MongoDB efficient way to search for objectId field with null using Index? - mongodb

Question

I am trying to find an efficient way to search for items in which specific field is null.
In the MongoDB, I have folder schema which has parent field of its parent folder's ObjectId and parent is indexed. For root folders, parent fields are null.
When I try to find all the root folders with parent:null, explain displays indexOnly: false
db.folders.find({parent: null}, {parent: 1, _id: 0}).explain()
{
"cursor" : "BtreeCursor parent_1",
"isMultiKey" : false,
"n" : 126,
"nscannedObjects" : 126,
"nscanned" : 126,
"nscannedObjectsAllPlans" : 126,
"nscannedAllPlans" : 126,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 1,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"parent" : [
[
null,
null
]
]
},
"server" : "c268.candidate.36:10268",
"filterSet" : false,
"stats" : {
"type" : "PROJECTION",
"works" : 128,
"yields" : 1,
"unyields" : 1,
"invalidates" : 0,
"advanced" : 126,
"needTime" : 0,
"needFetch" : 0,
"isEOF" : 1,
"children" : [
{
"type" : "KEEP_MUTATIONS",
"works" : 128,
"yields" : 1,
"unyields" : 1,
"invalidates" : 0,
"advanced" : 126,
"needTime" : 1,
"needFetch" : 0,
"isEOF" : 1,
"children" : [
{
"type" : "FETCH",
"works" : 128,
"yields" : 1,
"unyields" : 1,
"invalidates" : 0,
"advanced" : 126,
"needTime" : 1,
"needFetch" : 0,
"isEOF" : 1,
"alreadyHasObj" : 0,
"forcedFetches" : 0,
"matchTested" : 126,
"children" : [
{
"type" : "IXSCAN",
"works" : 127,
"yields" : 1,
"unyields" : 1,
"invalidates" : 0,
"advanced" : 126,
"needTime" : 1,
"needFetch" : 0,
"isEOF" : 1,
"keyPattern" : "{ parent: 1.0 }",
"isMultiKey" : 0,
"boundsVerbose" : "field #0['parent']: [null, null]",
"yieldMovedCursor" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0,
"matchTested" : 0,
"keysExamined" : 126,
"children" : []
}
]
}
]
}
]
}
}
I tried to use $type to find any ones which parent folder is not ObjectId. But still indexOnly:false.
db.folders.find({parent: {$ne: {$type: 7}}}, {parent: 1, _id: 0}).explain()
"indexOnly": false
Is there a way to search null value only using index? If not, is there a better value to store instead of null to be able to search with index?
Additional
example of root folder
{
"_id" : ObjectId("55a04a2d754971030059b7ad"),
"active" : true,
"modified" : ISODate("2016-02-05T22:30:08.053Z"),
"created" : ISODate("2015-07-10T22:41:49.009Z"),
"user" : ObjectId("54d3ae187a738c0300f59e61"),
"name" : "2nd Grade",
"parent" : null,
"clientModified" : ISODate("2016-02-05T22:30:07.872Z"),
"userCreated" : ISODate("2015-07-10T22:41:48.328Z"),
"ancestors" : [],
"__v" : 2
}
example of child folder
{
"_id" : ObjectId("56d0b4edb6f05e03009bcabc"),
"active" : true,
"modified" : ISODate("2016-02-26T20:26:21.328Z"),
"created" : ISODate("2016-02-26T20:26:21.328Z"),
"user" : ObjectId("54d3ae187a738c0300f59e61"),
"name" : "music",
"parent" : ObjectId("55a04a2d754971030059b7ad"),
"clientModified" : ISODate("2016-02-26T20:26:20.398Z"),
"userCreated" : ISODate("2016-02-26T20:26:20.398Z"),
"ancestors" : [
ObjectId("55a04a2d754971030059b7ad")
],
"__v" : 0
}
db.folders.getIndexes()
{
"0" : {
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "app29099188.folders"
},
"1" : {
"v" : 1,
"key" : {
"user" : 1,
"_fts" : "text",
"_ftsx" : 1
},
"name" : "user_1_name_text",
"ns" : "app29099188.folders",
"background" : true,
"safe" : null,
"weights" : {
"name" : 1
},
"default_language" : "english",
"language_override" : "language",
"textIndexVersion" : 2
},
"2" : {
"v" : 1,
"key" : {
"user" : 1,
"parent" : 1
},
"name" : "user_1_parent_1",
"ns" : "app29099188.folders",
"background" : true,
"safe" : null
},
"3" : {
"v" : 1,
"key" : {
"parent" : 1.0000000000000000
},
"name" : "parent_1",
"ns" : "app29099188.folders"
}
}

Answer 1

After comment - update:
The way to eliminate docScan is to have a value in parent field. It can be zeroed objectId or just "/" as a root.
db.satoko.insert({"test":"sdsf", parent: "/"})
db.satoko.insert({"test":"sds33f", parent: "/"})
db.satoko.insert({"parent":ObjectId("56d8b2879bd059e7247a6096"), "test":"sdsf"})
explain results:
db.satoko.find({parent:{$eq:"/"} }, {parent: 1, _id: 0}).explain("allPlansExec
ution")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.satoko",
"indexFilterSet" : false,
"parsedQuery" : {
"parent" : {
"$eq" : "/"
}
},
"winningPlan" : {
"stage" : "PROJECTION",
"transformBy" : {
"parent" : 1,
"_id" : 0
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"parent" : 1
},
"indexName" : "parent_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"parent" : [
"[\"/\", \"/\"]"
]
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 2,
"executionTimeMillis" : 0,
"totalKeysExamined" : 2,
"totalDocsExamined" : 0,
"executionStages" : {
"stage" : "PROJECTION",
"nReturned" : 2,
"executionTimeMillisEstimate" : 0,
"works" : 3,
"advanced" : 2,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"transformBy" : {
"parent" : 1,
"_id" : 0
},
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 2,
"executionTimeMillisEstimate" : 0,
"works" : 3,
"advanced" : 2,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"parent" : 1
},
"indexName" : "parent_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"parent" : [
"[\"/\", \"/\"]"
]
},
"keysExamined" : 2,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
},
"allPlansExecution" : [ ]
},
"serverInfo" : {
"host" : "greg",
"port" : 27017,
"version" : "3.2.3",
"gitVersion" : "b326ba837cf6f49d65c2f85e1b70f6f31ece7937"
},
"ok" : 1
}