Method applied_to

src/sql/src/plan/lowering.rs:243–948

Return a `MirRelationExpr` which evaluates `self` once for each row of `get_outer`. For uncorrelated `self`, this should be the cross-product between `get_outer` and `self`. When `self` references columns of `get_outer`, much more work needs to occur. The `col_map` argument contains mappings to some of the columns of `get_outer`, though perhaps not all of them. It should be used as the basis of

(
        self,
        id_gen: &mut mz_ore::id_gen::IdGen,
        get_outer: MirRelationExpr,
        col_map: &ColumnMap,
        cte_map: &mut CteMap,
        context: &Context,
    )

    /// The `get_outer` expression should be a `Get` with no duplicate rows, describing the distinct
    /// assignment of values to outer rows.
    fn applied_to(
        self,
        id_gen: &mut mz_ore::id_gen::IdGen,
        get_outer: MirRelationExpr,
        col_map: &ColumnMap,
        cte_map: &mut CteMap,
        context: &Context,
    ) -> Result<MirRelationExpr, PlanError> {
        maybe_grow(|| {
            use MirRelationExpr as SR;

            use HirRelationExpr::*;

            if let MirRelationExpr::Get { .. } = &get_outer {
            } else {
                panic!(
                    "get_outer: expected a MirRelationExpr::Get, found\n{}",
                    get_outer.pretty(),
                );
            }
            assert_eq!(col_map.len(), get_outer.arity());
            Ok(match self {
                Constant { rows, typ } => {
                    // Constant expressions are not correlated with `get_outer`, and should be cross-products.
                    get_outer.product(SR::Constant {
                        rows: Ok(rows.into_iter().map(|row| (row, Diff::ONE)).collect()),
                        typ: ReprRelationType::from(&typ),
                    })
                }
                Get { id, typ } => match id {
                    mz_expr::Id::Local(local_id) => {
                        let cte_desc = cte_map.get(&local_id).unwrap();
                        let get_cte = SR::Get {
                            id: mz_expr::Id::Local(cte_desc.new_id.clone()),
                            typ: cte_desc.relation_type.clone(),
                            access_strategy: AccessStrategy::UnknownOrLocal,
                        };
                        if get_outer == cte_desc.outer_relation {
                            // If the CTE was applied to the same exact relation, we can safely
                            // return a `Get` relation.
                            get_cte
                        } else {
                            // Otherwise, the new outer relation may contain more columns from some
                            // intermediate scope placed between the definition of the CTE and this
                            // reference of the CTE and/or more operations applied on top of the
                            // outer relation.
                            //
                            // An example of the latter is the following query:
                            //
                            // SELECT *
                            // FROM x,
                            //      LATERAL(WITH a(m) as (SELECT max(y.a) FROM y WHERE y.a < x.a)
                            //              SELECT (SELECT m FROM a) FROM y) b;
                            //
                            // When the CTE is lowered, the outer relation is `Get x`. But then,
                            // the reference of the CTE is applied to `Distinct(Join(Get x, Get y), x.*)`
                            // which has the same cardinality as `Get x`.
                            //