Sort of MongoDB using index - mongodb

Question

Below is the status of the index status of the collection that I want to let you see.
> db.histories.getIndexes();
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "development.histories"
},
{
"v" : 1,
"key" : {
"hoge_id" : 1,
"created_at" : 1
},
"name" : "hoge_id_1_created_at_1",
"ns" : "development.histories",
"background" : true
},
{
"v" : 1,
"key" : {
"created_at" : 1
},
"name" : "created_at_1",
"ns" : "development.histories",
"background" : true
}
]
And, I executed the following query.
> db.histories.find({hoge_id: ObjectId("5a5c171010ebfb1a2c901008")}).sort( { created_at: -1 } ).limit(1).explain("executionStats");
And, the result was below.
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "development.histories",
"indexFilterSet" : false,
"parsedQuery" : {
"hoge_id" : {
"$eq" : ObjectId("5a5c171010ebfb1a2c901008")
}
},
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 1,
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"hoge_id" : 1,
"created_at" : 1
},
"indexName" : "hoge_id_1_created_at_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "backward",
"indexBounds" : {
"hoge_id" : [
"[ObjectId('5a5c171010ebfb1a2c901008'), ObjectId('5a5c171010ebfb1a2c901008')]"
],
"created_at" : [
"[MaxKey, MinKey]"
]
}
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1,
"executionTimeMillis" : 0,
"totalKeysExamined" : 1,
"totalDocsExamined" : 1,
"executionStages" : {
"stage" : "LIMIT",
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 2,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"limitAmount" : 1,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 1,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 1,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 1,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"hoge_id" : 1,
"created_at" : 1
},
"indexName" : "hoge_id_1_created_at_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "backward",
"indexBounds" : {
"hoge_id" : [
"[ObjectId('5a5c171010ebfb1a2c901008'), ObjectId('5a5c171010ebfb1a2c901008')]"
],
"created_at" : [
"[MaxKey, MinKey]"
]
},
"keysExamined" : 1,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
}
},
"serverInfo" : {
"host" : "b9cb1b8d1fc1",
"port" : 27017,
"version" : "3.2.18",
"gitVersion" : "4c1bae566c0c00f996a2feb16febf84936ecaf6f"
},
"ok" : 1
}
The result is fast, I guess it's due to creating index on created_at.
ref. "totalDocsExamined" : 1, "executionTimeMillis" : 0
Then, I did exection the following query. The difference of previous is the field used for sort.
> db.histories.find({hoge_id: ObjectId("5a5c171010ebfb1a2c901008")}).sort( { id: -1 } ).limit(1).explain("executionStats");
And, the result was below.
> db.histories.find({hoge_id: ObjectId("5a5c171010ebfb1a2c901008")}).sort( { id: -1 } ).limit(1).explain("executionStats");
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "development.histories",
"indexFilterSet" : false,
"parsedQuery" : {
"hoge_id" : {
"$eq" : ObjectId("5a5c171010ebfb1a2c901008")
}
},
"winningPlan" : {
"stage" : "SORT",
"sortPattern" : {
"id" : -1
},
"limitAmount" : 1,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"hoge_id" : 1,
"created_at" : 1
},
"indexName" : "hoge_id_1_created_at_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"hoge_id" : [
"[ObjectId('5a5c171010ebfb1a2c901008'), ObjectId('5a5c171010ebfb1a2c901008')]"
],
"created_at" : [
"[MinKey, MaxKey]"
]
}
}
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1,
"executionTimeMillis" : 1215,
"totalKeysExamined" : 1034353,
"totalDocsExamined" : 1034353,
"executionStages" : {
"stage" : "SORT",
"nReturned" : 1,
"executionTimeMillisEstimate" : 1120,
"works" : 1034357,
"advanced" : 1,
"needTime" : 1034355,
"needYield" : 0,
"saveState" : 8080,
"restoreState" : 8080,
"isEOF" : 1,
"invalidates" : 0,
"sortPattern" : {
"id" : -1
},
"memUsage" : 297,
"memLimit" : 33554432,
"limitAmount" : 1,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"nReturned" : 0,
"executionTimeMillisEstimate" : 950,
"works" : 1034355,
"advanced" : 0,
"needTime" : 1,
"needYield" : 0,
"saveState" : 8080,
"restoreState" : 8080,
"isEOF" : 1,
"invalidates" : 0,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 1034353,
"executionTimeMillisEstimate" : 650,
"works" : 1034354,
"advanced" : 1034353,
"needTime" : 0,
"needYield" : 0,
"saveState" : 8080,
"restoreState" : 8080,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 1034353,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 1034353,
"executionTimeMillisEstimate" : 330,
"works" : 1034354,
"advanced" : 1034353,
"needTime" : 0,
"needYield" : 0,
"saveState" : 8080,
"restoreState" : 8080,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"hoge_id" : 1,
"created_at" : 1
},
"indexName" : "hoge_id_1_created_at_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"hoge_id" : [
"[ObjectId('5a5c171010ebfb1a2c901008'), ObjectId('5a5c171010ebfb1a2c901008')]"
],
"created_at" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 1034353,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
}
}
},
"serverInfo" : {
"host" : "b9cb1b8d1fc1",
"port" : 27017,
"version" : "3.2.18",
"gitVersion" : "4c1bae566c0c00f996a2feb16febf84936ecaf6f"
},
"ok" : 1
}
>
The result is late this time.
ref. "totalDocsExamined" : 1034353, "executionTimeMillis" : 1215
About totalDocsExamined, That's all in all documents.
Regardress that id is enable for index as created_at, but, when it is sorted using id, the result is late?

Answer 1

For your 1st query:
db.histories.find({hoge_id: ObjectId("5a5c171010ebfb1a2c901008")}).sort( { created_at: -1 } ).limit(1).explain("executionStats");
MongoDB is optimizing the performance by using the compound index on hoge_id and created_at. It firstly looks at the hoge_id and then it uses the index of created_at to sort the query results. In this way, the sort operation can be very fast because of efficient usage of compound index.
However, for your 2nd query:
db.histories.find({hoge_id: ObjectId("5a5c171010ebfb1a2c901008")}).sort( { id: -1 } ).limit(1).explain("executionStats");
Since there is no compound index on hoge_id and id(you only have a single index on id), MongoDB is actually manually sorting results by id.
More info on sorting with compound index can be found here.