aggregate¶

Command Fields命令字段¶

The aggregate command takes the following fields as arguments:聚合命令将以下字段作为参数：

collation: {
   locale: <string>,
   caseLevel: <boolean>,
   caseFirst: <string>,
   strength: <int>,
   numericOrdering: <boolean>,
   alternate: <string>,
   maxVariable: <string>,
   backwards: <boolean>
}

MongoDB 3.6 removes the use of aggregate command without the cursor option unless the command includes the explain option. MongoDB 3.6删除了不带cursor选项的聚合命令的使用，除非该命令包含解释选项。Unless you include the explain option, you must specify the cursor option.除非包含explain选项，否则必须指定游标选项。

• To indicate a cursor with the default batch size, specify cursor:{}.要使用默认的批处理大小指示游标，请指定cursor:{}。
• To indicate a cursor with a non-default batch size, use cursor: {batchSize: <num> }.要指示具有非默认批量大小的游标，请使用cursor: {batchSize: <num> }。

For more information about the aggregation pipeline Aggregation Pipeline, Aggregation Reference, and Aggregation Pipeline Limits.有关聚合管道聚合管道、聚合参考和聚合管道限制的更多信息。

Session Idle Timeout会话空闲超时¶

Starting in MongoDB 3.6, MongoDB drivers and the mongo shell associate all operations with a server session, with the exception of unacknowledged write operations. 从MongoDB 3.6开始，MongoDB驱动程序和mongo shell将所有操作与服务器会话相关联，但未确认的写入操作除外。For operations not explicitly associated with a session (i.e. using Mongo.startSession()), MongoDB drivers and the mongo shell creates an implicit session and associates it with the operation.对于与会话没有显式关联的操作（即使用Mongo.startSession()），MongoDB驱动程序和mongo shell会创建一个隐式会话，并将其与操作关联。

If a session is idle for longer than 30 minutes, the MongoDB server marks that session as expired and may close it at any time. 如果会话空闲时间超过30分钟，MongoDB服务器会将该会话标记为已过期，并可随时将其关闭。When the MongoDB server closes the session, it also kills any in-progress operations and open cursors associated with the session. 当MongoDB服务器关闭会话时，它还会终止任何正在进行的操作以及与会话相关的打开游标。This includes cursors configured with noCursorTimeout or a maxTimeMS greater than 30 minutes.这包括配置了noCursorTimeout或maxTimeMS（大于30分钟）的游标。

For operations that return a cursor, if the cursor may be idle for longer than 30 minutes, issue the operation within an explicit session using Session.startSession() and periodically refresh the session using the refreshSessions command. 对于返回游标的操作，如果游标空闲时间可能超过30分钟，请使用Session.startSession()在显式会话中发出该操作并使用refreshSessions命令定期刷新会话。See Session Idle Timeout for more information.有关更多信息，请参阅会话空闲超时。

Client Disconnection客户端断开¶

For aggregate operation that do not include the $out or $merge stages:对于不包括$out阶段或$merge阶段的聚合操作：

Starting in MongoDB 4.2, if the client that issued the aggregate disconnects before the operation completes, MongoDB marks the aggregate for termination (i.e. killOp on the operation).从MongoDB 4.2开始，如果发出聚合的客户端在操作完成之前断开连接，MongoDB会将聚合标记为终止（即操作上的killOp）。

Aggregate Data with Multi-Stage Pipeline使用多级管道聚合数据¶

A collection articles contains documents such as the following:包含以下文档的articles集合：

{
   _id: ObjectId("52769ea0f3dc6ead47c9a1b2"),
   author: "abc123",
   title: "zzz",
   tags: [ "programming", "database", "mongodb" ]
}

The following example performs an aggregate operation on the articles collection to calculate the count of each distinct element in the tags array that appears in the collection.下面的示例对articles集合执行聚合操作，以计算集合中出现的tags数组中每个不同元素的计数。

db.runCommand( {
   aggregate: "articles",
   pipeline: [
      { $project: { tags: 1 } },
      { $unwind: "$tags" },
      { $group: { _id: "$tags", count: { $sum : 1 } } }
   ],
   cursor: { }
} )

In the mongo shell, this operation can use the db.collection.aggregate() helper as in the following:在mongo shell中，此操作可以使用db.collection.aggregate()助手，如下所示：

db.articles.aggregate( [
   { $project: { tags: 1 } },
   { $unwind: "$tags" },
   { $group: { _id: "$tags", count: { $sum : 1 } } }
] )

Use $currentOp on an Admin Database在管理数据库上使用$currentOp¶

The following example runs a pipeline with two stages on the admin database. 下面的示例在管理数据库上运行一个带有两个阶段的管道。The first stage runs the $currentOp operation and the second stage filters the results of that operation.第一阶段运行$currentOp操作，第二阶段筛选该操作的结果。

db.adminCommand( {
   aggregate : 1,
   pipeline : [ {
      $currentOp : { allUsers : true, idleConnections : true } }, {
      $match : { shard : "shard01" }
      }
   ],
   cursor : { }
} )

Return Information on the Aggregation Operation返回有关聚合操作的信息¶

The following aggregation operation sets the optional field explain to true to return information about the aggregation operation.以下聚合操作将可选字段explain设置为true，以返回有关聚合操作的信息。

db.orders.aggregate([
      { $match: { status: "A" } },
      { $group: { _id: "$cust_id", total: { $sum: "$amount" } } },
      { $sort: { total: -1 } }
   ],
   { explain: true }
)

Aggregate Data using External Sort使用外部排序聚合数据¶

Aggregation pipeline stages have maximum memory use limit. 聚合管道阶段有最大内存使用限制。To handle large datasets, set allowDiskUse option to true to enable writing data to temporary files, as in the following example:要处理大型数据集，请将allowDiskUse选项设置为true，以允许将数据写入临时文件，如下例所示：

db.stocks.aggregate( [
      { $project : { cusip: 1, date: 1, price: 1, _id: 0 } },
      { $sort : { cusip : 1, date: 1 } }
   ],
   { allowDiskUse: true }
)

Starting in MongoDB 4.2, the profiler log messages and diagnostic log messages includes a usedDisk indicator if any aggregation stage wrote data to temporary files due to memory restrictions.从MongoDB 4.2开始，如果任何聚合阶段由于内存限制将数据写入临时文件，探查器日志消息和诊断日志消息将包括usedDisk指示符。

Aggregate Data Specifying Batch Size指定批量大小的聚合数据¶

To specify an initial batch size, specify the batchSize in the cursor field, as in the following example:要指定初始批次大小，请在cursor字段中指定batchSize（批次大小），如下例所示：

db.orders.aggregate( [
      { $match: { status: "A" } },
      { $group: { _id: "$cust_id", total: { $sum: "$amount" } } },
      { $sort: { total: -1 } },
      { $limit: 2 }
   ],
   { cursor: { batchSize: 0 } }
)

The {batchSize: 0 } document specifies the size of the initial batch size only. {batchSize:0}文档仅指定初始批大小的大小。Specify subsequent batch sizes to OP_GET_MORE operations as with other MongoDB cursors. 与其他MongoDB游标一样，指定后续批处理大小以执行OP_GET_MORE操作。A batchSize of 0 means an empty first batch and is useful if you want to quickly get back a cursor or failure message, without doing significant server-side work.batchSize为0表示第一批为空，如果希望在不做大量服务器端工作的情况下快速返回游标或失败消息，则该值非常有用。

Specify a Collation指定排序规则¶

Collation allows users to specify language-specific rules for string comparison, such as rules for lettercase and accent marks.排序规则允许用户为字符串比较指定特定于语言的规则，例如字母大小写和重音符号的规则。

A collection myColl has the following documents:myColl集合包含以下文档：

{ _id: 1, category: "café", status: "A" }
{ _id: 2, category: "cafe", status: "a" }
{ _id: 3, category: "cafE", status: "a" }

The following aggregation operation includes the Collation option:以下聚合操作包括排序规则选项：

db.myColl.aggregate(
   [ { $match: { status: "A" } }, { $group: { _id: "$category", count: { $sum: 1 } } } ],
   { collation: { locale: "fr", strength: 1 } }
);

For descriptions on the collation fields, see Collation Document.有关排序规则字段的说明，请参阅排序规则文档。

Hint an Index提示索引¶

Create a collection foodColl with the following documents:使用以下文档创建集合foodColl：

db.foodColl.insert([
   { _id: 1, category: "cake", type: "chocolate", qty: 10 },
   { _id: 2, category: "cake", type: "ice cream", qty: 25 },
   { _id: 3, category: "pie", type: "boston cream", qty: 20 },
   { _id: 4, category: "pie", type: "blueberry", qty: 15 }
])

Create the following indexes:创建以下索引：

db.foodColl.createIndex( { qty: 1, type: 1 } );
db.foodColl.createIndex( { qty: 1, category: 1 } );

The following aggregation operation includes the hint option to force the usage of the specified index:以下聚合操作包括hint选项，用于强制使用指定的索引：

db.foodColl.aggregate(
   [ { $sort: { qty: 1 }}, { $match: { category: "cake", qty: 10  } }, { $sort: { type: -1 } } ],
   { hint: { qty: 1, category: 1 } }
)

Override Default Read Concern覆盖默认读关注点¶

To override the default read concern level, use the readConcern option. 要覆盖默认的读取关注级别，请使用readConcern选项。The getMore command uses the readConcern level specified in the originating aggregate command.

You cannot use the $out or the $merge stage in conjunction with read concern "linearizable". That is, if you specify "linearizable" read concern for db.collection.aggregate(), you cannot include either stages in the pipeline.

The following operation on a replica set specifies a read concern of "majority" to read the most recent copy of the data confirmed as having been written to a majority of the nodes.以下对副本集的操作指定"majority"的读取关注点，以读取确认已写入多数Node的数据的最新副本。

db.restaurants.aggregate(
   [ { $match: { rating: { $lt: 5 } } } ],
   { readConcern: { level: "majority" } }
)

To ensure that a single thread can read its own writes, use "majority" read concern and "majority" write concern against the primary of the replica set.