Limit scans one extra

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    • Type: Bug
    • Resolution: Done
    • Priority: Minor - P4
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    • Affects Version/s: 2.6.1, 2.6.3
    • Component/s: Querying
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      Insert this set of test data:

      > db.rooms.find()
{ "_id" : ObjectId("53ad206e1d8f2d8351182830"), "id" : 1, "from" : ISODate("2014-06-26T00:00:00Z"), "to" : ISODate("2014-06-28T00:00:00Z") }
{ "_id" : ObjectId("53ad276f1d8f2d8351182831"), "id" : 1, "from" : ISODate("2014-06-24T00:00:00Z"), "to" : ISODate("2014-07-01T00:00:00Z") }
{ "_id" : ObjectId("53ad28ad1d8f2d8351182832"), "id" : 1, "from" : ISODate("2014-06-20T00:00:00Z"), "to" : ISODate("2014-06-28T00:00:00Z") }
{ "_id" : ObjectId("53ad28c61d8f2d8351182833"), "id" : 1, "from" : ISODate("2014-06-20T00:00:00Z"), "to" : ISODate("2014-07-03T00:00:00Z") }
{ "_id" : ObjectId("53ad29971d8f2d8351182834"), "id" : 1, "from" : ISODate("2014-06-20T00:00:00Z"), "to" : ISODate("2014-06-21T00:00:00Z") }

      Create these indexes:

      > db.rooms.getIndexes()
[
        {
                "v" : 1,
                "key" : {
                        "_id" : 1
                },
                "name" : "_id_",
                "ns" : "test.rooms"
        },
        {
                "v" : 1,
                "key" : {
                        "id" : 1
                },
                "name" : "id_1",
                "ns" : "test.rooms"
        }
]

      And then run this query:

      > db.rooms.find({id:1}).sort({id:1}).limit(2).explain()
{
        "cursor" : "BtreeCursor id_1",
        "isMultiKey" : false,
        "n" : 2,
        "nscannedObjects" : 2,
        "nscanned" : 3,
        "nscannedObjectsAllPlans" : 2,
        "nscannedAllPlans" : 3,
        "scanAndOrder" : false,
        "indexOnly" : false,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "millis" : 0,
        "indexBounds" : {
                "id" : [
                        [
                                1,
                                1
                        ]
                ]
        },
        "server" : "ubuntu:27017",
        "filterSet" : false
}

      with or without the sort will result in the same answer.

      Show
      Insert this set of test data: > db.rooms.find() { "_id" : ObjectId( "53ad206e1d8f2d8351182830" ), "id" : 1, "from" : ISODate( "2014-06-26T00:00:00Z" ), "to" : ISODate( "2014-06-28T00:00:00Z" ) } { "_id" : ObjectId( "53ad276f1d8f2d8351182831" ), "id" : 1, "from" : ISODate( "2014-06-24T00:00:00Z" ), "to" : ISODate( "2014-07-01T00:00:00Z" ) } { "_id" : ObjectId( "53ad28ad1d8f2d8351182832" ), "id" : 1, "from" : ISODate( "2014-06-20T00:00:00Z" ), "to" : ISODate( "2014-06-28T00:00:00Z" ) } { "_id" : ObjectId( "53ad28c61d8f2d8351182833" ), "id" : 1, "from" : ISODate( "2014-06-20T00:00:00Z" ), "to" : ISODate( "2014-07-03T00:00:00Z" ) } { "_id" : ObjectId( "53ad29971d8f2d8351182834" ), "id" : 1, "from" : ISODate( "2014-06-20T00:00:00Z" ), "to" : ISODate( "2014-06-21T00:00:00Z" ) } Create these indexes: > db.rooms.getIndexes() [ { "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_" , "ns" : "test.rooms" }, { "v" : 1, "key" : { "id" : 1 }, "name" : "id_1" , "ns" : "test.rooms" } ] And then run this query: > db.rooms.find({id:1}).sort({id:1}).limit(2).explain() { "cursor" : "BtreeCursor id_1" , "isMultiKey" : false, "n" : 2, "nscannedObjects" : 2, "nscanned" : 3, "nscannedObjectsAllPlans" : 2, "nscannedAllPlans" : 3, "scanAndOrder" : false, "indexOnly" : false, "nYields" : 0, "nChunkSkips" : 0, "millis" : 0, "indexBounds" : { "id" : [ [ 1, 1 ] ] }, "server" : "ubuntu:27017" , "filterSet" : false } with or without the sort will result in the same answer.
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      I have filed this under indexing since it only seems to happen when an index is used.

      I noticed that the nscanned on 2.6.1 for limit is always one extra than needed.

            Assignee:
            David Storch
            Reporter:
            Sam Millman
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              Created:
              Updated:
              Resolved: