MongoDB中索引的用法 转
索引能提高检索数据的速度，你可以想像成在MySQL中创建索引一样，同样索引也是用B-Tree也实现的。

1.单列索引

在字段x上创建索引，1 (ascending) or -1 (descending)

> db.data.ensureIndex({x:1}) 
显示表data里面的所有索引



> db.data.getIndexes() 
[ 
{ 
"name" : "_id_", 
"ns" : "recommender.data", 
"key" : { 
"_id" : 1 
} 
}, 
{ 
"_id" : ObjectId("4befb146b0e29ba1ce20e0bb"), 
"ns" : "recommender.data", 
"key" : { 
"x" : 1 
}, 
"name" : "x_1" 
} 
] 


查找字段x为6的值，此时已经用到索引了



> db.data.find({x:6}) 
{ "_id" : ObjectId("4bee804ba23d558eb6687117"), "x" : 6, "name" : "caihuafeng1" } 
{ "_id" : ObjectId("4bee804ba23d558eb6687118"), "x" : 6, "name" : "caihuafeng2" } 
{ "_id" : ObjectId("4bee804ba23d558eb6687119"), "x" : 6, "name" : "caihuafeng3" } 
{ "_id" : ObjectId("4bee804ba23d558eb668711a"), "x" : 6, "name" : "caihuafeng4" } 
{ "_id" : ObjectId("4bee804ba23d558eb668711b"), "x" : 6, "name" : "caihuafeng5" } 
{ "_id" : ObjectId("4bee804ba23d558eb668711c"), "x" : 6, "name" : "caihuafeng6" } 
{ "_id" : ObjectId("4bee804ba23d558eb668711d"), "x" : 6, "name" : "caihuafeng7" } 
{ "_id" : ObjectId("4bee804ba23d558eb668711e"), "x" : 6, "name" : "caihuafeng8" } 
{ "_id" : ObjectId("4bee804ba23d558eb668711f"), "x" : 6, "name" : "caihuafeng9" } 
{ "_id" : ObjectId("4bee804ba23d558eb6687120"), "x" : 6, "name" : "caihuafeng10" } 


2.默认索引

上述1中db.data.getIndexes()显示出来的一共有2个索引，其中_id是创建表的时候自动创建的索引，此索引是不能够删除的。

An index is always created on _id. This index is special and cannot be deleted. The _id index enforces uniqueness for its keys.

3.文档作为索引的键值

a.单列索引

MongoDB的官方文档上面是这样说的:

Documents as Keys

Indexed fields may be of any type, including documents:

往数据库recommender的表data中插入三条记录



> db.data.insert({name:"1616",info:{url:"http://www.1616.net/",city:"beijing"}}); 
> db.data.insert({name:"hao123",info:{url:"http://www.hao123.com/",city:"beijing"}}); 
> db.data.insert({name:"ll4la",info:{url:"http://www.114la.com/",city:"dongguan"}}); 


对字段info创建索引



> db.data.ensureIndex({info: 1}); 


显示表data上的所有索引



> db.data.getIndexes(); 
[ 
{ 
"name" : "_id_", 
"ns" : "recommender.data", 
"key" : { 
"_id" : 1 
} 
}, 
{ 
"_id" : ObjectId("4befb146b0e29ba1ce20e0bb"), 
"ns" : "recommender.data", 
"key" : { 
"x" : 1 
}, 
"name" : "x_1" 
}, 
{ 
"_id" : ObjectId("4befb76bb0e29ba1ce20e0bf"), 
"ns" : "recommender.data", 
"key" : { 
"info" : 1 
}, 
"name" : "info_1" 
} 
] 


查找指定的记录，此时会用到索引



> db.data.find({info: {url:"http://www.1616.net/",city:"beijing"}}); 
{ "_id" : ObjectId("4befb711b0e29ba1ce20e0bc"), "name" : "1616", "info" : { "url" : "http://www.1616.net/", "city" : "beijing" } }


b.组合索引

建立组合索引



> db.data.ensureIndex({"info.url":1, "info.city":1}); 
> db.data.getIndexes(); 
[ 
{ 
"name" : "_id_", 
"ns" : "recommender.data", 
"key" : { 
"_id" : 1 
} 
}, 
{ 
"_id" : ObjectId("4befb146b0e29ba1ce20e0bb"), 
"ns" : "recommender.data", 
"k