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Testing filter getting pushed into find during $lookup I created a case where no results are being returned rather than matching subset of results that only happens on Mac OS X build. I think we also are pushing $match into $lookup in spite an unwind which creates different results than lack of optimization would. Will put reproducer in comment to make it readable.

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xgen-internal-githook commented on Fri, 29 Apr 2016 16:19:34 +0000: Author: {u'username': u'benjaminmurphy', u'name': u'Benjamin Murphy', u'email': u'benjamin_murphy@me.com'} Message: SERVER-23922 Lookup correctly serializes an internalized match. Branch: master https://github.com/mongodb/mongo/commit/c1814956391a9c37f4f4e6cca93c08f7fcacc85d max.hirschhorn@10gen.com commented on Tue, 26 Apr 2016 22:02:24 +0000: I spent some more time with Ben looking into this issue. The problem is that DocumentSourceLookUp::serializeToArray() serializes the match expression absorbed into the $lookup after it has already descended on the "as" field. For example, we'll descend on the path "x" with the match expression {$match: {"x.b": "b"}} and execute it as {b: {$eq: "b"}} on the "from" collection. We should be serializing it back out {$match: {"x.b": {$eq: "b"}}}, but end up serializing it as {$match: {b: {$eq: "b"}}}. This issue affects debug builds of MongoDB and aggregations using $lookup in a sharded cluster. Process 38648 stopped * thread #2: tid = 0x1bf89c, 0x0000000100436db0 mongod`mongo::PipelineCommand::run(this=0x00000001030e4258, txn=0x0000000105b17e20, db=0x000000010bfcfa40, cmdObj=0x000000010bfcfa70, options=0, errmsg=0x000000010bfcfa58, result=0x000000010bfcfe68) + 2080 at pipeline_command.cpp:220, stop reason = step over frame #0: 0x0000000100436db0 mongod`mongo::PipelineCommand::run(this=0x00000001030e4258, txn=0x0000000105b17e20, db=0x000000010bfcfa40, cmdObj=0x000000010bfcfa70, options=0, errmsg=0x000000010bfcfa58, result=0x000000010bfcfe68) + 2080 at pipeline_command.cpp:220 217 // aggregations rely on this ability. Skipping when inShard because this has 218 // already been through the transformation (and this unsets pCtx->inShard). 219 parsed = pPipeline->serialize().toBson(); -> 220 pPipeline = Pipeline::parseCommand(errmsg, parsed, pCtx); 221 verify(pPipeline); 222 } 223 (lldb) p parsed.toString(false, false) (std::__1::string) $2 = "{ aggregate: \"a\", pipeline: [ { $lookup: { from: \"b\", as: \"x\", localField: \"a\", foreignField: \"a\" } }, { $unwind: { path: \"$x\" } }, { $match: { b: { $eq: \"b\" } } } ] }" asya commented on Tue, 26 Apr 2016 17:11:19 +0000: I can't reproduce this with builds for any platform other than OS X so far. asya commented on Tue, 26 Apr 2016 16:50:59 +0000: These are the binaries producing test failure for me: https://s3.amazonaws.com/mciuploads/mongodb-mongo-master/osx-107-debug/046441de98dd72689397a4a06f8a68ef58a4c226/binaries/mongo-mongodb_mongo_master_osx_107_debug_046441de98dd72689397a4a06f8a68ef58a4c226_16_04_25_21_26_45.tgz edit later Mac OS X binaries also reproduce the error. asya commented on Tue, 26 Apr 2016 15:50:09 +0000: My result with Max's test (I'm on Mac) test2@local:33599(3.3.5-21-g046441d) > (function() { db.dropDatabase(); assert.writeOK(db.a.insert([ {_id: ObjectId("571ea21117443c201ac21e0c"), a: "a", b: "b1"}, {_id: ObjectId("571ea21717443c201ac21e0d"), a: "a", b: "b2"}, {_id: ObjectId("571ea22117443c201ac21e0e"), a: "a3", b: "b3"}, ])); assert.writeOK(db.b.insert([ {_id: ObjectId("571ea22c17443c201ac21e0f"), a: "a", b: "b"}, {_id: ObjectId("571ea23117443c201ac21e10"), a: "a", b: "c"}, ])); var optimized = db.a.aggregate([ {$lookup: { from: "b", localField: "a", foreignField: "a", as:"x" }}, {$unwind: "$x"}, {$match: {"x.b": "b"}}, ]).toArray(); var nonOptimized = db.a.aggregate([ {$lookup: { from: "b", localField: "a", foreignField: "a", as:"x" }}, {$unwind: "$x"}, {$limit: 1000}, {$match: {"x.b": "b"}}, ]).toArray(); assert.eq(optimized, nonOptimized); })(); assert: [[ ]] != [[ { "_id" : ObjectId("571ea21117443c201ac21e0c"), "a" : "a", "b" : "b1", "x" : { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" } }, { "_id" : ObjectId("571ea21717443c201ac21e0d"), "a" : "a", "b" : "b2", "x" : { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" } } ]] are not equal : undefined doassert@src/mongo/shell/assert.js:15:14 assert.eq@src/mongo/shell/assert.js:51:5 @(shell):1:1020 @(shell):1:2 2016-04-26T08:47:31.835-0700 E QUERY [thread1] Error: [[ ]] != [[ { "_id" : ObjectId("571ea21117443c201ac21e0c"), "a" : "a", "b" : "b1", "x" : { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" } }, { "_id" : ObjectId("571ea21717443c201ac21e0d"), "a" : "a", "b" : "b2", "x" : { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" } } ]] are not equal : undefined : doassert@src/mongo/shell/assert.js:15:14 assert.eq@src/mongo/shell/assert.js:51:5 @(shell):1:1020 @(shell):1:2 test2@local:33599(3.3.5-21-g046441d) > db.serverBuildInfo() { "version" : "3.3.5-21-g046441d", "gitVersion" : "046441de98dd72689397a4a06f8a68ef58a4c226", "modules" : [ ], "allocator" : "system", "javascriptEngine" : "mozjs", "sysInfo" : "deprecated", "versionArray" : [ 3, 3, 6, -100 ], "openssl" : { "running" : "disabled", "compiled" : "disabled" }, "buildEnvironment" : { "distmod" : "", "distarch" : "x86_64", "cc" : "gcc: Apple LLVM version 7.0.2 (clang-700.1.81)", "ccflags" : "-fno-omit-frame-pointer -fPIC -fno-strict-aliasing -ggdb -pthread -Wall -Wsign-compare -Wno-unknown-pragmas -Winvalid-pch -Werror -O2 -Wno-unused-local-typedefs -Wno-unused-function -Wno-unused-private-field -Wno-deprecated-declarations -Wno-tautological-constant-out-of-range-compare -Wno-unused-const-variable -Wno-missing-braces -Wno-inconsistent-missing-override -Wno-potentially-evaluated-expression -mmacosx-version-min=10.7 -fno-builtin-memcmp", "cxx" : "g++: Apple LLVM version 7.0.2 (clang-700.1.81)", "cxxflags" : "-Wnon-virtual-dtor -Woverloaded-virtual -stdlib=libc++ -std=c++11", "linkflags" : "-fPIC -pthread -Wl,-bind_at_load -mmacosx-version-min=10.7 -stdlib=libc++", "target_arch" : "x86_64", "target_os" : "osx" }, "bits" : 64, "debug" : true, "maxBsonObjectSize" : 16777216, "storageEngines" : [ "devnull", "ephemeralForTest", "mmapv1", "wiredTiger" ], "ok" : 1 } test2@local:33599(3.3.5-21-g046441d) > asya commented on Tue, 26 Apr 2016 15:35:13 +0000: I had no other indexes when it happened. I did add some indexes to see if that would change behavior but it didn't. I will try to figure out the difference between my reproducer and yours. max.hirschhorn@10gen.com commented on Tue, 26 Apr 2016 04:12:20 +0000: asya, I put together the following test script based on the documents inserted and aggregation pipelines shown above. However, I cannot reproduce the issue you described when running MongoDB version 3.3.5-23-g632dfe3. Are you able to reliably reproduce this issue, and if so, are there any additional indexes that you have created on either the test.a or test.b collections? (function() { db.dropDatabase(); assert.writeOK(db.a.insert([ {_id: ObjectId("571ea21117443c201ac21e0c"), a: "a", b: "b1"}, {_id: ObjectId("571ea21717443c201ac21e0d"), a: "a", b: "b2"}, {_id: ObjectId("571ea22117443c201ac21e0e"), a: "a3", b: "b3"}, ])); assert.writeOK(db.b.insert([ {_id: ObjectId("571ea22c17443c201ac21e0f"), a: "a", b: "b"}, {_id: ObjectId("571ea23117443c201ac21e10"), a: "a", b: "c"}, ])); var optimized = db.a.aggregate([ {$lookup: { from: "b", localField: "a", foreignField: "a", as:"x" }}, {$unwind: "$x"}, {$match: {"x.b": "b"}}, ]).toArray(); var nonOptimized = db.a.aggregate([ {$lookup: { from: "b", localField: "a", foreignField: "a", as:"x" }}, {$unwind: "$x"}, {$limit: 1000}, {$match: {"x.b": "b"}}, ]).toArray(); assert.eq(optimized, nonOptimized); })(); asya commented on Mon, 25 Apr 2016 23:12:23 +0000: "version" : "3.3.5-21-g046441d", "gitVersion" : "046441de98dd72689397a4a06f8a68ef58a4c226", asya commented on Mon, 25 Apr 2016 23:11:22 +0000: > db.a.find() { "_id" : ObjectId("571ea21117443c201ac21e0c"), "a" : "a", "b" : "b1" } { "_id" : ObjectId("571ea21717443c201ac21e0d"), "a" : "a", "b" : "b2" } { "_id" : ObjectId("571ea22117443c201ac21e0e"), "a" : "a3", "b" : "b3" } > db.b.find() { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" } { "_id" : ObjectId("571ea23117443c201ac21e10"), "a" : "a", "b" : "c" } > db.a.aggregate({$lookup:{from:"b",localField:"a",foreignField:"a",as:"x"}}) { "_id" : ObjectId("571ea21117443c201ac21e0c"), "a" : "a", "b" : "b1", "x" : [ { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" }, { "_id" : ObjectId("571ea23117443c201ac21e10"), "a" : "a", "b" : "c" } ] } { "_id" : ObjectId("571ea21717443c201ac21e0d"), "a" : "a", "b" : "b2", "x" : [ { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" }, { "_id" : ObjectId("571ea23117443c201ac21e10"), "a" : "a", "b" : "c" } ] } { "_id" : ObjectId("571ea22117443c201ac21e0e"), "a" : "a3", "b" : "b3", "x" : [ ] } > db.a.aggregate({$lookup:{from:"b",localField:"a",foreignField:"a",as:"x"}},{$match:{"x.b":"b"}}) { "_id" : ObjectId("571ea21117443c201ac21e0c"), "a" : "a", "b" : "b1", "x" : [ { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" }, { "_id" : ObjectId("571ea23117443c201ac21e10"), "a" : "a", "b" : "c" } ] } { "_id" : ObjectId("571ea21717443c201ac21e0d"), "a" : "a", "b" : "b2", "x" : [ { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" }, { "_id" : ObjectId("571ea23117443c201ac21e10"), "a" : "a", "b" : "c" } ] } > db.a.aggregate({$lookup:{from:"b",localField:"a",foreignField:"a",as:"x"}},{$match:{"x.b":"b"}},{$unwind:"$x"}) { "_id" : ObjectId("571ea21117443c201ac21e0c"), "a" : "a", "b" : "b1", "x" : { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" } } { "_id" : ObjectId("571ea21117443c201ac21e0c"), "a" : "a", "b" : "b1", "x" : { "_id" : ObjectId("571ea23117443c201ac21e10"), "a" : "a", "b" : "c" } } { "_id" : ObjectId("571ea21717443c201ac21e0d"), "a" : "a", "b" : "b2", "x" : { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" } } { "_id" : ObjectId("571ea21717443c201ac21e0d"), "a" : "a", "b" : "b2", "x" : { "_id" : ObjectId("571ea23117443c201ac21e10"), "a" : "a", "b" : "c" } } > db.a.aggregate({$lookup:{from:"b",localField:"a",foreignField:"a",as:"x"}},{$unwind:"$x"},{$match:{"x.b":"b"}}) // no results! add $limit to block optimization of $match > db.a.aggregate({$lookup:{from:"b",localField:"a",foreignField:"a",as:"x"}},{$unwind:"$x"},{$limit:1000},{$match:{"x.b":"b"}}) { "_id" : ObjectId("571ea21117443c201ac21e0c"), "a" : "a", "b" : "b1", "x" : { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" } } { "_id" : ObjectId("571ea21717443c201ac21e0d"), "a" : "a", "b" : "b2", "x" : { "_id" : ObjectId("571ea22c17443c201ac21e0f"), "a" : "a", "b" : "b" } } Increasing debugging makes it seem like the extra $match condition may be applied to source collection ("a" rather than "b" but it's hard to tell). Explain for problematic query db.a.aggregate([{$lookup:{from:"b",localField:"a",foreignField:"a",as:"x"}},{$unwind:"$x"},{$match:{"x.b":"b"}}],{explain:true}) { "waitedMS" : NumberLong(0), "stages" : [ { "$cursor" : { "query" : { }, "queryPlanner" : { "plannerVersion" : 1, "namespace" : "test.a", "indexFilterSet" : false, "parsedQuery" : { }, "winningPlan" : { "stage" : "COLLSCAN", "direction" : "forward" }, "rejectedPlans" : [ ] } } }, { "$lookup" : { "from" : "b", "as" : "x", "localField" : "a", "foreignField" : "a", "unwinding" : { "preserveNullAndEmptyArrays" : false }, "matching" : { "b" : { "$eq" : "b" } } } } ], "ok" : 1 } Logs show: Canonical query: ns=test.aTree: b == "b" Sort: {} Proj: {} ============================= 2016-04-25T16:12:50.793-0700 D QUERY [conn1] Index 0 is kp: { _id: 1 } unique name: '_id_' io: { v: 1, key: { _id: 1 }, name: "_id_", ns: "test.a" } 2016-04-25T16:12:50.793-0700 D QUERY [conn1] Predicate over field 'b' 2016-04-25T16:12:50.793-0700 D QUERY [conn1] Rated tree: b == "b" || First: notFirst: full path: b 2016-04-25T16:12:50.793-0700 D QUERY [conn1] Planner: outputted 0 indexed solutions. 2016-04-25T16:12:50.793-0700 D QUERY [conn1] Planner: outputting a collscan: COLLSCAN ---ns = test.a ---filter = b == "b" ---fetched = 1 ---sortedByDiskLoc = 0 ---getSort = [] 2016-04-25T16:12:50.793-0700 D QUERY [conn1] Only one plan is available; it will be run but will not be cached. query: { b: { $eq: "b" } } sort: {} projection: {}, planSummary: COLLSCAN which seems to be applying the condition to the wrong collection (a instead of b)

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