1. create index
db.students.ensureIndex({class:1,student_name:1})
http://docs.mongodb.org/manual/tutorial/create-an-index/
2. show all indexes in current DB
db.system.indexes.find()
3. show index for a collection
db.grades.getIndexes()
4. delete index
db.grades.dropIndex({"student_id":1})
原文：http://www.cnblogs.com/phoenix13suns/p/4003135.html