Here's an example collection's index (besides _id): { "v" : 1, "key" : { "c" : 1 } , "ns" : "test.test", "name" : "c_1", "background" : true, "sparse" : true } There are 6 items: db.test.insert( {c: 5, b: 1} ); db.test.insert( {c: 5} ); db.test.insert( {c: 4} ); db.test.insert( {c: 3} ); db.test.insert( {c: 5} ); db.test.insert( {c: 1} ); db.test.find({c: 5, b: {$exists: false}}).explain() { "cursor" : "BasicCursor", "isMultiKey" : false, "n" : 2, "nscannedObjects" : 6, "nscanned" : 6, "nscannedObjectsAllPlans" : 6, "nscannedAllPlans" : 6, "scanAndOrder" : false, "indexOnly" : false, "nYields" : 0, "nChunkSkips" : 0, "millis" : 0, "indexBounds" : { }, "server" : "test:27017" } Even if I add an index on {c: 1, b: 1} it won't use that index either. With hint (where did the "b": 1 go on the first item?): db.test.find({c: 5, b: {$exists: false}}).hint( {c: 1} ) { "_id" : ObjectId("5216bcf5f2aa2ebe3c6ed298"), "c" : 5 } { "_id" : ObjectId("5216bcf5f2aa2ebe3c6ed295"), "c" : 5 }