【MongoDB学习笔记22】MongoDB的索引管理

使用ensureIndex函数创建集合的索引。

对集合而言，每个索引只需要创建一次，重复创建没有任何作用；

> show collections   system.indexes    users

所有的数据库的索引元数据都存放在system.indexs的集合中；

使用getIndexes函数查看给定的集合上的索引信息：

> db.users.getIndexes()   [        {            "v" : 1,            "key" : {                "_id" : 1            },            "name" : "_id_",            "ns" : "blog.users"        },        {            "v" : 1,            "key" : {                "username" : 1            },            "name" : "username_1",            "ns" : "blog.users"        },        {            "v" : 1,            "key" : {                "age" : 1,                "username" : 1            },            "name" : "age_1_username_1",            "ns" : "blog.users"        }    ]    >

可以适应dropIndex函数删除指定的索引：

> db.users.dropIndex({"username":1})   { "nIndexesWas" : 3, "ok" : 1 }    > db.users.getIndexes()    [        {            "v" : 1,            "key" : {                "_id" : 1            },            "name" : "_id_",            "ns" : "blog.users"        },        {            "v" : 1,            "key" : {                "age" : 1,                "username" : 1            },            "name" : "age_1_username_1",            "ns" : "blog.users"        }    ]    >

上述的例子中可以看到索引的标识默认格式为keyname1_dir1_keyname2_dir….keynameN_DirN;

也可以指定标识名：

> db.users.ensureIndex({"username":1},{"name":"firstname"})   {        "createdCollectionAutomatically" : false,        "numIndexesBefore" : 2,        "numIndexesAfter" : 3,        "ok" : 1    }    > db.users.getIndexes()    [        {            "v" : 1,            "key" : {                "_id" : 1            },            "name" : "_id_",            "ns" : "blog.users"        },        {            "v" : 1,            "key" : {                "age" : 1,                "username" : 1            },            "name" : "age_1_username_1",            "ns" : "blog.users"        },        {            "v" : 1,            "key" : {                "username" : 1            },            "name" : "firstname",            "ns" : "blog.users"        }    ]    >

修改索引的步骤：删除索引->创建索引；

删除索引可以直接指定索引标识，例如：

> db.users.dropIndex("firstname")   { "nIndexesWas" : 3, "ok" : 1 }    > db.users.getIndexes()    [        {            "v" : 1,            "key" : {                "_id" : 1            },            "name" : "_id_",            "ns" : "blog.users"        },        {            "v" : 1,            "key" : {                "age" : 1,                "username" : 1            },            "name" : "age_1_username_1",            "ns" : "blog.users"        }    ]    >