pub fn optimize_dataflow_demand_inner<'a, I>(
    view_sequence: I,
    demand: &mut HashMap<Id, BTreeSet<usize>>
) -> Result<(), TransformError> where
    I: Iterator<Item = (Id, &'a mut MirRelationExpr)>, 
Expand description

Pushes demand through views in view_sequence in order, removing columns not demanded.

This method is made public for the sake of testing. TODO: make this private once we allow multiple exports per dataflow.