Method action

src/transform/src/predicate_pushdown.rs:159–799

Predicate pushdown This method looks for opportunities to push predicates toward sources of data. Primarily, this is the `Filter` expression, and moving its predicates through the operators it contains. In addition, the method accumulates the intersection of predicates applied to each `Get` expression, so that the predicate can then be pushed through to a `Let` binding, or to the external source

(
        &self,
        relation: &mut MirRelationExpr,
        get_predicates: &mut BTreeMap<Id, BTreeSet<MirScalarExpr>>,
    )

    /// then be pushed through to a `Let` binding, or to the external
    /// source of the data if the `Get` binds to another view.
    pub fn action(
        &self,
        relation: &mut MirRelationExpr,
        get_predicates: &mut BTreeMap<Id, BTreeSet<MirScalarExpr>>,
    ) -> Result<(), TransformError> {
        self.checked_recur(|_| {
            // In the case of Filter or Get we have specific work to do;
            // otherwise we should recursively descend.
            match relation {
                MirRelationExpr::Filter { input, predicates } => {
                    // Reduce the predicates to determine as best as possible
                    // whether they are literal errors before working with them.
                    let input_type = input.typ();
                    for predicate in predicates.iter_mut() {
                        predicate.reduce(&input_type.column_types);
                    }

                    // It can be helpful to know if there are any non-literal errors,
                    // as this is justification for not pushing down literal errors.
                    let all_errors = predicates.iter().all(|p| p.is_literal_err());
                    // Depending on the type of `input` we have different
                    // logic to apply to consider pushing `predicates` down.
                    match &mut **input {
                        MirRelationExpr::Let { body, .. }
                        | MirRelationExpr::LetRec { body, .. } => {
                            // Push all predicates to the body.
                            **body = body
                                .take_dangerous()
                                .filter(std::mem::replace(predicates, Vec::new()));

                            self.action(input, get_predicates)?;
                        }
                        MirRelationExpr::Get { id, .. } => {
                            // We can report the predicates upward in `get_predicates`,
                            // but we are not yet able to delete them from the
                            // `Filter`.
                            get_predicates
                                .entry(*id)
                                .or_insert_with(|| predicates.iter().cloned().collect())
                                .retain(|p| predicates.contains(p));
                        }
                        MirRelationExpr::Join {
                            inputs,
                            equivalences,
                            ..
                        } => {
                            // We want to scan `predicates` for any that can
                            // 1) become join variable constraints
                            // 2) apply to individual elements of `inputs`.
                            // Figuring out the set of predicates that belong to
                            //    the latter group requires 1) knowing which predicates
                            //    are in the former group and 2) that the variable
                            //    constraints be in canonical form.
                            // Thus, there is a first scan across `predicates` to
                            //    populate the join variable constraints
                            //    and a second scan across the remaining predicates
                            //    to see which ones can become individual elements of
                            //    `inputs`.