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// 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}}]});
}());
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