MongoDB索引

索引的优点：
1、大大减少了服务器需要扫描的数据量；
2、索引可以帮助服务器避免排序或使用临时表；
3、索引可以将随机I/O转换为顺序I/O；

mongodb的索引类型
https://docs.mongodb.com/manual/indexes/#index-types

单字段索引
组合索引（多字段索引）
多键索引
空间索引
文本索引（全文索引）
hash索引

> use testdb
> for (i=1;i<=10000;i++) db.students.insert({name: "student"+i, age: (i%120), address: "#85 Wenhua Road, Zhengzhou, China"})
WriteResult({ "nInserted" : 1 })
> db.students.find().count()
10000
> db.students.find()

创建索引

语法：
> db.mycoll.ensureIndex({KEY: 1})
Key为你要创建的索引字段，1为按升序创建索引，-1为按降序创建索引。

也可以使用多个字段创建索引（关系型数据库中称作复合索引）
> db.mycoll.ensureIndex({"title": 1, "description": -1})

> db.mycoll.help()
> db.students.ensureIndex({name: 1}) 		
{
	"createdCollectionAutomatically" : false,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}

查看索引

> db.students.getIndexes() 		
[
	{
		"v" : 1,
		"key" : {
			"_id" : 1
		},
		"name" : "_id_",
		"ns" : "testdb.students"
	},
	{
		"v" : 1,
		"key" : {
			"name" : 1
		},
		"name" : "name_1",
		"ns" : "testdb.students"
	}
]

删除指定字段索引

> db.students.dropIndex("name_1") 		
{ "nIndexesWas" : 2, "ok" : 1 }
> db.students.getIndexes() 

创建一个惟一键索引

> db.students.ensureIndex({name: 1}, {unique: true}) 		
> db.students.getIndexes() 
[
	{
		"v" : 1,
		"key" : {
			"_id" : 1
		},
		"name" : "_id_",
		"ns" : "testdb.students"
	},
	{
		"v" : 1,
		"unique" : true,
		"key" : {
			"name" : 1
		},
		"name" : "name_1",
		"ns" : "testdb.students"
	}
]

> db.mycoll.dropIndexes() 		删除指定collection里的所有索引
> db.mycoll.reIndex() 			重建索引

查询分析

> db.students.find({name: "student5000"})
{ "_id" : ObjectId("58bd9ab8661d8c90936418bf"), "name" : "student5000", "age" : 80, "address" : "#85 Wenhua Road, Zhengzhou, China" }

> db.students.find({name: "student5000"}).explain()         
{
	"cursor" : "BtreeCursor name_1",
	"isMultiKey" : false,
	"n" : 1,
	"nscannedObjects" : 1,
	"nscanned" : 1,
	"nscannedObjectsAllPlans" : 1,
	"nscannedAllPlans" : 1,
	"scanAndOrder" : false,
	"indexOnly" : false,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"millis" : 0,
	"indexBounds" : {
		"name" : [
			[
				"student5000",
				"student5000"
			]
		]
	},
	"server" : "aliyun:27017",
	"filterSet" : false
}