mongodb学习（2）索引有关

1. _id索引
   _id索引是绝大多数集合默认建立的索引，对于每个插入的数据，MongDB都会自动生成一条唯一的_id字段，db.posts.getIndexes()可以获取到该集合的索引，创建一个集合之后，系统会默认给这个集合创建一个_id索引

db.posts.getIndexes()
[
    {
        "v" : 2,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "posts.posts"
    }
]

2. 普通索引
   创建普通索引：

db.collection.createIndex(keys, options)

语法中 Key 值为你要创建的索引字段，1 为指定按升序创建索引，如果你想按降序来创建索引指定为 -1 即可。创建索引后可以加快查询速度。

可以对文档中的多个键单独创建索引或者创建联合索引。

3. MongoDB 高级索引
   1)索引子文档字段：可以对文档的子文档创建索引

> db.col.insert({
...    "address": {
...       "city": "Los Angeles",
...       "state": "California",
...       "pincode": "123"
...    },
...    "tags": [
...       "music",
...       "cricket",
...       "blogs"
...    ],
...    "name": "Tom Benzamin"
... })
WriteResult({ "nInserted" : 1 })

> db.col.ensureIndex({"address.city":1,"address.state":1,"address.pincode":1})
{
    "createdCollectionAutomatically" : false,
    "numIndexesBefore" : 5,
    "numIndexesAfter" : 6,
    "ok" : 1
}

> db.col.find({"address.city":"Los Angeles"})
{ "_id" : ObjectId("5c9c581279239019d92fa21a"), "address" : { "city" : "Los Angeles", "state" : "California", "pincode" : "123" }, "tags" : [ "music", "cricket", "blogs" ], "name" : "Tom Benzamin" }

2)数组索引
可以对文档中的数据建立索引：

> db.posts.find()
{ "_id" : ObjectId("5c9c5b8a79239019d92fa21b"), "post_text" : "enjoy the mongodb articles on Runoob", "tags" : [ "mongodb", "runoob" ] }
{ "_id" : ObjectId("5c9c5c2079239019d92fa21c"), "post_text" : "enjoy the mongodb articles on monday" }
{ "_id" : ObjectId("5c9c6f1d79239019d92fa21e"), "name" : "hi,today is sunny day,today is Monday" }
{ "_id" : ObjectId("5c9c729879239019d92fa21f"), "name" : "lihong" }
{ "_id" : ObjectId("5c9c780a79239019d92fa220"), "name" : "liming" }
> db.posts.ensureIndex({"tags":1})
{
    "createdCollectionAutomatically" : false,
    "numIndexesBefore" : 2,
    "numIndexesAfter" : 2,
    "note" : "all indexes already exist",
    "ok" : 1
}
> db.posts.find({tags:"mongodb"})
{ "_id" : ObjectId("5c9c5b8a79239019d92fa21b"), "post_text" : "enjoy the mongodb articles on Runoob", "tags" : [ "mongodb", "runoob" ] }

4. 全文检索
   全文检索对每一个词建立一个索引，指明该词在文章中出现的次数和位置，当用户查询时，检索程序就根据事先建立的索引进行查找，并将查找的结果反馈给用户的检索方式。
   开启全文检索

> db.adminCommand({setParameter:true,textSearchEnabled:true})
{
    "ok" : 0,
    "errmsg" : "attempted to set unrecognized parameter [textSearchEnabled], use help:true to see options "
}

创建全文索引：先在集合posts中插入两个文档，再对文档中的post_text字段建立索引，然后执行搜索

> db.posts.insert({
...    "post_text": "enjoy the mongodb articles on Runoob",
...    "tags": [
...       "mongodb",
...       "runoob"
...    ]
... })
WriteResult({ "nInserted" : 1 })
> db.posts.insert({"post_text": "enjoy the mongodb articles on monday"})
WriteResult({ "nInserted" : 1 })
> db.posts.ensureIndex({post_text:"text"})
{
    "createdCollectionAutomatically" : false,
    "numIndexesBefore" : 1,
    "numIndexesAfter" : 2,
    "ok" : 1
}
> db.posts.find({$text:{$search:"enjoy"}})
{ "_id" : ObjectId("5c9c5b8a79239019d92fa21b"), "post_text" : "enjoy the mongodb articles on Runoob", "tags" : [ "mongodb", "runoob" ] }
{ "_id" : ObjectId("5c9c5c2079239019d92fa21c"), "post_text" : "enjoy the mongodb articles on monday" }

查找索引名：

> db.posts.getIndexes()
[
    {
        "v" : 2,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "posts.posts"
    },
    {
        "v" : 2,
        "key" : {
            "_fts" : "text",
            "_ftsx" : 1
        },
        "name" : "post_text_text",
        "ns" : "posts.posts",
        "weights" : {
            "post_text" : 1
        },
        "default_language" : "english",
        "language_override" : "language",
        "textIndexVersion" : 3
    }
]

删除全文检索：

db.posts.dropIndex("post_text_text")

索引是特殊的数据结构，以易于遍历的形式存储数据集的一小部分。 索引存储特定字段或一组字段的值，按照索引中指定的字段值排序。
使用索引，可以加快索引相关的查询，也相应地带来一些坏处：
每个索引占据一定的存储空间，在进行插入，更新和删除操作时也需要对索引进行操作。所以，如果你很少对集合进行读取操作，建议不使用索引。