Function handle_mutation_using_clause

src/sql/src/plan/query.rs:852–941

Adjust `get` to perform an existential subquery on `using` accounting for `selection`. If `USING`, we essentially want to rewrite the query as a correlated existential subquery, i.e. ``` ...WHERE EXISTS (SELECT 1 FROM WHERE ) ``` However, we can't do that directly because of esoteric rules w/r/t `lateral` subqueries. https://github.com/postgres/postgres/commit/158b7fa6a34006bdc

(
    qcx: &QueryContext,
    selection: Option<Expr<Aug>>,
    using: Vec<TableWithJoins<Aug>>,
    get: HirRelationExpr,
    outer_scope: Scope,
)

// subqueries.
// https://github.com/postgres/postgres/commit/158b7fa6a34006bdc70b515e14e120d3e896589b
fn handle_mutation_using_clause(
    qcx: &QueryContext,
    selection: Option<Expr<Aug>>,
    using: Vec<TableWithJoins<Aug>>,
    get: HirRelationExpr,
    outer_scope: Scope,
) -> Result<HirRelationExpr, PlanError> {
    // Plan `USING` as a cross-joined `FROM` without knowledge of the
    // statement's `FROM` target. This prevents `lateral` subqueries from
    // "seeing" the `FROM` target.
    let (mut using_rel_expr, using_scope) =
        using.into_iter().try_fold(plan_join_identity(), |l, twj| {
            let (left, left_scope) = l;
            plan_join(
                qcx,
                left,
                left_scope,
                &Join {
                    relation: TableFactor::NestedJoin {
                        join: Box::new(twj),
                        alias: None,
                    },
                    join_operator: JoinOperator::CrossJoin,
                },
            )
        })?;

    if let Some(expr) = selection {
        // Join `FROM` with `USING` tables, like `USING..., FROM`. This gives us
        // PG-like semantics e.g. expressing ambiguous column references. We put
        // `USING...` first for no real reason, but making a different decision
        // would require adjusting the column references on this relation
        // differently.
        let on = HirScalarExpr::literal_true();
        let joined = using_rel_expr
            .clone()
            .join(get.clone(), on, JoinKind::Inner);
        let joined_scope = using_scope.product(outer_scope)?;
        let joined_relation_type = qcx.relation_type(&joined);

        let ecx = &ExprContext {
            qcx,
            name: "WHERE clause",
            scope: &joined_scope,
            relation_type: &joined_relation_type,
            allow_aggregates: false,
            allow_subqueries: true,
            allow_parameters: true,
            allow_windows: false,
        };

        // Plan the filter expression on `FROM, USING...`.
        let mut expr = plan_expr(ecx, &expr)?.type_as(ecx, &SqlScalarType::Bool)?;

        // Rewrite all column referring to the `FROM` section of `joined` (i.e.
        // those to the right of `using_rel_expr`) to instead be correlated to
        // the outer relation, i.e. `get`.
        let using_rel_arity = qcx.relation_type(&using_rel_expr).arity();