MongoDB executionStats understanding - mongodb

Question

I am currently trying to integrate indexes in one of my collection. All of my tests worked and I can really see the improvments (number of scanned document etc) however one of my result is a bit weird (or maybe I did not understand it well).
Could you please explain me?
I have a collection of owner/holder/product (here is an example and by the same the concerned document):
{
"_id": ObjectId("59ef7c84f545c8278c8cd967"),
"owner": "59ef7c83f545c8278c8cd965",
"holder": "59ef7c84f545c8278c8cd967",
"product": 1,
"date" : ISODate("2017-10-24T17:46:44.367Z")
}
Here are the indexes:
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "mydb.relation"
},
{
"v" : 1,
"key" : {
"owner" : 1
},
"name" : "onlyOwner",
"ns" : "mydb.relation"
},
{
"v" : 1,
"key" : {
"holder" : 1
},
"name" : "onlyHolder",
"ns" : "mydb.relation"
},
{
"v" : 1,
"key" : {
"owner" : 1,
"holder" : 1
},
"name" : "ownerAndHolder",
"ns" : "mydb.relation"
},
{
"v" : 1,
"key" : {
"owner" : 1,
"product" : 1
},
"name" : "ownerAndProduct",
"ns" : "mydb.relation"
}
I executed the following query:
db.relation.find({ $and : [{"owner" : "59ef7c83f545c8278c8cd965"}, {"holder" : "59ef7c84f545c8278c8cd967"}]})
And my relation is well returned (the one mentionned earlier). So I decided to check executionStats to see the status and here is the result:
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "mydb.relation",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"holder" : {
"$eq" : "59ef7c84f545c8278c8cd967"
}
},
{
"owner" : {
"$eq" : "59ef7c83f545c8278c8cd965"
}
}
]
},
"winningPlan" : {
"stage" : "FETCH",
"filter" : {
"holder" : {
"$eq" : "59ef7c84f545c8278c8cd967"
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"owner" : 1,
"product" : 1
},
"indexName" : "ownerAndProduct",
"isMultiKey" : false,
"multiKeyPaths" : {
"owner" : [ ],
"product" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"owner" : [
"[\"59ef7c83f545c8278c8cd965\", \"59ef7c83f545c8278c8cd965\"]"
],
"product" : [
"[MinKey, MaxKey]"
]
}
}
},
"rejectedPlans" : [
{
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"owner" : 1,
"holder" : 1
},
"indexName" : "ownerAndHolder",
"isMultiKey" : false,
"multiKeyPaths" : {
"owner" : [ ],
"holder" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"owner" : [
"[\"59ef7c83f545c8278c8cd965\", \"59ef7c83f545c8278c8cd
],
"holder" : [
"[\"59ef7c84f545c8278c8cd967\", \"59ef7c84f545c8278c8cd
]
}
}
},
{
"stage" : "FETCH",
"filter" : {
"holder" : {
"$eq" : "59ef7c84f545c8278c8cd967"
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"owner" : 1
},
"indexName" : "onlyOwner",
"isMultiKey" : false,
"multiKeyPaths" : {
"owner" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"owner" : [
"[\"59ef7c83f545c8278c8cd965\", \"59ef7c83f545c8278c8cd
]
}
}
},
{
"stage" : "FETCH",
"filter" : {
"owner" : {
"$eq" : "59ef7c83f545c8278c8cd965"
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"holder" : 1
},
"indexName" : "onlyHolder",
"isMultiKey" : false,
"multiKeyPaths" : {
"holder" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"holder" : [
"[\"59ef7c84f545c8278c8cd967\", \"59ef7c84f545c8278c8cd
]
}
}
}
]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1,
"executionTimeMillis" : 0,
"totalKeysExamined" : 1,
"totalDocsExamined" : 1,
"executionStages" : {
"stage" : "FETCH",
"filter" : {
"holder" : {
"$eq" : "59ef7c84f545c8278c8cd967"
}
},
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 3,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 1,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 2,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"owner" : 1,
"product" : 1
},
"indexName" : "ownerAndProduct",
"isMultiKey" : false,
"multiKeyPaths" : {
"owner" : [ ],
"product" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"owner" : [
"[\"59ef7c83f545c8278c8cd965\", \"59ef7c83f545c8278c8cd965\"]"
],
"product" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 1,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
"serverInfo" : {
"host" : "NCEL15605",
"port" : 27017,
"version" : "3.4.6",
"gitVersion" : "c55eb86ef46ee7aede3b1e2a5d184a7df4bfb5b5"
},
"ok" : 1
}
So I understand that onlyHolder onlyOwner are rejected but I don't understand why ownerAndHolder is rejected and ownerAndProduct is used as I am not using it as variable.
Can someone help me to understand this result (the response is correct from the find, however this is the choice of MongoDB I did not understand) ?
Thanks in advance
Regards

Answer 1

Look at these two fields:
"nReturned" : 1,
"totalDocsExamined" : 1,
It means that Mongo had to examine only one document to return one. Then the chosen query plan, although counter-intuitively, achieved the best result possible. It looks like at least one other plan could have achieved the same result (the owner holder index). In such cases Mongo is free to choose a plan.
If you want to know the details of query optimizations read this material.