SERVER-39241 Apply noFetchBonus iff no FETCH stage; disregard PROJECTION

3 files changed, 110 insertions, 5 deletions

diff --git a/jstests/noPassthrough/noFetchBonus.js b/jstests/noPassthrough/noFetchBonus.js
new file mode 100644
index 00000000000..7035eac5aea
--- /dev/null
+++ b/jstests/noPassthrough/noFetchBonus.js
@@ -0,0 +1,29 @@
+// @tags: [requires_sharding, requires_replication]
+(function() {
+    "use strict";
+
+    load('jstests/libs/analyze_plan.js');
+
+    const st = new ShardingTest({shards: 1, rs: {nodes: 1}, config: 1});
+    const db = st.s.getDB("test");
+    const coll = db.getCollection('coll');
+
+    assert.commandWorked(coll.createIndex({a: 1}));
+    assert.commandWorked(coll.createIndex({a: 1, b: 1}));
+    st.shardColl(coll, {a: 1, b: 1}, false);
+
+    // SERVER-39241 One plan uses the {a: 1} index, which isn't enough to cover the query, because
+    // the
+    // sharding filter needs check the value of b. The other plan uses the {a: 1, b: 1}, which does
+    // cover the query. Assert the covered plan wins.
+    let explain = coll.explain().count({a: 1});
+    assert(planHasStage(db, explain.queryPlanner.winningPlan, 'SHARDING_FILTER'), explain);
+    assert(isIndexOnly(db, explain.queryPlanner.winningPlan), explain);
+
+    let rejected = explain.queryPlanner.winningPlan.shards[0].rejectedPlans;
+    assert.eq(rejected.length, 1, rejected);
+    assert(planHasStage(db, rejected[0], 'SHARDING_FILTER'), explain);
+    assert(planHasStage(db, rejected[0], 'FETCH'), rejected);
+
+    st.stop();
+}());
diff --git a/src/mongo/db/query/plan_ranker.cpp b/src/mongo/db/query/plan_ranker.cpp
index a8260b22c7f..d8d5ff809f9 100644
--- a/src/mongo/db/query/plan_ranker.cpp
+++ b/src/mongo/db/query/plan_ranker.cpp
@@ -209,12 +209,9 @@ double PlanRanker::scoreTree(const PlanStageStats* stats) {
     // plan doesn't lose to a less productive plan due to tie breaking.
     const double epsilon = std::min(1.0 / static_cast<double>(10 * workUnits), 1e-4);
 
-    // We prefer covered projections.
-    //
-    // We only do this when we have a projection stage because we have so many jstests that
-    // check bounds even when a collscan plan is just as good as the ixscan'd plan :(
+    // We prefer queries that don't require a fetch stage.
     double noFetchBonus = epsilon;
-    if (hasStage(STAGE_PROJECTION, stats) && hasStage(STAGE_FETCH, stats)) {
+    if (hasStage(STAGE_FETCH, stats)) {
         noFetchBonus = 0;
     }
 
diff --git a/src/mongo/db/query/plan_ranker_test.cpp b/src/mongo/db/query/plan_ranker_test.cpp
new file mode 100644
index 00000000000..8f51a0edab1
--- /dev/null
+++ b/src/mongo/db/query/plan_ranker_test.cpp
@@ -0,0 +1,79 @@
+/**
+ *    Copyright (C) 2019-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+/**
+ * This file contains tests for mongo/db/query/plan_ranker.h
+ */
+
+#include "mongo/db/query/plan_ranker.h"
+
+#include "mongo/unittest/unittest.h"
+#include "mongo/util/assert_util.h"
+
+using namespace mongo;
+
+namespace {
+
+using std::make_unique;
+using std::string;
+using std::unique_ptr;
+using std::vector;
+
+unique_ptr<PlanStageStats> makeStats(const char* name,
+                                     StageType type,
+                                     unique_ptr<SpecificStats> specific) {
+    auto stats = make_unique<PlanStageStats>(name, type);
+    stats->common.works = 1;
+    stats->specific = std::move(specific);
+    return stats;
+}
+
+TEST(PlanRankerTest, NoFetchBonus) {
+    // Two plans: one does a fetch, one does not. Assert the plan without the fetch has a higher
+    // score. Note there is no projection involved: before SERVER-39241 was fixed we would give
+    // these two plans the same score.
+
+    auto goodPlan =
+        makeStats("SHARDING_FILTER", STAGE_SHARDING_FILTER, make_unique<ShardingFilterStats>());
+    goodPlan->children.emplace_back(
+        makeStats("IXSCAN", STAGE_IXSCAN, make_unique<IndexScanStats>()));
+
+    auto badPlan =
+        makeStats("SHARDING_FILTER", STAGE_SHARDING_FILTER, make_unique<ShardingFilterStats>());
+    badPlan->children.emplace_back(makeStats("FETCH", STAGE_FETCH, make_unique<IndexScanStats>()));
+    badPlan->children[0]->children.emplace_back(
+        makeStats("IXSCAN", STAGE_IXSCAN, make_unique<IndexScanStats>()));
+
+    auto goodScore = PlanRanker::scoreTree(goodPlan.get());
+    auto badScore = PlanRanker::scoreTree(badPlan.get());
+
+    ASSERT_GT(goodScore, badScore);
+}
+
+};  // namespace