// Tests that an aggregation that only needs a finite set of fields to do its computations can
// sometimes use the query system to provide a covered plan which only returns those fields without
// fetching the full document.
//
// Relies on the initial $match being pushed into the query system in order for the planner to
// consider an index scan, so the pipelines cannot be wrapped in facet stages.
// @tags: [
//   do_not_wrap_aggregations_in_facets,
// ]
(function() {
"use strict";

load("jstests/libs/analyze_plan.js");  // For 'aggPlanHasStage' and other explain helpers.

const coll = db.use_query_projection;
coll.drop();

const bulk = coll.initializeUnorderedBulkOp();
for (let i = 0; i < 100; ++i) {
    bulk.insert({_id: i, x: "string", a: -i, y: i % 2});
}
assert.commandWorked(bulk.execute());

function assertQueryCoversProjection(
    {pipeline = [], pipelineOptimizedAway = true, options = {}} = {}) {
    const explainOutput = coll.explain().aggregate(pipeline, options);

    if (pipelineOptimizedAway) {
        assert(isQueryPlan(explainOutput), explainOutput);
        assert(!planHasStage(db, explainOutput, "FETCH"), explainOutput);
        assert(planHasStage(db, explainOutput, "IXSCAN"), explainOutput);
    } else {
        assert(isAggregationPlan(explainOutput), explainOutput);
        assert(!aggPlanHasStage(explainOutput, "FETCH"), explainOutput);
        assert(aggPlanHasStage(explainOutput, "IXSCAN"), explainOutput);
    }
    assert(!hasRejectedPlans(explainOutput), explainOutput);
    return explainOutput;
}

function assertQueryDoesNotCoverProjection({pipeline = [], pipelineOptimizedAway = true} = {}) {
    const explainOutput = coll.explain().aggregate(pipeline);

    if (pipelineOptimizedAway) {
        assert(isQueryPlan(explainOutput), explainOutput);
        assert(planHasStage(db, explainOutput, "FETCH") || aggPlanHasStage("COLLSCAN"),
               explainOutput);
    } else {
        assert(isAggregationPlan(explainOutput), explainOutput);
        assert(aggPlanHasStage(explainOutput, "FETCH") || aggPlanHasStage("COLLSCAN"),
               explainOutput);
    }
    assert(!hasRejectedPlans(explainOutput), explainOutput);
    return explainOutput;
}

assert.commandWorked(coll.createIndex({x: 1, a: -1, _id: 1}));

// Test that a pipeline requiring a subset of the fields in a compound index can use that index
// to cover the query.
assertQueryCoversProjection({pipeline: [{$match: {x: "string"}}, {$project: {_id: 1, x: 1}}]});
assertQueryCoversProjection({pipeline: [{$match: {x: "string"}}, {$project: {_id: 0, x: 1}}]});
assertQueryCoversProjection(
    {pipeline: [{$match: {x: "string"}}, {$project: {_id: 0, x: 1, a: 1}}]});
assertQueryCoversProjection(
    {pipeline: [{$match: {x: "string"}}, {$project: {_id: 1, x: 1, a: 1}}]});
assertQueryCoversProjection({
    pipeline: [{$match: {_id: 0, x: "string"}}, {$project: {_id: 1, x: 1, a: 1}}],
    options: {hint: {x: 1, a: -1, _id: 1}}
});

// Test that a pipeline requiring a field that is not in the index cannot use a covered plan.
assertQueryDoesNotCoverProjection({pipeline: [{$match: {x: "string"}}, {$project: {notThere: 1}}]});

// Test that a covered plan is the only plan considered, even if another plan would be equally
// selective. Add an equally selective index, then rely on assertQueryCoversProjection() to
// assert that there is only one considered plan, and it is a covered plan.
assert.commandWorked(coll.createIndex({x: 1}));
assertQueryCoversProjection({
    pipeline: [
        {$match: {_id: 0, x: "string"}},
        {$sort: {x: 1, a: 1}},  // Note: not indexable, but doesn't add any additional dependencies.
        {$project: {_id: 1, x: 1, a: 1}},
    ],
    options: {hint: {x: 1, a: -1, _id: 1}}
});

// Test that a multikey index will prevent a covered plan.
assert.commandWorked(coll.dropIndex({x: 1}));  // Make sure there is only one plan considered.
assert.commandWorked(coll.insert({x: ["an", "array!"]}));
assertQueryDoesNotCoverProjection(
    {pipeline: [{$match: {x: "string"}}, {$project: {_id: 1, x: 1}}]});
assertQueryDoesNotCoverProjection(
    {pipeline: [{$match: {x: "string"}}, {$project: {_id: 1, x: 1, a: 1}}]});
}());