Method remove_undemanded

src/expr/src/linear.rs:1383–1433

Removes unused expressions from `self.expressions`. Expressions are "used" if they are relied upon by any output columns or any predicates, even transitively. Any expressions that are not relied upon in this way can be discarded. # Example ```rust use mz_expr::{func, MapFilterProject, MirScalarExpr, UnaryFunc, BinaryFunc}; // Use the output from `inline_expression` example. let mut map_filter_p

(&mut self)

    /// );
    /// ```
    pub fn remove_undemanded(&mut self) {
        // Determine the demanded expressions to remove irrelevant ones.
        let mut demand = BTreeSet::new();
        for (_index, pred) in self.predicates.iter() {
            demand.extend(pred.support());
        }
        // Start from the output columns as presumed demanded.
        // If this is not the case, the caller should project some away.
        demand.extend(self.projection.iter().cloned());
        // Proceed in *reverse* order, as expressions may depend on other
        // expressions that precede them.
        for index in (0..self.expressions.len()).rev() {
            if demand.contains(&(self.input_arity + index)) {
                demand.extend(self.expressions[index].support());
            }
        }

        // Maintain a map from initial column identifiers to locations
        // once we have removed undemanded expressions.
        let mut remap = BTreeMap::new();
        // This map only needs to map elements of `demand` to a new location,
        // but the logic is easier if we include all input columns (as the
        // new position is then determined by the size of the map).
        for index in 0..self.input_arity {
            remap.insert(index, index);
        }
        // Retain demanded expressions, and record their new locations.
        let mut new_expressions = Vec::new();
        for (index, expr) in self.expressions.drain(..).enumerate() {
            if demand.contains(&(index + self.input_arity)) {
                remap.insert(index + self.input_arity, remap.len());
                new_expressions.push(expr);
            }
        }
        self.expressions = new_expressions;

        // Update column identifiers; rebuild `Self` to re-establish any invariants.
        // We mirror `self.permute(&remap)` but we specifically want to remap columns
        // that are produced by `self.expressions` after the input columns.
        let (expressions, predicates, projection) = self.as_map_filter_project();
        *self = Self::new(self.input_arity)
            .map(expressions.into_iter().map(|mut e| {
                e.permute_map(&remap);
                e
            }))
            .filter(predicates.into_iter().map(|mut p| {
                p.permute_map(&remap);
                p
            }))
            .project(projection.into_iter().map(|c| remap[&c]));
    }
}

// TODO: move this elsewhere?