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