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"
}
]
>
