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Type:
Task
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Resolution: Won't Fix
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Priority:
Major - P3
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None
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Affects Version/s: None
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Component/s: None
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None
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Query Optimization
Here is the explain output for this aggregate command. That is there is FETCH although no fetching is necessary.
const cmdObj = {
"aggregate": "testColl",
"pipeline": [
{"$project": {"_id": 0, "key": {"$meta": "indexKey"}}},
{"$limit": 10000},
],
"allowDiskUse": true,
"cursor": {"batchSize": 101},
"collation": {"locale": "simple"},
"hint": {"a": 1}
};
{
"explainVersion" : "1",
"stages" : [
{
"$cursor" : {
"queryPlanner" : {
"namespace" : "testDb.testColl",
"indexFilterSet" : false,
"parsedQuery" : {
},
"queryHash" : "FCBE9F38",
"planCacheKey" : "FCBE9F38",
"maxIndexedOrSolutionsReached" : false,
"maxIndexedAndSolutionsReached" : false,
"maxScansToExplodeReached" : false,
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 10000,
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"a" : 1
},
"indexName" : "a_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"a" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"a" : [
"[MinKey, MaxKey]"
]
}
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 10000,
"executionTimeMillis" : 23,
"totalKeysExamined" : 10000,
"totalDocsExamined" : 10000,
"executionStages" : {
"stage" : "LIMIT",
"nReturned" : 10000,
"executionTimeMillisEstimate" : 3,
"works" : 10001,
"advanced" : 10000,
"needTime" : 0,
"needYield" : 0,
"saveState" : 11,
"restoreState" : 11,
"isEOF" : 1,
"limitAmount" : 10000,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 10000,
"executionTimeMillisEstimate" : 3,
"works" : 10000,
"advanced" : 10000,
"needTime" : 0,
"needYield" : 0,
"saveState" : 11,
"restoreState" : 11,
"isEOF" : 0,
"docsExamined" : 10000,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 10000,
"executionTimeMillisEstimate" : 2,
"works" : 10000,
"advanced" : 10000,
"needTime" : 0,
"needYield" : 0,
"saveState" : 11,
"restoreState" : 11,
"isEOF" : 0,
"keyPattern" : {
"a" : 1
},
"indexName" : "a_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"a" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"a" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 10000,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
},
"allPlansExecution" : [ ]
}
},
"nReturned" : NumberLong(10000),
"executionTimeMillisEstimate" : NumberLong(17)
},
{
"$project" : {
"key" : {
"$meta" : "indexKey"
},
"_id" : false
},
"nReturned" : NumberLong(10000),
"executionTimeMillisEstimate" : NumberLong(17)
}
],
"serverInfo" : {
"host" : "ip-",
"port" : 20040,
"version" : "7.1.0-alpha-1667-g39b59fc",
"gitVersion" : "39b59fc567cb4e69a948ebcf119a70bb98fa682d"
},
"serverParameters" : {
"internalQueryFacetBufferSizeBytes" : 104857600,
"internalQueryFacetMaxOutputDocSizeBytes" : 104857600,
"internalLookupStageIntermediateDocumentMaxSizeBytes" : 104857600,
"internalDocumentSourceGroupMaxMemoryBytes" : 104857600,
"internalQueryMaxBlockingSortMemoryUsageBytes" : 104857600,
"internalQueryProhibitBlockingMergeOnMongoS" : 0,
"internalQueryMaxAddToSetBytes" : 104857600,
"internalDocumentSourceSetWindowFieldsMaxMemoryBytes" : 104857600,
"internalQueryFrameworkControl" : "trySbeEngine"
},
"command" : {
"aggregate" : "testColl",
"pipeline" : [
{
"$project" : {
"_id" : 0,
"key" : {
"$meta" : "indexKey"
}
}
},
{
"$limit" : 10000
}
],
"allowDiskUse" : true,
"cursor" : {
"batchSize" : 101
},
"collation" : {
"locale" : "simple"
},
"hint" : {
"a" : 1
},
"$db" : "testDb"
},
"ok" : 1,
"$clusterTime" : {
"clusterTime" : Timestamp(1687800460, 19000),
"signature" : {
"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
"keyId" : NumberLong(0)
}
},
"operationTime" : Timestamp(1687800460, 19000)
}