/** * JOO wrong results: an indexed nested loop join that probes a *multikey* compound index returns * the same foreign document once per array element. * * createIndexProbeQSN() builds the IndexEntry with isMultikey= false regardless of the catalog, so * buildIndexedJoinIndexProbe() never adds the UniqueRoaring dedup stage. The probe seeks * [x, MinKey] .. [x, MaxKey] on {y: 1, tags: 1}, which matches one key per element of "tags" . * The join predicate field itself (y) is scalar, so PathArrayness does not block the plan. * * The join method is forced here for determinism; the same plan is chosen naturally e.g. with * 3 local documents and 20000 foreign documents. */ const conn = MongoRunner.runMongod({ setParameter: { internalEnableJoinOptimization: true , featureFlagPathArrayness: true , featureFlagCostBasedRanker: true , internalEnableJoinPlanCache: false , internalJoinMethod: "INLJ" , }, }); const db = conn.getDB( "test" ); try { assert.commandWorked(db.a.insert({_id: 1, x: 1})); assert.commandWorked(db.b.insert({_id: 1, y: 1, tags: [1, 2, 3]})); assert.commandWorked(db.a.createIndex({x: 1})); assert.commandWorked(db.b.createIndex({y: 1, tags: 1})); const pipeline = [{$lookup: {from: "b" , localField: "x" , foreignField: "y" , as: "b" }}, {$unwind: "$b" }]; function run(joinOpt) { assert.commandWorked(db.adminCommand({setParameter: 1, internalEnableJoinOptimization: joinOpt})); return db.a.aggregate(pipeline).toArray(); } const expected = run( false ); assert.eq(expected.length, 1, tojson(expected)); const actual = run( true ); // assert: expected 3 to equal 1 : join optimization returned duplicates: [ { "_id" : 1, "x" : 1, "b" : { "_id" : 1, "y" : 1, "tags" : [ 1, 2, 3 ] } }, ... ] assert.eq(actual.length, 1, "join optimization returned duplicates: " + tojson(actual)); } finally { MongoRunner.stopMongod(conn); }