Method lower_subqueries

src/sql/src/plan/lowering.rs:1571–1666

Applies the subqueries in the given list of scalar expressions to every distinct value of the given relation and returns a join of the given relation with all the subqueries found, and the mapping of scalar expressions with columns projected by the returned join that will hold their results.

(
        exprs: &[Self],
        id_gen: &mut mz_ore::id_gen::IdGen,
        col_map: &ColumnMap,
        cte_map: &mut CteMap,
        inner: MirRelationExpr,
        context: &Context,
    )

    /// the subqueries found, and the mapping of scalar expressions with columns projected
    /// by the returned join that will hold their results.
    fn lower_subqueries(
        exprs: &[Self],
        id_gen: &mut mz_ore::id_gen::IdGen,
        col_map: &ColumnMap,
        cte_map: &mut CteMap,
        inner: MirRelationExpr,
        context: &Context,
    ) -> Result<(MirRelationExpr, BTreeMap<HirScalarExpr, usize>), PlanError> {
        let mut subquery_map = BTreeMap::new();
        let output = inner.let_in(id_gen, |id_gen, get_inner| {
            let mut subqueries = Vec::new();
            let distinct_inner = get_inner.clone().distinct();
            for expr in exprs.iter() {
                expr.visit_pre_post(
                    &mut |e| match e {
                        // For simplicity, subqueries within a conditional statement will be
                        // lowered when lowering the conditional expression.
                        HirScalarExpr::If { .. } => Some(vec![]),
                        _ => None,
                    },
                    &mut |e| match e {
                        HirScalarExpr::Select(expr, _name) => {
                            let apply_requires_distinct_outer = false;
                            let subquery = apply_scalar_subquery(
                                id_gen,
                                distinct_inner.clone(),
                                col_map,
                                cte_map,
                                (**expr).clone(),
                                apply_requires_distinct_outer,
                                context,
                            )
                            .unwrap();

                            subqueries.push((e.clone(), subquery));
                        }
                        HirScalarExpr::Exists(expr, _name) => {
                            let apply_requires_distinct_outer = false;
                            let subquery = apply_existential_subquery(
                                id_gen,
                                distinct_inner.clone(),
                                col_map,
                                cte_map,
                                (**expr).clone(),
                                apply_requires_distinct_outer,
                                context,
                            )
                            .unwrap();
                            subqueries.push((e.clone(), subquery));
                        }
                        _ => {}
                    },
                );
            }

            if subqueries.is_empty() {
                Ok::<MirRelationExpr, PlanError>(get_inner)
            } else {