MongoDB indexing results on array fields - mongodb

Question

I have a collection as { student_id :1, teachers : [ "....",...]}
steps done in sequence as : 1) find by {teachers : "gore"}
2) set the index as { student_id : 1 }
3) find by {teachers : "gore"}
4) set the index as { teachers : 1 }
5) find by {teachers : "gore"}
and the results(time taken) are not that much effective by indexing teachers(array) Please someone explain what is happening? I may be doing something wrong here please correct me. The results are as :
d.find({teachers : "gore"}).explain()
{ "cursor" : "BasicCursor", "nscanned" : 999999, "nscannedObjects" : 999999, "n" : 447055, "millis" : 1623, "nYields" : 0, "nChunkSkips" : 0, "isMultiKey" : false, "indexOnly" : false, "indexBounds" : {
} }
d.ensureIndex({student_id : 1})
d.find({teachers : "gore"}).explain() { "cursor" : "BasicCursor", "nscanned" : 999999, "nscannedObjects" : 999999, "n" : 447055, "millis" : 1300, "nYields" : 0, "nChunkSkips" : 0, "isMultiKey" : false, "indexOnly" : false, "indexBounds" : {
} }
d.ensureIndex({teachers : 1})
d.find({teachers : "gore"}).explain() { "cursor" : "BtreeCursor teachers_1", "nscanned" : 447055, "nscannedObjects" : 447055, "n" : 447055, "millis" : 1501, "nYields" : 0, "nChunkSkips" : 0, "isMultiKey" : true, "indexOnly" : false, "indexBounds" : { "teachers" : [ [ "gore", "gore" ] ] } }

Answer 1

Do you have the same data inserted over and over? The fact that it is showing a BtreeCursor is a positive, but the number of nscannedObjects is too large. Do you have the same data inserted over and over again? Is it possible that you have 447055 "gore" values? If so, thats why its taking such a long time.