Indexing(Chapter 5 of MongoDB The Definitive Guild)

　　explain is an incredibly handy tool that will give you lots of information about your　queries. You can run it on any query by tacking it on to a cursor. explain returns a　document, not the cursor itself, unlike most cursor methods:

　　> db.foo.find().explain()

　　explain will return information about the indexes used for the query (if any) and stats　about timing and the number of documents scanned.