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// 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().
//
// Cannot implicitly shard accessed collections because the explain output from a mongod when run
// against a sharded collection is wrapped in a "shards" object with keys for each shard.
//
// This test assumes that an initial $match will be absorbed by the query system, which will not
// happen if the $match is wrapped within a $facet stage.
// @tags: [do_not_wrap_aggregations_in_facets,assumes_unsharded_collection]
load('jstests/libs/analyze_plan.js');
(function() {
"use strict";
var coll = db.countscan;
coll.drop();
for (var i = 0; i < 3; i++) {
for (var j = 0; j < 10; j += 2) {
coll.insert({foo: i, bar: j});
}
}
coll.ensureIndex({foo: 1});
var simpleGroup = coll.aggregate([{$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: {$sum: 1}}}]);
assert(planHasStage(db, explained.stages[0].$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(db, explained.stages[0].$cursor.queryPlanner.winningPlan, "COUNT_SCAN"));
// Make sure a $count stage can use the COUNT_SCAN optimization.
explained = coll.explain().aggregate([{$match: {foo: {$gt: 0}}}, {$count: "count"}]);
assert(planHasStage(db, explained.stages[0].$cursor.queryPlanner.winningPlan, "COUNT_SCAN"));
// A $match that is not a single range cannot use the COUNT_SCAN optimization.
explained = coll.explain().aggregate([{$match: {foo: {$in: [0, 1]}}}, {$count: "count"}]);
assert(!planHasStage(db, explained.stages[0].$cursor.queryPlanner.winningPlan, "COUNT_SCAN"));
}());
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