Mongo Query

from Optimizing MongoDB Compound Indexes  

•  For indexed queries
• nscanned: the number of index keys in the range that Mongo scanned
• nscannedObjects: the number of documents it looked at to get the final result
• n: matched documents
• "optimal index" for a query
• include all the query's filtered fields and sort fields
• any range-filtered or sort fields in the query must come after equality fields
• if there are multiple optimal indexes, Mongo chooses one arbitrarily