（08）ElasticSearch 批量获取文档MultiGet

　　使用Multi Get 可以通过索引名、类型名、文档id一次得到一个文档集合，文档可以来自同一个索引库，也可以来自不同索引库。

　　1、准备数据

PUT /lib/user/1
{
    "first_name":"Jane1",
    "last_name":"Smith1",
    "age":31,
    "about":"I like to collect rock albums1",
    "interests":[ "music1" ]
}

PUT /lib/user/2
{
    "first_name":"Jane2",
    "last_name":"Smith2",
    "age":32,
    "about":"I like to collect rock albums2",
    "interests":[ "music2" ]
}

PUT /lib/user/3
{
    "first_name":"Jane3",
    "last_name":"Smith3",
    "age":33,
    "about":"I like to collect rock albums1",
    "interests":[ "music3" ]
}

PUT /lib2/user2/1
{
    "first_name":"Jane1",
    "last_name":"Smith1",
    "age":31,
    "about":"I like to collect rock albums1",
    "interests":[ "music1" ]
}

PUT /lib2/user2/2
{
    "first_name":"Jane2",
    "last_name":"Smith2",
    "age":32,
    "about":"I like to collect rock albums2",
    "interests":[ "music2" ]
}

PUT /lib2/user2/3
{
    "first_name":"Jane3",
    "last_name":"Smith3",
    "age":31,
    "about":"I like to collect rock albums3",
    "interests":[ "music3" ]
}

　　2、操作演示

　　同时查询到文档id是1、2、3的文档。会查出6条数据

GET /_mget
  {
      "docs":[
         {
              "_index":"lib",
              "_type":"user",
              "_id":1
          },
          {
             "_index":"lib",
             "_type":"user",
             "_id":2
         },
         {
             "_index":"lib",
             "_type":"user",
             "_id":3
         },
          {
              "_index":"lib2",
              "_type":"user2",
              "_id":1
          },
          {
             "_index":"lib2",
             "_type":"user2",
             "_id":2
         },
         {
             "_index":"lib2",
             "_type":"user2",
             "_id":3
         }
     ]
 }

　　指定每个文档显示的字段：

GET /_mget
{
    "docs":[
        {
              "_index":"lib",
              "_type":"user",
              "_id":1,
              "_source":"interests"
          },
         {
             "_index":"lib",
             "_type":"user",
             "_id":2,
             "_source":["age","interests"]
         }
     ]
}

　　批量获取文档的简单写法有以下两种

GET /lib/user/_mget
  {
    "docs":[
        {
            "_id":1
        },
        {
            "_id":2
        }
    ]
}

GET /lib/user/_mget
{
    "ids":["1","2"]
}