MongoDB 索引

1. ensureIndex添加索引  ensureIndex 函数帮助文档  db.blog.ensureIndex(keypattern[,options]) - options is an object with these 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" : "aaa", "age" : 23, "sex" : "male" }  { "_id" : ObjectId("4fc6d0e5387a7fee4eb6bfaa"), "name" : "bbb", "age" : 25, "sex" : "male" }  { "_id" : ObjectId("4fc6d0f4387a7fee4eb6bfab"), "name" : "ccc", "age" : 25, "sex" : "male" }  { "_id" : ObjectId("4fc6d100387a7fee4eb6bfac"), "name" : "ddd", "age" : 25, "sex" : "male" }  { "_id" : ObjectId("4fc6d110387a7fee4eb6bfad"), "name" : "eee", "age" : 23, "sex" : "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. 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. hint强制使用某个索引  > db.users.find({"age":23,"name":/.*/}).hint({"name":1,"age":-1})  { "_id" : ObjectId("4fc6d110387a7fee4eb6bfad"), "name" : "aaa", "age" : 23, "sex  " : "male" }  { "_id" : ObjectId("4fc6d121387a7fee4eb6bfaf"), "name" : "bbb", "age" : 23, "se  x" : "male" }  { "_id" : ObjectId("4fc6d11b387a7fee4eb6bfae"), "name" : "ccc", "age" : 23, "sex  " : "male" }  { "_id" : ObjectId("4fc6d12b387a7fee4eb6bfb0"), "name" : "ddd", "age" : 23, "se  x" : "male" }  { "_id" : ObjectId("4fc6d0c9387a7fee4eb6bfa9"), "name" : "eee", "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。 

参考推荐：

Create a Unique Index（官方文档）

MongoDB 索引

MongoDB基本管理命令

MongoDB 常用命令

MongoDB 基本操作