SERVER-22093 Aggregation uses a COUNT plan when no fields are needed …

…from input documents.

jstests/aggregation/bugs/server22093.js

@@ -0,0 +1,35 @@
+// From the work done for SERVER-22093, an aggregation pipeline that does not require any fields
+// from the input documents will tell the query planner to use a count scan, which is faster than an
+// index scan. In this test file, we check this behavior through explain().
+load('jstests/libs/analyze_plan.js');
+
+(function() {
+    "use strict";
+
+    var coll = db.countscan;
+    coll.drop();
+
+    for (var i = 0; i < 3; i++) {
+        for (var j = 0; j < 10; j += 2) {
+            coll.insert({foo: i, bar: j});
+        }
+    }
+
+    coll.ensureIndex({foo: 1});
+
+    var simpleGroup = coll.aggregate([{$group: {_id: null, count: {$sum: 1}}}]).toArray();
+
+    assert.eq(simpleGroup.length, 1);
+    assert.eq(simpleGroup[0]["count"], 15);
+
+    var explained = coll.explain().aggregate([{$match: {foo: {$gt: 0}}},
+            {$group: {_id: null, count: {$sum: 1}}}]);
+
+    assert(planHasStage(explained.stages[0].$cursor.queryPlanner.winningPlan, "COUNT_SCAN"));
+
+    explained = coll.explain().aggregate([{$match: {foo: {$gt: 0}}},
+            {$project: {_id: 0, a: {$literal: null}}},
+            {$group: {_id: null, count: {$sum: 1}}}]);
+
+    assert(planHasStage(explained.stages[0].$cursor.queryPlanner.winningPlan, "COUNT_SCAN"));
+}());