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  1. Core Server
  2. SERVER-18119

Covered query over compound index using the second field does not results in an index-only query

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      Have a collection with this index: 

      {
    "a" : 1,
    "b" : -1,
    "c" : 1
}

      query this: 

      { b : { $lte : 1262343601554}}, {b : 1, _id : 0}

      will not even use the index… so we hint the index. and limit to 10.
      An explain tells now:

      {
    "cursor" : "BtreeCursor indexName",
    "isMultiKey" : false,
    "n" : 10,
    "nscannedObjects" : 72,
    "nscanned" : 72,
    "nscannedObjectsAllPlans" : 72,
    "nscannedAllPlans" : 72,
    "scanAndOrder" : false,
    "indexOnly" : false,
…
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      Have a collection with this index: { "a" : 1, "b" : -1, "c" : 1 } query this: { b : { $lte : 1262343601554}}, {b : 1, _id : 0} will not even use the index… so we hint the index. and limit to 10. An explain tells now: { "cursor" : "BtreeCursor indexName" , "isMultiKey" : false , "n" : 10, "nscannedObjects" : 72, "nscanned" : 72, "nscannedObjectsAllPlans" : 72, "nscannedAllPlans" : 72, "scanAndOrder" : false , "indexOnly" : false , …

      We have a non sharded collection with a compound index.
      On a query using only the second field of the compound index and restricting the returned fields to those of the index the query will not be a covered query.

      But even as it has to go through more elements of the index it should only use the index.

            Assignee:
            Unassigned Unassigned
            Reporter:
            breitlauch Daniel Breitlauch
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              Created:
              Updated:
              Resolved: