nscanned nscannedObjects and n attribute values are equal in mongos explian command - mongodb

Question

I am using mongodb for our Application .
I used mongodb profiler by setting up system.profiling level to 2 , did all my application operations , exported all the records from system.profile and finally set the indexes on the collections based upon the profiler result .
Now when i did explain on those queries
Query 1.
db.stocks.find({ symbol: "GOOG", date: "2013-09-13",type: "O",Mini: false, rootsymbol: "GOOG" }).sort( { "price": 1,"call":1} ).explain();
{
"cursor" : "BtreeCursor symbol_1_date_1_type_1_Mini_1_rootsymbol_1_price_1",
"nscanned" : 80,
"nscannedObjects" : 80,
"n" : 80,
"scanAndOrder" : true,
"millis" : 2,
Query 2.
db.stocks.find({ symbol: "NVO" }).explain()
{
"cursor" : "BtreeCursor symbol_1",
"nscanned" : 1,
"nscannedObjects" : 1,
"n" : 1,
"millis" : 0,
"indexBounds" : {
"symbol" : [
[
"NVO",
"NVO"
]
]
}
}
I was confused of the results as the nscanned , nscannedObjects and n are always equal on all of my queries .
Please let me know if its an issue if the nscanned , nscannedObjects and n values have equal values ??
Please let me know if i am missing something or if its a issue ??

Answer 1

The fact that they are all equal mean you have good, uncovered, index usage.
I shall break down the results:
"nscanned" : 80,
This is number scanned in the index
"nscannedObjects" : 80,
This is the number of documents loaded after scanning the index, if this figure is higher than scanned then you could have bad index usage (depends on scenario).
The times nscannedObjects might be lower than nscanned is on a covered index or ( http://docs.mongodb.org/manual/reference/method/cursor.explain/#explain.nscannedObjects ):
in the case of multikey index on an array field with duplicate documents.
"n" : 80,
This is the amount returned.
However, this does not mean you had a good result for the sort as shown in the first explain by:
"scanAndOrder" : true,

Answer 2

Your first query is good - it is using the index to find the 80 matches that exist. "n" is the number of documents returned; "nscannedRecords" is the number it needed to scan during the query (it could be less than 'scanned' if you have a covered query). "nscanned" is the number of index entries scanned (if an index could be used). Sorting 80 records should be fairly quick; you don't have an index that you could use for it (from what you've shown us).
The second query is also great - it uses an index, and finds the one document that matches.
For more details on explain(), see http://docs.mongodb.org/manual/reference/method/cursor.explain/#explain-output-fields-core.