
    prev_schema: &Arc<DFSchema>,
) -> Result<()>

These are invariants which should hold true before and after LogicalPlan optimization.

This differs from LogicalPlan::check_invariants, which addresses if a singular LogicalPlan is valid. Instead this address if the optimization was valid based upon permitted changes.