mongodb index find index sort - mongodb

Question

I would like to ask about mongodb indexes. Can I use a different index in the find and the sort. By example I have two indexes:
(a:-1)
(b:1,c:1)
What indexes uses this sentence?
({a:[$gt30}},{a:[$lt50}}]}.sort({c:1})

Answer 1

Can I use a different index in the find and the sort.
After reading some more into this you will see at the bottom of the documentation page on index intersectioning: http://docs.mongodb.org/manual/core/index-intersection/#index-intersection-and-sort
Index intersection does not apply when the sort() operation requires an index completely separate from the query predicate.
So no, even if ypou created an index of {c:1} it could not be used independantly to intersect {a:1}
What indexes uses this sentence?
In this case only {a:1} will be used.

Answer 2

Creating an index on a single field called Single Field Index.
Creating multiple Single Field indexes to boost your query and the sort performance won't help much!. You should use Compound Indexes instead.
Check the documentation on MongoDB: https://docs.mongodb.com/manual/core/index-compound/
If you want to learn how to index your fields and how to measure the performance of your queries.
And Check this tutorial on Youtube: https://dplink.app/nxLgvk7lR

Answer 3

Suppose I am having persons collection having documents like below :
{
dob:
{ age : 50} ,
gender : "male" ,
phone : ""
}
Now i create indexes as below .
1 : db.persons.createIndex({"dob.age" : -1})
2 : db.persons.createIndex({phone : 1 , gender : 1})
Now If i execute below query like yours
db.persons.explain("executionStats").find({$and : [ {"dob.age" : {$lt : 50} } ,
{"dob.age" : {$gt : 30} } ] } ).sort({gender : 1 })
I will get below execution stats :
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "college.persons",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"dob.age" : {
"$lt" : 50
}
},
{
"dob.age" : {
"$gt" : 30
}
}
]
},
"queryHash" : "22FEA299",
"planCacheKey" : "5E8F38C1",
"winningPlan" : {
"stage" : "SORT",
"sortPattern" : {
"phone" : 1
},
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"dob.age" : -1
},
"indexName" : "dob.age_-1",
"isMultiKey" : false,
"multiKeyPaths" : {
"dob.age" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"dob.age" : [
"(50.0, 30.0)"
]
}
}
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1734,
"executionTimeMillis" : 10,
"totalKeysExamined" : 1734,
"totalDocsExamined" : 1734,
"executionStages" : {
"stage" : "SORT",
"nReturned" : 1734,
"executionTimeMillisEstimate" : 0,
"works" : 3471,
"advanced" : 1734,
"needTime" : 1736,
"needYield" : 0,
"saveState" : 27,
"restoreState" : 27,
"isEOF" : 1,
"sortPattern" : {
"phone" : 1
},
"memUsage" : 1914799,
"memLimit" : 33554432,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"nReturned" : 1734,
"executionTimeMillisEstimate" : 0,
"works" : 1736,
"advanced" : 1734,
"needTime" : 1,
"needYield" : 0,
"saveState" : 27,
"restoreState" : 27,
"isEOF" : 1,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 1734,
"executionTimeMillisEstimate" : 0,
"works" : 1735,
"advanced" : 1734,
"needTime" : 0,
"needYield" : 0,
"saveState" : 27,
"restoreState" : 27,
"isEOF" : 1,
"docsExamined" : 1734,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 1734,
"executionTimeMillisEstimate" : 0,
"works" : 1735,
"advanced" : 1734,
"needTime" : 0,
"needYield" : 0,
"saveState" : 27,
"restoreState" : 27,
"isEOF" : 1,
"keyPattern" : {
"dob.age" : -1
},
"indexName" : "dob.age_-1",
"isMultiKey" : false,
"multiKeyPaths" : {
"dob.age" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"dob.age" : [
"(50.0, 30.0)"
]
},
"keysExamined" : 1734,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
}
}
},
"serverInfo" : {
"host" : "RGGYSLT-0483",
"port" : 27017,
"version" : "4.2.0",
"gitVersion" : "a4b751dcf51dd249c5865812b390cfd1c0129c30"
},
"ok" : 1
}
This Means Data was fetched with IXScan on single field first and then sorted on
third field separately .
But the moment i change the query to sort on fields for which the index is already created things change . Now {"dob.age" : -1} index gets rejected .
In Mongo db Winning plan is the one for which 100 docs can be fetched early and Mongo db caches that plan for a query . Now this cache will be purged after 1000 docs insertions , index rebuild , server restarts or new index insertions.
Hence which index will be used depends upon winning plans .
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "college.persons",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"dob.age" : {
"$lt" : 50
}
},
{
"dob.age" : {
"$gt" : 30
}
}
]
},
"queryHash" : "DA8248FA",
"planCacheKey" : "E779554F",
"winningPlan" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"dob.age" : {
"$lt" : 50
}
},
{
"dob.age" : {
"$gt" : 30
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"gender" : 1,
"phone" : 1
},
"indexName" : "gender_1_phone_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"gender" : [ ],
"phone" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"gender" : [
"[MinKey, MaxKey]"
],
"phone" : [
"[MinKey, MaxKey]"
]
}
}
},
"rejectedPlans" : [
{
"stage" : "SORT",
"sortPattern" : {
"gender" : 1,
"phone" : 1
},
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"dob.age" : -1
},
"indexName" : "dob.age_-1",
"isMultiKey" : false,
"multiKeyPaths" : {
"dob.age" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"dob.age" : [
"(50.0, 30.0)"
]
}
}
}
}
}
]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1734,
"executionTimeMillis" : 12,
"totalKeysExamined" : 5002,
"totalDocsExamined" : 5002,
"executionStages" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"dob.age" : {
"$lt" : 50
}
},
{
"dob.age" : {
"$gt" : 30
}
}
]
},
"nReturned" : 1734,
"executionTimeMillisEstimate" : 0,
"works" : 5003,
"advanced" : 1734,
"needTime" : 3268,
"needYield" : 0,
"saveState" : 41,
"restoreState" : 41,
"isEOF" : 1,
"docsExamined" : 5002,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 5002,
"executionTimeMillisEstimate" : 0,
"works" : 5003,
"advanced" : 5002,
"needTime" : 0,
"needYield" : 0,
"saveState" : 41,
"restoreState" : 41,
"isEOF" : 1,
"keyPattern" : {
"gender" : 1,
"phone" : 1
},
"indexName" : "gender_1_phone_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"gender" : [ ],
"phone" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"gender" : [
"[MinKey, MaxKey]"
],
"phone" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 5002,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
},
"serverInfo" : {
"host" : "RGGYSLT-0483",
"port" : 27017,
"version" : "4.2.0",
"gitVersion" : "a4b751dcf51dd249c5865812b390cfd1c0129c30"
},
"ok" : 1
}