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MongoDB在创建集合的时候就在_id字段创建了一个唯一性索引. 这个索引防止客户端插入两个拥有相同_id的文档，并且这个索引不可被删除。
MongoDB索引使用的是B-tree数据结构。

创建索引
db.collection.createIndex()

单个字段创建索引

db.books.createIndex({name:1},{ unique: true })#在name上创建唯一性索引

复合索引

MongoDB规定限制复合的字段最多只能是31个。
db.userlist.createIndex({name:1,score:1})

多键索引

1.数组元素
db.userlist.createIndex({"habbit":1});
2.含有文档的数组元素
db.userlist.createIndex({"books.name":1,"books.pages":1})

文本索引

1.创建文本索引
db.news.createIndex({title:'text',content:'text'})
db.collection.createIndex( { "$**": "text" } )#创建多个
指定索引名称：
db.news.createIndex({title:'text',content:'text'},{name:'tilte_content_index'})
根据索引名称删除索引:
db.collection.dropIndex("tilte_content_index")

3.数据准备

db.news.insertMany([
{title:'aa bb cc ',content:'cc rice'},
{title:'bb cc ',content:'dd ee'},
{title:'ee bb aa ',content:'cc'},
{title:'aa dd',content:'ee'},
{title:'aa bb',content:'ae'},
{title:'rice cc dd ',content:'aa'}
])

4.检索：

• 单个关键字
  db.news.find({$text:{$search:"aa"}})
  查找结果：

{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa1"), "title" : "aa bb cc ", "content" : "cc rice" }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa6"), "title" : "rice cc dd ", "content" : "aa" }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa5"), "title" : "aa bb", "content" : "ae" }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa4"), "title" : "aa dd", "content" : "ee" }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa3"), "title" : "ee bb aa ", "content" : "cc" }

• 多个关键字(或关系):空格隔开
  db.news.find({$text:{$search:"aa cc"}})
  显示结果：

{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa2"), "title" : "bb cc ", "content" : "dd ee" }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa1"), "title" : "aa bb cc ", "content" : "cc rice" }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa6"), "title" : "rice cc dd ", "content" : "aa" }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa5"), "title" : "aa bb", "content" : "ae" }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa4"), "title" : "aa dd", "content" : "ee" }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa3"), "title" : "ee bb aa ", "content" : "cc" }

• 多个关键字（与关系）: \"包住关键字
  db.news.find({$text:{$search:"\"aa\" \"cc\""}})
  查询结果：

{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa1"), "title" : "aa bb cc ", "content" : "cc rice" }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa6"), "title" : "rice cc dd ", "content" : "aa" }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa3"), "title" : "ee bb aa ", "content" : "cc" }

• 查询多个关键，排除某个关键字
  db.news.find({$text:{$search:"\"aa\" \"cc\" -rice"}})
  显示结果：

{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa3"), "title" : "ee bb aa ", "content" : "cc" }

相似度查询：

db.news.find(
  {"$text":{"$search":"aa cc"}},
  {"score":{"$meta":"textScore"}}
).sort({"score":
  {"$meta":"textScore"}
});

查询结果：

{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa1"), "title" : "aa bb cc ", "content" : "cc rice", "score" : 2.083333333333333 }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa3"), "title" : "ee bb aa ", "content" : "cc", "score" : 1.7666666666666666 }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa6"), "title" : "rice cc dd ", "content" : "aa", "score" : 1.7666666666666666 }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa2"), "title" : "bb cc ", "content" : "dd ee", "score" : 0.75 }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa4"), "title" : "aa dd", "content" : "ee", "score" : 0.75 }
{ "_id" : ObjectId("599f03272aa12cb8dc4dcaa5"), "title" : "aa bb", "content" : "ae", "score" : 0.75 }

2d球面索引

2d索引

1.创建索引
db.shop.createIndex({"loc":"2d"})
2.插入数据

db.shop.insertMany([{name:'兰州拉面',loc:[11,12]},
{name:'番茄妹米线',loc:[11,11]},
{name:'香格里拉',loc:[10,12]},
{name:'座上客',loc:[11,52]},
{name:'苏果超市',loc:[41,42]},
{name:'一鸣奶品',loc:[31,32]},
{name:'和善园',loc:[21,22]}]);

3.检索：
db.shop.find({loc:{$near:[11,11],$maxDistance:5}});
4.检索结果：

{ "_id" : ObjectId("599f192d2aa12cb8dc4dcaaf"), "name" : "番茄妹米线", "loc" : [ 11, 11 ] }
{ "_id" : ObjectId("599f192d2aa12cb8dc4dcaae"), "name" : "兰州拉面", "loc" : [ 11, 12 ] }
{ "_id" : ObjectId("599f192d2aa12cb8dc4dcab0"), "name" : "香格里拉", "loc" : [ 10, 12 ] }

geoHaystack索引

哈希索引