Mongo入门 - 基本使用：索引和聚合

在了解MongoDB的基本CRUD操作后，常用的其它操作还有对字段的索引以及对字段的聚合操作。@pdai

聚合 - Aggregation Pipline

类似于将SQL中的group by + order by + left join ... 等操作管道化。

常规使用

图例理解

[MongoDB] Mongo入门 - 基本使用：索引和聚合插图

准备数据

> db.orders.insertMany( [
...     { _id: 1, cust_id: "abc1", ord_date: ISODate("2012-11-02T17:04:11.102Z"), status: "A", amount: 50 },
...     { _id: 2, cust_id: "xyz1", ord_date: ISODate("2013-10-01T17:04:11.102Z"), status: "A", amount: 100 },
...     { _id: 3, cust_id: "xyz1", ord_date: ISODate("2013-10-12T17:04:11.102Z"), status: "D", amount: 25 },
...     { _id: 4, cust_id: "xyz1", ord_date: ISODate("2013-10-11T17:04:11.102Z"), status: "D", amount: 125 },
...     { _id: 5, cust_id: "abc1", ord_date: ISODate("2013-11-12T17:04:11.102Z"), status: "A", amount: 25 }
... ] );
{ "acknowledged" : true, "insertedIds" : [ 1, 2, 3, 4, 5 ] }
> db.orders.find({})
{ "_id" : 1, "cust_id" : "abc1", "ord_date" : ISODate("2012-11-02T17:04:11.102Z"), "status" : "A", "amount" : 50 }
{ "_id" : 2, "cust_id" : "xyz1", "ord_date" : ISODate("2013-10-01T17:04:11.102Z"), "status" : "A", "amount" : 100 }
{ "_id" : 3, "cust_id" : "xyz1", "ord_date" : ISODate("2013-10-12T17:04:11.102Z"), "status" : "D", "amount" : 25 }
{ "_id" : 4, "cust_id" : "xyz1", "ord_date" : ISODate("2013-10-11T17:04:11.102Z"), "status" : "D", "amount" : 125 }
{ "_id" : 5, "cust_id" : "abc1", "ord_date" : ISODate("2013-11-12T17:04:11.102Z"), "status" : "A", "amount" : 25 }
>

聚合操作

> db.orders.aggregate([
...                      { $match: { status: "A" } },
...                      { $group: { _id: "$cust_id", total: { $sum: "$amount" } } },
...                      { $sort: { total: -1 } }
...                    ])
{ "_id" : "xyz1", "total" : 100 }
{ "_id" : "abc1", "total" : 75 }

官网还有两个例子：

Pipline操作

MongoDB的聚合管道（Pipline）将MongoDB文档在一个阶段（Stage）处理完毕后将结果传递给下一个阶段（Stage）处理。阶段（Stage）操作是可以重复的。

表达式：处理输入文档并输出。表达式是无状态的，只能用于计算当前聚合管道的文档，不能处理其它的文档。

这里我们介绍一下聚合框架中常用的几个Stages：

$project：修改输入文档的结构。可以用来重命名、增加或删除域，也可以用于创建计算结果以及嵌套文档。
$match：用于过滤数据，只输出符合条件的文档。$match使用MongoDB的标准查询操作。
$limit：用来限制MongoDB聚合管道返回的文档数。
$skip：在聚合管道中跳过指定数量的文档，并返回余下的文档。
$unwind：将文档中的某一个数组类型字段拆分成多条，每条包含数组中的一个值。
$group：将集合中的文档分组，可用于统计结果。
$sort：将输入文档排序后输出。
$geoNear：输出接近某一地理位置的有序文档。
$bucket: 分组（分桶）计算。
$facet : 多次分组计算。
$out: 将结果集输出，必须是Pipline最后一个Stage。

举几个例子

$project

> db.orders.aggregate(
...     { $project : {
...         _id : 0 , // 默认不显示_id
...         cust_id : 1 ,
...         status : 1
...     }});
{ "cust_id" : "abc1", "status" : "A" }
{ "cust_id" : "xyz1", "status" : "A" }
{ "cust_id" : "xyz1", "status" : "D" }
{ "cust_id" : "xyz1", "status" : "D" }
{ "cust_id" : "abc1", "status" : "A" }
>

$skip

 db.orders.aggregate(
...     { $skip : 4 });
{ "_id" : 5, "cust_id" : "abc1", "ord_date" : ISODate("2013-11-12T17:04:11.102Z"), "status" : "A", "amount" : 25 }
>

$unwind

> db.inventory2.insertOne({ "_id" : 1, "item" : "ABC1", sizes: [ "S", "M", "L"] })
{ "acknowledged" : true, "insertedId" : 1 }
> db.inventory2.aggregate( [ { $unwind : "$sizes" } ] )
{ "_id" : 1, "item" : "ABC1", "sizes" : "S" }
{ "_id" : 1, "item" : "ABC1", "sizes" : "M" }
{ "_id" : 1, "item" : "ABC1", "sizes" : "L" }

$bucket

> db.artwork.insertMany([
... { "_id" : 1, "title" : "The Pillars of Society", "artist" : "Grosz", "year" : 1926,
...     "price" : NumberDecimal("199.99") },
... { "_id" : 2, "title" : "Melancholy III", "artist" : "Munch", "year" : 1902,
...     "price" : NumberDecimal("280.00") },
... { "_id" : 3, "title" : "Dancer", "artist" : "Miro", "year" : 1925,
...     "price" : NumberDecimal("76.04") },
... { "_id" : 4, "title" : "The Great Wave off Kanagawa", "artist" : "Hokusai",
...     "price" : NumberDecimal("167.30") },
... { "_id" : 5, "title" : "The Persistence of Memory", "artist" : "Dali", "year" : 1931,
...     "price" : NumberDecimal("483.00") },
... { "_id" : 6, "title" : "Composition VII", "artist" : "Kandinsky", "year" : 1913,
...     "price" : NumberDecimal("385.00") },
... { "_id" : 7, "title" : "The Scream", "artist" : "Munch", "year" : 1893 },
... { "_id" : 8, "title" : "Blue Flower", "artist" : "O'Keefe", "year" : 1918,
...     "price" : NumberDecimal("118.42") }
... ])
{
        "acknowledged" : true,
        "insertedIds" : [
                1,
                2,
                3,
                4,
                5,
                6,
                7,
                8
        ]
}
> db.artwork.find({})
{ "_id" : 1, "title" : "The Pillars of Society", "artist" : "Grosz", "year" : 1926, "price" : NumberDecimal("199.99") }
{ "_id" : 2, "title" : "Melancholy III", "artist" : "Munch", "year" : 1902, "price" : NumberDecimal("280.00") }
{ "_id" : 3, "title" : "Dancer", "artist" : "Miro", "year" : 1925, "price" : NumberDecimal("76.04") }
{ "_id" : 4, "title" : "The Great Wave off Kanagawa", "artist" : "Hokusai", "price" : NumberDecimal("167.30") }
{ "_id" : 5, "title" : "The Persistence of Memory", "artist" : "Dali", "year" : 1931, "price" : NumberDecimal("483.00") }
{ "_id" : 6, "title" : "Composition VII", "artist" : "Kandinsky", "year" : 1913, "price" : NumberDecimal("385.00") }
{ "_id" : 7, "title" : "The Scream", "artist" : "Munch", "year" : 1893 } // 注意这里没有price，聚合结果中为Others
{ "_id" : 8, "title" : "Blue Flower", "artist" : "O'Keefe", "year" : 1918, "price" : NumberDecimal("118.42") }
> db.artwork.aggregate( [
...   {
...     $bucket: {
...       groupBy: "$price",
...       boundaries: [ 0, 200, 400 ],
...       default: "Other",
...       output: {
...         "count": { $sum: 1 },
...         "titles" : { $push: "$title" }
...       }
...     }
...   }
... ] )
{ "_id" : 0, "count" : 4, "titles" : [ "The Pillars of Society", "Dancer", "The Great Wave off Kanagawa", "Blue Flower" ] }
{ "_id" : 200, "count" : 2, "titles" : [ "Melancholy III", "Composition VII" ] }
{ "_id" : "Other", "count" : 2, "titles" : [ "The Persistence of Memory", "The Scream" ] }

$bucket + $facet

非常常用！

db.artwork.aggregate( [
  {
    $facet: {
      "price": [
        {
          $bucket: {
              groupBy: "$price",
              boundaries: [ 0, 200, 400 ],
              default: "Other",
              output: {
                "count": { $sum: 1 },
                "artwork" : { $push: { "title": "$title", "price": "$price" } }
              }
          }
        }
      ],
      "year": [
        {
          $bucket: {
            groupBy: "$year",
            boundaries: [ 1890, 1910, 1920, 1940 ],
            default: "Unknown",
            output: {
              "count": { $sum: 1 },
              "artwork": { $push: { "title": "$title", "year": "$year" } }
            }
          }
        }
      ]
    }
  }
] )

// 输出
{
  "year" : [
    {
      "_id" : 1890,
      "count" : 2,
      "artwork" : [
        {
          "title" : "Melancholy III",
          "year" : 1902
        },
        {
          "title" : "The Scream",
          "year" : 1893
        }
      ]
    },
    {
      "_id" : 1910,
      "count" : 2,
      "artwork" : [
        {
          "title" : "Composition VII",
          "year" : 1913
        },
        {
          "title" : "Blue Flower",
          "year" : 1918
        }
      ]
    },
    {
      "_id" : 1920,
      "count" : 3,
      "artwork" : [
        {
          "title" : "The Pillars of Society",
          "year" : 1926
        },
        {
          "title" : "Dancer",
          "year" : 1925
        },
        {
          "title" : "The Persistence of Memory",
          "year" : 1931
        }
      ]
    },
    {
      // Includes the document without a year, e.g., _id: 4
      "_id" : "Unknown",
      "count" : 1,
      "artwork" : [
        {
          "title" : "The Great Wave off Kanagawa"
        }
      ]
    }
  ],
      "price" : [
    {
      "_id" : 0,
      "count" : 4,
      "artwork" : [
        {
          "title" : "The Pillars of Society",
          "price" : NumberDecimal("199.99")
        },
        {
          "title" : "Dancer",
          "price" : NumberDecimal("76.04")
        },
        {
          "title" : "The Great Wave off Kanagawa",
          "price" : NumberDecimal("167.30")
        },
        {
          "title" : "Blue Flower",
          "price" : NumberDecimal("118.42")
        }
      ]
    },
    {
      "_id" : 200,
      "count" : 2,
      "artwork" : [
        {
          "title" : "Melancholy III",
          "price" : NumberDecimal("280.00")
        },
        {
          "title" : "Composition VII",
          "price" : NumberDecimal("385.00")
        }
      ]
    },
    {
      // Includes the document without a price, e.g., _id: 7
      "_id" : "Other",
      "count" : 2,
      "artwork" : [
        {
          "title" : "The Persistence of Memory",
          "price" : NumberDecimal("483.00")
        },
        {
          "title" : "The Scream"
        }
      ]
    }
  ]
}

聚合操作使用的比较频繁，在实际的工作中可以参考官方文档 - Aggregation Pipeline Stages (opens new window)。

[MongoDB] Mongo入门 - 基本使用：索引和聚合插图1

Aggregation Options参数

举一个explain参数为例，更多的相关Options可以参考官方文档，Aggregrate相关配置 (opens new window)

explain

> db.orders.aggregate(
...                      [
...                        { $match: { status: "A" } },
...                        { $group: { _id: "$cust_id", total: { $sum: "$amount" } } },
...                        { $sort: { total: -1 } }
...                      ],
...                      {
...                        explain: true
...                      }
...                    )
{
        "serverInfo" : {
                "host" : "pdai",
                "port" : 27017,
                "version" : "3.6.19",
                "gitVersion" : "41b289ff734a926e784d6ab42c3129f59f40d5b4"
        },
        "stages" : [
                {
                        "$cursor" : {
                                "query" : {
                                        "status" : "A"
                                },
                                "fields" : {
                                        "amount" : 1,
                                        "cust_id" : 1,
                                        "_id" : 0
                                },
                                "queryPlanner" : {
                                        "plannerVersion" : 1,
                                        "namespace" : "testdb.orders",
                                        "indexFilterSet" : false,
                                        "parsedQuery" : {
                                                "status" : {
                                                        "$eq" : "A"
                                                }
                                        },
                                        "winningPlan" : {
                                                "stage" : "COLLSCAN",
                                                "filter" : {
                                                        "status" : {
                                                                "$eq" : "A"
                                                        }
                                                },
                                                "direction" : "forward"
                                        },
                                        "rejectedPlans" : [ ]
                                }
                        }
                },
                {
                        "$group" : {
                                "_id" : "$cust_id",
                                "total" : {
                                        "$sum" : "$amount"
                                }
                        }
                },
                {
                        "$sort" : {
                                "sortKey" : {
                                        "total" : -1
                                }
                        }
                }
        ],
        "ok" : 1
}

聚合 - Map Reduce

图例理解

[MongoDB] Mongo入门 - 基本使用：索引和聚合插图2

官网给了个例子

准备数据

{
     _id: ObjectId("50a8240b927d5d8b5891743c"),
     cust_id: "abc123",
     ord_date: new Date("Oct 04, 2012"),
     status: 'A',
     price: 25,
     items: [ { sku: "mmm", qty: 5, price: 2.5 },
              { sku: "nnn", qty: 5, price: 2.5 } ]
}

计算每个顾客总花费：

map

var mapFunction1 = function() {
                       emit(this.cust_id, this.price);
                   };

reduce

var reduceFunction1 = function(keyCustId, valuesPrices) {
                          return Array.sum(valuesPrices);
                      };

out

db.orders.mapReduce(
                     mapFunction1,
                     reduceFunction1,
                     { out: "map_reduce_example" }
                   )

计算每个订单中Items的均价

map

var mapFunction2 = function() {
                       for (var idx = 0; idx < this.items.length; idx++) {
                           var key = this.items[idx].sku;
                           var value = {
                                         count: 1,
                                         qty: this.items[idx].qty
                                       };
                           emit(key, value);
                       }
                    };

reduce

var reduceFunction2 = function(keySKU, countObjVals) {
                     reducedVal = { count: 0, qty: 0 };

                     for (var idx = 0; idx < countObjVals.length; idx++) {
                         reducedVal.count += countObjVals[idx].count;
                         reducedVal.qty += countObjVals[idx].qty;
                     }

                     return reducedVal;
                  };

finalize

var finalizeFunction2 = function (key, reducedVal) {

                       reducedVal.avg = reducedVal.qty/reducedVal.count;

                       return reducedVal;

                    };

索引

索引即为提升查询等的效率，默认是对_id进行索引的。

图例理解

以对users中score进行索引时查询的效果

[MongoDB] Mongo入门 - 基本使用：索引和聚合插图3

索引的类型

对于索引，这里简单介绍下常用的类型，其它类型和例子可以参考官网文档 - 索引 (opens new window)

单一索引

[MongoDB] Mongo入门 - 基本使用：索引和聚合插图4

复合索引

[MongoDB] Mongo入门 - 基本使用：索引和聚合插图5

多键索引

[MongoDB] Mongo入门 - 基本使用：索引和聚合插图6

对索引的操作

查看集合索引

db.col.getIndexes()

查看集合索引大小

db.col.totalIndexSize()

删除集合所有索引

db.col.dropIndexes()

删除集合指定索引

db.col.dropIndex("索引名称")

[MongoDB] Mongo入门 - 基本使用：索引和聚合