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  1. Core Server
  2. SERVER-44309

Not full index use in aggregation pipelines

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      I could be wrong, but it seems to me that when running aggregation pipelines, indexes aren't used as well as for find queries with sort/limit. Example:

      I have documents like this:

      _id: ObjectId(...),
id: "someid",
status: "published",
version: 123123123,
foo: "bar"

      Then there's an index for 

      {id: 1, status: 1, version: 1} 

      If I run

       myCollection.find({id:'someid', status:'published'}).sort({version: -1}).limit(1).explain()

      ...I get the following inputStage indicating that this is done without document fetching.

      "inputStage" :
{ 
"stage" : "SORT", 
"sortPattern" : { "id" : 1.0, "status" : 1.0, "version" : -1.0 }, 
 "limitAmount" : 1.0, 
 "inputStage" : {
 "stage" : "SORT_KEY_GENERATOR", 
 "inputStage" :
{ 
"stage" : "IXSCAN", 
"keyPattern" : { "id" : 1.0, "status" : 1.0, "version" : 1.0 }, 
 "indexName" : "contentId_1_status_1_version_1", 
 "isMultiKey" : false, 
 "multiKeyPaths" : \{ "id" : [ ], "status" : [ ], "version" : [ ] }, 
 "isUnique" : false, 
 "isSparse" : false, 
 "isPartial" : false, 
 "indexVersion" : 2.0, 
 "direction" : "forward", 
 "indexBounds" : { "id" : [ "[\"someid\", \"someid\"]" ], "status" : [ "[\"published\", \"published\"]" ], "version" : [ "[MinKey, MaxKey]" ] }
 }
 }
 }

       However, If I do the same with aggregation pipeline. I.e. something like

       myCollection.explain().aggregate([
 
 {$match: {id: 'content12', status:'published'}},
 
 {$sort: {id: 1, status: 1, version: -1} },
 
 {$limit: 1}])

       ....then it seems like it's fetching more. I get 

      "inputStage" :
{ 
"stage" : "IXSCAN", 
"keyPattern" : { "contentId" : 1.0, "status" : 1.0, "version" : 1.0 }, 
 "indexName" : "contentId_1_status_1_version_1", 
 "isMultiKey" : false, 
 "multiKeyPaths" : \{ "contentId" : [ ], "status" : [ ], "version" : [ ] }, 
 "isUnique" : false, 
 "isSparse" : false, 
 "isPartial" : false, 
 "indexVersion" : 2.0, 
 "direction" : "forward", 
 "indexBounds" : { "contentId" : [ "[\"content12\", \"content12\"]" ], 
"status" : [ "[\"published\", \"published\"]" ], 
"version" : [ "[MinKey, MaxKey]" ] }
 }

       This seems like a miss. Shouldn't an aggregation pipeline like that be able to use the same index handling?

            Assignee:
            carl.champain@mongodb.com Carl Champain (Inactive)
            Reporter:
            staffan.eketorp@gmail.com Staffan Eketorp
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              Created:
              Updated:
              Resolved: