db.stu.find(query).explain()
	"cursor" : "BasicCursor",   --- 说明索引没有发挥作用
	"nscannedObjects" : 1000,    --- 理论上要扫描多少行
	"cursor" : "BtreeCursor sn_1",  -- 用到了btree索引

db.student.getIndexes();

db.student.ensureIndex({name:-1}) -- 降序创建索引

db.student.dropIndex({name: -1}) -- 删除单个索引

db.student.dropIndexes();  -- 删除所有索引

db.student.ensureIndex({sn:1, name:1})

> db.teacher.find()
{ "_id" : ObjectId("5678c99fff917ea632e5a6ca"), "email" : "a@gmail.com" }
{ "_id" : ObjectId("5678c9a3ff917ea632e5a6cb"), "email" : "b@gmail.com" }
{ "_id" : ObjectId("5678ca14ff917ea632e5a6cd"), "email" : "c@gmail.com" }
{ "_id" : ObjectId("5678ca5fff917ea632e5a6cf") }