MongoDB uses wrong index, even after clearing the query plan cache - mongodb

Question

I have the following command:
db.mycol.find({sourceId:ObjectId("596bac5a6f473e1a042bFFFF"),myFlag:false}).count()
It was taking a lot of time.
So I decided to add an index { "sourceId" : -1, "myFlag" : -1 }
MongoDB still kept taking the same time.
When I ran explain() on the find command, I noticed that mongoDB still kept using my old indexes.
db.mycol.find({sourceId:ObjectId("596bac5a6f473e1a042bFFFF"),myFlag:false}).explain(true)
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "mydb.mycol",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"sourceId" : {
"$eq" : ObjectId("596bac5a6f473e1a042bFFFF")
}
},
{
"myFlag" : {
"$eq" : false
}
}
]
},
"winningPlan" : {
"stage" : "FETCH",
"filter" : {
"myFlag" : {
"$eq" : false
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"sourceId" : 1
},
"indexName" : "sourceId_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"sourceId" : [
"[ObjectId('596bac5a6f473e1a042bFFFF'), ObjectId('596bac5a6f473e1a042bFFFF')]"
]
}
}
},
"rejectedPlans" : [
{
"stage" : "FETCH",
"filter" : {
"sourceId" : {
"$eq" : ObjectId("596bac5a6f473e1a042bFFFF")
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"myFlag" : -1
},
"indexName" : "myFlag_-1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"myFlag" : [
"[false, false]"
]
}
}
},
{
"stage" : "FETCH",
"filter" : {
"myFlag" : {
"$eq" : false
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"sourceId" : -1,
"timestamp" : -1
},
"indexName" : "sourceId_-1_timestamp_-1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"sourceId" : [
"[ObjectId('596bac5a6f473e1a042bFFFF'), ObjectId('596bac5a6f473e1a042bFFFF')]"
],
"timestamp" : [
"[MaxKey, MinKey]"
]
}
}
},
{
"stage" : "FETCH",
"filter" : {
"myFlag" : {
"$eq" : false
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"sourceId" : -1,
"timestamp" : 1
},
"indexName" : "sourceId_-1_timestamp_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"sourceId" : [
"[ObjectId('596bac5a6f473e1a042bFFFF'), ObjectId('596bac5a6f473e1a042bFFFF')]"
],
"timestamp" : [
"[MinKey, MaxKey]"
]
}
}
},
{
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"sourceId" : -1,
"myFlag" : -1
},
"indexName" : "sourceId_-1_myFlag_-1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"sourceId" : [
"[ObjectId('596bac5a6f473e1a042bFFFF'), ObjectId('596bac5a6f473e1a042bFFFF')]"
],
"myFlag" : [
"[false, false]"
]
}
}
}
]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 2074,
"executionTimeMillis" : 33936,
"totalKeysExamined" : 2074,
"totalDocsExamined" : 2074,
"executionStages" : {
"stage" : "FETCH",
"filter" : {
"myFlag" : {
"$eq" : false
}
},
"nReturned" : 2074,
"executionTimeMillisEstimate" : 31710,
"works" : 2075,
"advanced" : 2074,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1554,
"restoreState" : 1554,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 2074,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 2074,
"executionTimeMillisEstimate" : 0,
"works" : 2075,
"advanced" : 2074,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1554,
"restoreState" : 1554,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"sourceId" : 1
},
"indexName" : "sourceId_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"sourceId" : [
"[ObjectId('596bac5a6f473e1a042bFFFF'), ObjectId('596bac5a6f473e1a042bFFFF')]"
]
},
"keysExamined" : 2074,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
},
"allPlansExecution" : [
{
"nReturned" : 0,
"executionTimeMillisEstimate" : 1630,
"totalKeysExamined" : 101,
"totalDocsExamined" : 101,
"executionStages" : {
"stage" : "FETCH",
"filter" : {
"sourceId" : {
"$eq" : ObjectId("596bac5a6f473e1a042bFFFF")
}
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 1630,
"works" : 101,
"advanced" : 0,
"needTime" : 101,
"needYield" : 0,
"saveState" : 1554,
"restoreState" : 1554,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 101,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1554,
"restoreState" : 1554,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"myFlag" : -1
},
"indexName" : "myFlag_-1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"myFlag" : [
"[false, false]"
]
},
"keysExamined" : 101,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
{
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"totalKeysExamined" : 101,
"totalDocsExamined" : 101,
"executionStages" : {
"stage" : "FETCH",
"filter" : {
"myFlag" : {
"$eq" : false
}
},
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1554,
"restoreState" : 1554,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 101,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1554,
"restoreState" : 1554,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"sourceId" : -1,
"timestamp" : -1
},
"indexName" : "sourceId_-1_timestamp_-1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"sourceId" : [
"[ObjectId('596bac5a6f473e1a042bFFFF'), ObjectId('596bac5a6f473e1a042bFFFF')]"
],
"timestamp" : [
"[MaxKey, MinKey]"
]
},
"keysExamined" : 101,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
{
"nReturned" : 101,
"executionTimeMillisEstimate" : 10,
"totalKeysExamined" : 101,
"totalDocsExamined" : 101,
"executionStages" : {
"stage" : "FETCH",
"filter" : {
"myFlag" : {
"$eq" : false
}
},
"nReturned" : 101,
"executionTimeMillisEstimate" : 10,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1554,
"restoreState" : 1554,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 101,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1554,
"restoreState" : 1554,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"sourceId" : -1,
"timestamp" : 1
},
"indexName" : "sourceId_-1_timestamp_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"sourceId" : [
"[ObjectId('596bac5a6f473e1a042bFFFF'), ObjectId('596bac5a6f473e1a042bFFFF')]"
],
"timestamp" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 101,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
{
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"totalKeysExamined" : 101,
"totalDocsExamined" : 101,
"executionStages" : {
"stage" : "FETCH",
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1554,
"restoreState" : 1554,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 101,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1554,
"restoreState" : 1554,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"sourceId" : -1,
"myFlag" : -1
},
"indexName" : "sourceId_-1_myFlag_-1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"sourceId" : [
"[ObjectId('596bac5a6f473e1a042bFFFF'), ObjectId('596bac5a6f473e1a042bFFFF')]"
],
"myFlag" : [
"[false, false]"
]
},
"keysExamined" : 101,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
{
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"totalKeysExamined" : 101,
"totalDocsExamined" : 101,
"executionStages" : {
"stage" : "FETCH",
"filter" : {
"myFlag" : {
"$eq" : false
}
},
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 88,
"restoreState" : 88,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 101,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 101,
"needTime" : 0,
"needYield" : 0,
"saveState" : 88,
"restoreState" : 88,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"sourceId" : 1
},
"indexName" : "sourceId_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"sourceId" : [
"[ObjectId('596bac5a6f473e1a042bFFFF'), ObjectId('596bac5a6f473e1a042bFFFF')]"
]
},
"keysExamined" : 101,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
}
]
},
"serverInfo" : {
"host" : "gdm3d",
"port" : 27017,
"version" : "3.2.15",
"gitVersion" : "e11e3c1b9c9ce3f7b4a79493e16f5e4504e01140"
},
"ok" : 1
}
For some reason, it rejects the plan to use sourceId_-1_myFlag_-1 and goes with sourceId_1 and then filters myFlag document by document.
When I forced it to use the index, it returned the result immediately!
db.mycol.find({sourceId:ObjectId("596bac5a6f473e1a042bFFFF"),myFlag:false}).hint("sourceId_-1_myFlag_-1").count()
I read that periodically mongoDB runs all the plans and checks which one is faster, then uses that one in future.
I even tried restarting the mongodb, clearing query plan cache, restarting the computer. For some sourceIds with very less data, it did use the correct index automatically (I checked the index usage) but then it again started using the old ones.
I don't want to force it to use the index. How do I make sure it automatically does that?
Update:
It started using the correct one after some time. Atleast thats what the indexStats says.
However, the command still takes time, and when i give it hint, it gives output immediately.