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  1. Core Server
  2. SERVER-10472

find(a).sort(b,_id) doesn't use index(a,b) to minimize scans

          Details

          • Type: Bug
          • Status: Open
          • Priority: Major - P3
          • Resolution: Unresolved
          • Affects Version/s: 2.4.4
          • Fix Version/s: Backlog
          • Component/s: Indexing, Querying
          • Labels:
          • Environment:
            OSX, MongoLabs
          • Operating System:
            ALL
          • Steps To Reproduce:
            Hide 

            db.temp.insert({_id:1, a:1, b:1})
            		 
            db.temp.insert({_id:2, a:1, b:2})
            		 
            db.temp.insert({_id:3, a:1, b:3})
            		 
            db.temp.insert({_id:4, a:1, b:4})
            		 
            db.temp.insert({_id:5, a:1, b:5})
            		 
            db.temp.insert({_id:6, a:1, b:6})
            		 
             
            		 
            db.temp.ensureIndex({a:1, b:1})
            		 
             
            		 
            // Scans 3 objects
            		 
            db.temp.find({a:1}).limit(3).sort({b:1}).explain()
            		 
             
            		 
            // Include _id as secondary sort field
            		 
            // Scans 6 objects.  Should only need to scan 4  (3 + enough to reach next unique b value)
            		 
            db.temp.find({a:1}).limit(3).sort({b:1, _id:1}).explain()

            Show
            db.temp.insert({_id:1, a:1, b:1}) db.temp.insert({_id:2, a:1, b:2}) db.temp.insert({_id:3, a:1, b:3}) db.temp.insert({_id:4, a:1, b:4}) db.temp.insert({_id:5, a:1, b:5}) db.temp.insert({_id:6, a:1, b:6})   db.temp.ensureIndex({a:1, b:1})   // Scans 3 objects db.temp.find({a:1}).limit(3).sort({b:1}).explain()   // Include _id as secondary sort field // Scans 6 objects. Should only need to scan 4 (3 + enough to reach next unique b value) db.temp.find({a:1}).limit(3).sort({b:1, _id:1}).explain()

            Description

            I have a collection of documents with keys _id, a, and b. I have an index {a:1,b:1}.

            When I do find(a).sort(b).limit(5) the index is used, and only 5 documents are scanned.

            When I do find(a).sort(b,_id).limit(5) (with or without a hint) it appears the index is only used to match a - as all documents with the matching value for a are scanned (nscanned and nscannedObjects are high).

            The behavior I would expect is that it would scan through a and b values enough to get to the limit (5), and then it would stop scanning when it found the next unique b (or a). It would then sort on _id within the scanned records.

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                Issue Links

                depends on

                Improvement - An improvement or enhancement to an existing feature or task. SERVER-3310 Query optimizer should efficiently handle $in and sort with compound index.

                • Major - P3 - Major loss of function.
                • Closed
                is related to

                Improvement - An improvement or enhancement to an existing feature or task. SERVER-12533 Include sort (spec) for sort stage in full explain output

                • Trivial - P5 - Cosmetic problem like misspelt words or misaligned text.
                • Closed

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                      Dates

                      • Created:
                        Updated: