void run() {
            // We insert lots of copies of {a:1, b:1, c: 20}.  We have the indices {a:1} and {b:1},
            // and the query is {a:1, b:1, c: 999}.  No data that matches the query but we won't
            // know that during plan ranking.  We don't want to choose an intersection plan here.
            for (int i = 0; i < N; ++i) {
                insert(BSON("a" << 1 << "b" << 1 << "c" << 20));
            }

            addIndex(BSON("a" << 1));
            addIndex(BSON("b" << 1));

            // There is no data that matches this query but we don't know that until EOF.
            CanonicalQuery* cq;
            BSONObj queryObj = BSON("a" << 1 << "b" << 1 << "c" << 99);
            ASSERT(CanonicalQuery::canonicalize(ns, queryObj, &cq).isOK());
            ASSERT(NULL != cq);

            // Takes ownership of cq.
            QuerySolution* soln = pickBestPlan(cq);

            // Anti-prefer the intersection plan.
            bool bestIsScanOverA = QueryPlannerTestLib::solutionMatches(
                        "{fetch: {node: {ixscan: {pattern: {a: 1}}}}}",
                        soln->root.get());
            bool bestIsScanOverB = QueryPlannerTestLib::solutionMatches(
                        "{fetch: {node: {ixscan: {pattern: {b: 1}}}}}",
                        soln->root.get());
            ASSERT(bestIsScanOverA || bestIsScanOverB);
        }

        void run() {
            // Set up the data so that for the query {a: 1, b: 1, c: 1}, the intersection
            // between 'b' and 'c' is small, and the other intersections are larger.
            for (int i = 0; i < 10; ++i) {
                insert(BSON("a" << 1 << "b" << 1 << "c" << 1));
            }
            for (int i = 0; i < 10; ++i) {
                insert(BSON("a" << 2 << "b" << 1 << "c" << 1));
            }
            for (int i = 0; i < N/2; ++i) {
                insert(BSON("a" << 1 << "b" << 1 << "c" << 2));
                insert(BSON("a" << 1 << "b" << 2 << "c" << 1));
            }

            // Add indices on 'a', 'b', and 'c'.
            addIndex(BSON("a" << 1));
            addIndex(BSON("b" << 1));
            addIndex(BSON("c" << 1));

            CanonicalQuery* cq;
            ASSERT(CanonicalQuery::canonicalize(ns,
                                                fromjson("{a: 1, b: 1, c: 1}"),
                                                &cq).isOK());
            ASSERT(NULL != cq);

            // Intersection between 'b' and 'c' should hit EOF while the
            // other plans are busy fetching documents.
            QuerySolution* soln = pickBestPlan(cq);
            ASSERT(QueryPlannerTestLib::solutionMatches(
                "{fetch: {filter: {a:1}, node: {andSorted: {nodes: ["
                    "{ixscan: {filter: null, pattern: {b:1}}},"
                    "{ixscan: {filter: null, pattern: {c:1}}}]}}}}",
                soln->root.get()));
        }

    Runner::RunnerState MultiPlanRunner::getNext(BSONObj* objOut) {
        if (_failure || _killed) {
            return Runner::RUNNER_DEAD;
        }

        // If we haven't picked the best plan yet...
        if (NULL == _bestPlan) {
            // TODO: Consider rewriting pickBestPlan to return results as it iterates.
            if (!pickBestPlan(NULL)) {
                verify(_failure);
                return Runner::RUNNER_DEAD;
            }
        }

        if (!_alreadyProduced.empty()) {
            WorkingSetID id = _alreadyProduced.front();
            _alreadyProduced.pop();

            WorkingSetMember* member = _bestPlan->getWorkingSet()->get(id);
            // TODO: getOwned?
            verify(WorkingSetCommon::fetch(member));
            *objOut = member->obj;
            _bestPlan->getWorkingSet()->free(id);
            return Runner::RUNNER_ADVANCED;
        }

        return _bestPlan->getNext(objOut);
    }

    Runner::RunnerState MultiPlanRunner::getNext(BSONObj* objOut, DiskLoc* dlOut) {
        if (_killed) { return Runner::RUNNER_DEAD; }
        if (_failure) { return Runner::RUNNER_ERROR; }

        // If we haven't picked the best plan yet...
        if (NULL == _bestPlan) {
            if (!pickBestPlan(NULL)) {
                verify(_failure || _killed);
                if (_killed) { return Runner::RUNNER_DEAD; }
                if (_failure) { return Runner::RUNNER_ERROR; }
            }
        }

        if (!_alreadyProduced.empty()) {
            WorkingSetID id = _alreadyProduced.front();
            _alreadyProduced.pop_front();

            WorkingSetMember* member = _bestPlan->getWorkingSet()->get(id);
            // Note that this copies code from PlanExecutor.
            if (NULL != objOut) {
                if (WorkingSetMember::LOC_AND_IDX == member->state) {
                    if (1 != member->keyData.size()) {
                        _bestPlan->getWorkingSet()->free(id);
                        return Runner::RUNNER_ERROR;
                    }
                    *objOut = member->keyData[0].keyData;
                }
                else if (member->hasObj()) {
                    *objOut = member->obj;
                }
                else {
                    // TODO: Checking the WSM for covered fields goes here.
                    _bestPlan->getWorkingSet()->free(id);
                    return Runner::RUNNER_ERROR;
                }
            }

            if (NULL != dlOut) {
                if (member->hasLoc()) {
                    *dlOut = member->loc;
                }
                else {
                    _bestPlan->getWorkingSet()->free(id);
                    return Runner::RUNNER_ERROR;
                }
            }
            _bestPlan->getWorkingSet()->free(id);
            return Runner::RUNNER_ADVANCED;
        }

        return _bestPlan->getNext(objOut, dlOut);
    }

    Runner::RunnerState MultiPlanRunner::getNext(BSONObj* objOut, DiskLoc* dlOut) {
        if (_killed) { return Runner::RUNNER_DEAD; }
        if (_failure) { return Runner::RUNNER_ERROR; }

        // If we haven't picked the best plan yet...
        if (NULL == _bestPlan) {
            if (!pickBestPlan(NULL, objOut)) {
                verify(_failure || _killed);
                if (_killed) { return Runner::RUNNER_DEAD; }
                if (_failure) { return Runner::RUNNER_ERROR; }
            }
        }

        // Look for an already produced result that provides the data the caller wants.
        while (!_alreadyProduced.empty()) {
            WorkingSetID id = _alreadyProduced.front();
            _alreadyProduced.pop_front();

            WorkingSetMember* member = _bestPlan->getWorkingSet()->get(id);

            // Note that this copies code from PlanExecutor.
            if (NULL != objOut) {
                if (WorkingSetMember::LOC_AND_IDX == member->state) {
                    if (1 != member->keyData.size()) {
                        _bestPlan->getWorkingSet()->free(id);
                        // If the caller needs the key data and the WSM doesn't have it, drop the
                        // result and carry on.
                        continue;
                    }
                    *objOut = member->keyData[0].keyData;
                }
                else if (member->hasObj()) {
                    *objOut = member->obj;
                }
                else {
                    // If the caller needs an object and the WSM doesn't have it, drop and
                    // try the next result.
                    _bestPlan->getWorkingSet()->free(id);
                    continue;
                }
            }

            if (NULL != dlOut) {
                if (member->hasLoc()) {
                    *dlOut = member->loc;
                }
                else {
                    // If the caller needs a DiskLoc and the WSM doesn't have it, drop and carry on.
                    _bestPlan->getWorkingSet()->free(id);
                    continue;
                }
            }

            // If we're here, the caller has all the data needed and we've set the out
            // parameters.  Remove the result from the WorkingSet.
            _bestPlan->getWorkingSet()->free(id);
            return Runner::RUNNER_ADVANCED;
        }

        RunnerState state = _bestPlan->getNext(objOut, dlOut);

        if (Runner::RUNNER_ERROR == state && (NULL != _backupSolution)) {
            QLOG() << "Best plan errored out switching to backup\n";
            // Uncache the bad solution if we fall back
            // on the backup solution.
            //
            // XXX: Instead of uncaching we should find a way for the
            // cached plan runner to fall back on a different solution
            // if the best solution fails. Alternatively we could try to
            // defer cache insertion to be after the first produced result.
            Database* db = cc().database();
            verify(NULL != db);
            Collection* collection = db->getCollection(_query->ns());
            verify(NULL != collection);
            PlanCache* cache = collection->infoCache()->getPlanCache();
            cache->remove(*_query);

            _bestPlan.reset(_backupPlan);
            _backupPlan = NULL;
            _bestSolution.reset(_backupSolution);
            _backupSolution = NULL;
            _alreadyProduced = _backupAlreadyProduced;
            return getNext(objOut, dlOut);
        }

        if (NULL != _backupSolution && Runner::RUNNER_ADVANCED == state) {
            QLOG() << "Best plan had a blocking sort, became unblocked, deleting backup plan\n";
            delete _backupSolution;
            delete _backupPlan;
            _backupSolution = NULL;
            _backupPlan = NULL;
            // TODO: free from WS?
            _backupAlreadyProduced.clear();
        }

        return state;
    }