Method demand

src/expr/src/linear.rs:875–888

Lists input columns whose values are used in outputs. You can use `BTreeSet::last()` to extract the maximum demanded column from the set. It is entirely appropriate to determine the demand of an instance and then both apply a projection to the subject of the instance and `self.permute` this instance.

(&self)

    /// and then both apply a projection to the subject of the instance and
    /// `self.permute` this instance.
    pub fn demand(&self) -> BTreeSet<usize> {
        let mut demanded = BTreeSet::new();
        for (_index, pred) in self.predicates.iter() {
            demanded.extend(pred.support());
        }
        demanded.extend(self.projection.iter().cloned());
        for index in (0..self.expressions.len()).rev() {
            if demanded.contains(&(self.input_arity + index)) {
                demanded.extend(self.expressions[index].support());
            }
        }
        demanded.retain(|col| col < &self.input_arity);
        demanded
    }

    /// Update input column references, due to an input projection or permutation.
    ///