mongodb 3.2.7 sorting not using compound index - mongodb

Question

I'm using mongodb 3.2.7, docs like:
{
"finished" : true,
"buildNo": 1,
"tryTimes" : 1,
"createdTime" : ISODate("2019-05-16T19:00:50.604+08:00"),
"modifiedTime" : ISODate("2019-12-27T18:33:25.682+08:00"),
}
I created my index using:
db.getCollection('builds').createIndex({"createdTime": 1})
db.getCollection('builds').createIndex({"tryTimes": 1, "createdTime": -1})
and my query is :
db.getCollection('builds').find({
"createdTime": {$lte: new Date("2020-04-15T00:00:00.000Z"), $gte: new Date("2020-04-01T00:00:00.000Z")},
"buildNo": 1,
"finished": {$ne: true}
})
.sort({"tryTimes": 1, "createdTime": -1})
.limit(200)
But this query doesn't use the index I created before. the explain() output:
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "builds",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"buildNo" : {
"$eq" : 1.0
}
},
{
"createdTime" : {
"$lte" : ISODate("2020-04-15T08:00:00.000+08:00")
}
},
{
"createdTime" : {
"$gte" : ISODate("2020-04-01T08:00:00.000+08:00")
}
},
{
"$not" : {
"finished" : {
"$eq" : true
}
}
}
]
},
"winningPlan" : {
"stage" : "SORT",
"sortPattern" : {
"tryTimes" : 1.0,
"createdTime" : -1.0
},
"limitAmount" : 200,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"buildNo" : {
"$eq" : 1.0
}
},
{
"$not" : {
"finished" : {
"$eq" : true
}
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"createdTime" : 1.0
},
"indexName" : "createdTime_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"createdTime" : [
"[new Date(1585699200000), new Date(1586908800000)]"
]
}
}
}
}
},
"rejectedPlans" : [
{
"stage" : "LIMIT",
"limitAmount" : 200,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"buildNo" : {
"$eq" : 1.0
}
},
{
"createdTime" : {
"$lte" : ISODate("2020-04-15T08:00:00.000+08:00")
}
},
{
"createdTime" : {
"$gte" : ISODate("2020-04-01T08:00:00.000+08:00")
}
},
{
"$not" : {
"finished" : {
"$eq" : true
}
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"tryTimes" : 1.0,
"createdTime" : -1.0
},
"indexName" : "tryTimes_1_createdTime_-1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"tryTimes" : [
"[MinKey, MaxKey]"
],
"createdTime" : [
"[MaxKey, MinKey]"
]
}
}
}
}
]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 200,
"executionTimeMillis" : 352,
"totalKeysExamined" : 48256,
"totalDocsExamined" : 48256,
"executionStages" : {
"stage" : "SORT",
"nReturned" : 200,
"executionTimeMillisEstimate" : 200,
"works" : 48459,
"advanced" : 200,
"needTime" : 48258,
"needYield" : 0,
"saveState" : 757,
"restoreState" : 757,
"isEOF" : 1,
"invalidates" : 0,
"sortPattern" : {
"tryTimes" : 1.0,
"createdTime" : -1.0
},
"memUsage" : 198706,
"memLimit" : 33554432,
"limitAmount" : 200,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"nReturned" : 0,
"executionTimeMillisEstimate" : 180,
"works" : 48258,
"advanced" : 0,
"needTime" : 24539,
"needYield" : 0,
"saveState" : 757,
"restoreState" : 757,
"isEOF" : 1,
"invalidates" : 0,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"buildNo" : {
"$eq" : 1.0
}
},
{
"$not" : {
"finished" : {
"$eq" : true
}
}
}
]
},
"nReturned" : 23718,
"executionTimeMillisEstimate" : 160,
"works" : 48257,
"advanced" : 23718,
"needTime" : 24538,
"needYield" : 0,
"saveState" : 757,
"restoreState" : 757,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 48256,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 48256,
"executionTimeMillisEstimate" : 20,
"works" : 48257,
"advanced" : 48256,
"needTime" : 0,
"needYield" : 0,
"saveState" : 757,
"restoreState" : 757,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"createdTime" : 1.0
},
"indexName" : "createdTime_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"createdTime" : [
"[new Date(1585699200000), new Date(1586908800000)]"
]
},
"keysExamined" : 48256,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
}
}
},
"serverInfo" : {
"host" : "...",
"port" : 9999,
"version" : "3.2.7",
"gitVersion" : "4249c1d2b5999ebbf1fdf3bc0e0e3b3ff5c0aaf2"
},
"ok" : 1.0}
even if I try to simplify the query to
db.getCollection('builds')
.find({ "buildNo": 1, "finished": true })
.sort({"tryTimes": 1, "createdTime": -1})
.limit(200)
, the index still doesn't work.
The wierd things is: I use the same index and query on another mongondb ver. 3.4.14.3. The index works...
Any ideas is welcomed and appreciated.
Thanks a lot!

Answer 1

If a query can be satisfied by multiple indexes (satisfied is used losely as Mongo actually chooses all possibly relevant indexes) defined in the collection, MongoDB will then test all the applicable indexes in parallel (meaning Mongo's performs a "race"). The first index that can returns 101 results will be selected by the query planner.
Meaning that for that certain query your using the "wrong" index wins.
What can you do?:
You can use $hint, hint basically forces Mongo to use a specific index, however Mongo this is not recommended because if changes occur Mongo will not adapt to those.
Like so:
db.getCollection('builds')
.find({ "buildNo": 1, "finished": true })
.sort({"tryTimes": 1, "createdTime": -1})
.limit(200)
.hint({tryTimes: 1, createdTime: -1})