Index management with MongoDB - mongodb

Question

I have several questions about the way mongoDb choose the index which will be more efficient to retrieve datas.
I have the following query :
db.getCollection('myCollection').find({
"TenantId":"1a1a1a1a",
"ContractId": 1111,
"Lists":1111,
"Email":{"$exists":true,"$nin":["",null]},
"Optouts.Media":{"$nin":["EMAIL",null]},
"Deleted":false
})
This query is used in a mapReduce which failed. When I run it with count(), I'll get the result of 290000 in 47s.
So, to understand what happen, I execute the query again with explain mode. And indeed, the index used seems to don't be the best (TenantId_1_ContractId_1_GUID_1_Deleted_1).
I re-run the same query with hint(TenantId_1_ContractId_1_Lists_1_Deleted_1) and this time, I'll get result in 2 sec.
How mongo select the index to retrieve datas?
Good to know:
For the current contractId I've 1,6 million entry.
When filter on the lists, I've got 700 000 entries
In documents, lists is an array of values
I have retrieve index planExecution to try to get more informations, but I can't figure out. See below result of explain('allPlansExecution').
{
"queryPlanner" : {
"mongosPlannerVersion" : 1,
"winningPlan" : {
"stage" : "SINGLE_SHARD",
"shards" : [
{
"shardName" : "rs1",
"connectionString" : "...",
"serverInfo" : {...},
"plannerVersion" : 1,
"namespace" : "",
"indexFilterSet" : false,
"parsedQuery" : {...},
"winningPlan" : {
"stage" : "FETCH",
"filter" : {...},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"TenantId" : 1.0,
"ContractId" : 1.0,
"GUID" : 1.0,
"Deleted" : 1.0
},
"indexName" : "TenantId_1_ContractId_1_GUID_1_Deleted_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"TenantId" : [],
"ContractId" : [],
"GUID" : [],
"Deleted" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {...}
}
},
"rejectedPlans" : [
{
"stage" : "FETCH",
"filter" : {...},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"TenantId" : 1.0,
"ContractId" : 1.0,
"Lists" : 1.0,
"Deleted" : 1.0
},
"indexName" : "TenantId_1_ContractId_1_Lists_1_Deleted_1",
"isMultiKey" : true,
"multiKeyPaths" : {
"TenantId" : [],
"ContractId" : [],
"Lists" : [
"Lists"
],
"Deleted" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {...}
}
},
{...}
]
}
]
}
},
"executionStats" : {...},
"allPlansExecution" : [
undefined,
{
"shardName" : "rs1",
"allPlans" : [
{
"nReturned" : 0,
"executionTimeMillisEstimate" : 51,
"totalKeysExamined" : 10618,
"totalDocsExamined" : 10618,
"executionStages" : {
"stage" : "FETCH",
"filter" : {...},
"nReturned" : 0,
"executionTimeMillisEstimate" : 51,
"works" : 10618,
"advanced" : 0,
"needTime" : 10618,
"needYield" : 0,
"saveState" : 664,
"restoreState" : 664,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 10618,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 10618,
"executionTimeMillisEstimate" : 20,
"works" : 10618,
"advanced" : 10618,
"needTime" : 0,
"needYield" : 0,
"saveState" : 664,
"restoreState" : 664,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"TenantId" : 1.0,
"ContractId" : 1.0,
"Lists" : 1.0,
"Deleted" : 1.0
},
"indexName" : "TenantId_1_ContractId_1_Lists_1_Deleted_1",
"isMultiKey" : true,
"multiKeyPaths" : {
"TenantId" : [],
"ContractId" : [],
"Lists" : [
"Lists"
],
"Deleted" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {...},
"keysExamined" : 10618,
"seeks" : 1,
"dupsTested" : 10618,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
{
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"totalKeysExamined" : 10618,
"totalDocsExamined" : 10616,
"executionStages" : {
"stage" : "FETCH",
"filter" : {...},
"nReturned" : 101,
"executionTimeMillisEstimate" : 0,
"works" : 10618,
"advanced" : 101,
"needTime" : 10517,
"needYield" : 0,
"saveState" : 664,
"restoreState" : 664,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 10616,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 10616,
"executionTimeMillisEstimate" : 0,
"works" : 10618,
"advanced" : 10616,
"needTime" : 2,
"needYield" : 0,
"saveState" : 664,
"restoreState" : 664,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"TenantId" : 1.0,
"ContractId" : 1.0,
"GUID" : 1.0,
"Deleted" : 1.0
},
"indexName" : "TenantId_1_ContractId_1_GUID_1_Deleted_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"TenantId" : [],
"ContractId" : [],
"GUID" : [],
"Deleted" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {...},
"keysExamined" : 10618,
"seeks" : 3,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
}
]
}
]
}
}
I have remove some lines {...}, if you want more details, don't hesitate to ask me.
Thanks a lot for your help!