// 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.writeOK(bulk.execute());

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

    if (pipelineOptimizedAway) {
        assert(isQueryPlan(explainOutput));
        assert(
            !planHasStage(db, explainOutput, "FETCH"),
            "Expected pipeline " + tojsononeline(pipeline) +
                " *not* to include a FETCH stage in the explain output: " + tojson(explainOutput));
        assert(planHasStage(db, explainOutput, "IXSCAN"),
               "Expected pipeline " + tojsononeline(pipeline) +
                   " to include an index scan in the explain output: " + tojson(explainOutput));
    } else {
        assert(isAggregationPlan(explainOutput));
        assert(
            !aggPlanHasStage(explainOutput, "FETCH"),
            "Expected pipeline " + tojsononeline(pipeline) +
                " *not* to include a FETCH stage in the explain output: " + tojson(explainOutput));
        assert(aggPlanHasStage(explainOutput, "IXSCAN"),
               "Expected pipeline " + tojsononeline(pipeline) +
                   " to include an index scan in the explain output: " + tojson(explainOutput));
    }
    assert(!hasRejectedPlans(explainOutput),
           "Expected pipeline " + tojsononeline(pipeline) +
               " not to have any rejected plans in the explain output: " + tojson(explainOutput));
    return explainOutput;
}

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

    if (pipelineOptimizedAway) {
        assert(isQueryPlan(explainOutput));
        assert(planHasStage(db, explainOutput, "FETCH") || aggPlanHasStage("COLLSCAN"),
               "Expected pipeline " + tojsononeline(pipeline) +
                   " to include a FETCH or COLLSCAN stage in the explain output: " +
                   tojson(explainOutput));
        assert(
            !hasRejectedPlans(explainOutput),
            "Expected pipeline " + tojsononeline(pipeline) +
                " not to have any rejected plans in the explain output: " + tojson(explainOutput));
    } else {
        assert(isAggregationPlan(explainOutput));
        assert(aggPlanHasStage(explainOutput, "FETCH") || aggPlanHasStage("COLLSCAN"),
               "Expected pipeline " + tojsononeline(pipeline) +
                   " to include a FETCH or COLLSCAN stage in the explain output: " +
                   tojson(explainOutput));
        assert(
            !hasRejectedPlans(explainOutput),
            "Expected pipeline " + tojsononeline(pipeline) +
                " not to have any rejected plans in the explain output: " + tojson(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}}]});

// 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}}], pipelineOptimizedAway: false});

// 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}},
    ],
    pipelineOptimizedAway: false
});

// 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.writeOK(coll.insert({x: ["an", "array!"]}));
assertQueryDoesNotCoverProjection({
    pipeline: [{$match: {x: "string"}}, {$project: {_id: 1, x: 1}}],
    pipelineOptimizedAway: false
});
assertQueryDoesNotCoverProjection({
    pipeline: [{$match: {x: "string"}}, {$project: {_id: 1, x: 1, a: 1}}],
    pipelineOptimizedAway: false
});
}());