MongoDB 索引

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1.使用ensureIndex添加索引

ensureIndex函数帮助文档

db.blog.ensureIndex(keypattern[,options]) - options is an object with th

ese possible fields: name, unique, dropDups

name：指定索引名称

unique:是否唯一索引

dropDups：是否删除重复

创建索引的缺点：每次插入、更新、删除时都会产生额外的开销，要尽可能少创建索引。每个集合默认的最大索引个数为64个。

如果没有对应的键，索引会将其作为null存储，所以，如果对某个建立了唯一索引，但插入了多个缺少该索引键的文档，则由于文档包含null值而导致插入失败。

例子

> db.users.find()

{ "_id" : ObjectId("4fc6d0c9387a7fee4eb6bfa9"), "name" : "wilian", "age" : 23, "

sex" : "male" }

{ "_id" : ObjectId("4fc6d0e5387a7fee4eb6bfaa"), "name" : "rcsd", "age" : 25, "se

x" : "male" }

{ "_id" : ObjectId("4fc6d0f4387a7fee4eb6bfab"), "name" : "sds", "age" : 25, "sex

" : "male" }

{ "_id" : ObjectId("4fc6d100387a7fee4eb6bfac"), "name" : "qff", "age" : 25, "sex

" : "male" }

{ "_id" : ObjectId("4fc6d110387a7fee4eb6bfad"), "name" : "qff", "age" : 23, "sex

" : "male" }

{ "_id" : ObjectId("4fc6d11b387a7fee4eb6bfae"), "name" : "ssf", "age" : 23, "sex

" : "male" }

{ "_id" : ObjectId("4fc6d121387a7fee4eb6bfaf"), "name" : "qwwf", "age" : 23, "se

x" : "male" }

{ "_id" : ObjectId("4fc6d12b387a7fee4eb6bfb0"), "name" : "ttwf", "age" : 23, "se

x" : "male" }

> db.users.ensureIndex({"name":1,"age":-1},{"name":"userIndex"})

//1，-1代表索引方向

//查找索引

> db.system.indexes.find()

{ "name" : "_id_", "ns" : "blog.users", "key" : { "_id" : 1 }, "v" : 0 }

{ "_id" : ObjectId("4fc6d1d0387a7fee4eb6bfb1"), "ns" : "blog.users", "key" : { "

name" : 1, "age" : -1 }, "name" : "userIndex", "v" : 0 }

2.使用explain()监控索引使用情况

> db.users.find({"name":"qff","age":23}).explain()

{

"cursor" : "BtreeCursor userIndex",//没有使用索引时为BasicCursor,索引存储在B树结构中，所以使用查询时候会使用BtreeCursor类型的游标，userIndex为使用索引的名称

"nscanned" : 1,//查找了多少个文档

"nscannedObjects" : 1,

"n" : 1,//返回文档的数量

"millis" : 50,//数据库执行查询时间

"nYields" : 0,

"nChunkSkips" : 0,

"isMultiKey" : false,

"indexOnly" : false,

"indexBounds" : {

"name" : [

[

"qff",

"qff"

]

],

"age" : [

[

23,

23

]

]

}

}

3.使用hint强制使用某个索引

> db.users.find({"age":23,"name":/.*/}).hint({"name":1,"age":-1})

{ "_id" : ObjectId("4fc6d110387a7fee4eb6bfad"), "name" : "qff", "age" : 23, "sex

" : "male" }

{ "_id" : ObjectId("4fc6d121387a7fee4eb6bfaf"), "name" : "qwwf", "age" : 23, "se

x" : "male" }

{ "_id" : ObjectId("4fc6d11b387a7fee4eb6bfae"), "name" : "ssf", "age" : 23, "sex

" : "male" }

{ "_id" : ObjectId("4fc6d12b387a7fee4eb6bfb0"), "name" : "ttwf", "age" : 23, "se

x" : "male" }

{ "_id" : ObjectId("4fc6d0c9387a7fee4eb6bfa9"), "name" : "wilian", "age" : 23, "

sex" : "male" }

MongoDB的查询优化器非常智能，会替你选择该用哪个索引，多数情况下不需要指定的。