$bsonSize (aggregation)¶

Return Sizes of Documents文件返回大小¶

From the mongo shell, create a sample collection named employees with the following documents:

 db.employees.insertMany([
   {
     "_id": 1,
     "name": "Alice", "email": "alice@company.com", "position": "Software Developer",
     "current_task": {
       "project_id": 1,
       "project_name": "Aggregation Improvements",
       "project_duration": 5,
       "hours": 20
     }
   },
   {
     "_id": 2,
     "name": "Bob", "email": "bob@company.com", "position": "Sales",
     "current_task": {
       "project_id": 2,
       "project_name": "Write Blog Posts",
       "project_duration": 2,
       "hours": 10,
       "notes": "Progress is slow. Waiting for feedback."
     }
   },
   {
     "_id": 3,
     "name": "Charlie", "email": "charlie@company.com", "position": "HR (On Leave)",
     "current_task": null
   },
   {
     "_id": 4,
     "name": "Dianne", "email": "diane@company.com", "position": "Web Designer",
     "current_task": {
       "project_id": 3,
       "project_name": "Update Home Page",
       "notes": "Need to scope this project."
     }
   }
]);

The following aggregation projects:

• The name field
• The object_size field, which uses $bsonSize to return the size of the document in bytes. The $$ROOT variable references the document currently being processed by the pipeline. To learn more about variables in the aggregation pipeline, see Variables in Aggregation Expressions.

db.employees.aggregate([
  {
    "$project": {
      "name": 1,
      "object_size": { $bsonSize: "$$ROOT" }
    }
  }
])

The operation returns the following result:操作返回以下结果：

{ "_id" : 1, "name" : "Alice", "object_size" : 222 }
{ "_id" : 2, "name" : "Bob", "object_size" : 248 }
{ "_id" : 3, "name" : "Charlie", "object_size" : 112 }
{ "_id" : 4, "name" : "Dianne", "object_size" : 207 }

Return Combined Size of All Documents in a Collection返回集合中所有文档的组合大小¶

The following pipeline returns the combined size of all of the documents in the employees collection:以下管道返回employees集合中所有文档的合并大小：

db.employees.aggregate([
  {
    "$group": {
      "_id": null,
      "combined_object_size": { $sum: { $bsonSize: "$$ROOT" } }
    }
  }
])

When you specify an $group _id value of null, or any other constant value, the $group stage calculates accumulated values for all the input documents as a whole.当您将$group _id值指定为null或任何其他常量值时，$group阶段将计算所有输入文档作为一个整体的累积值。

The operation uses the $sum operator to calculate the combined $bsonSize of each document in the collection. The $$ROOT variable references the document currently being processed by the pipeline. To learn more about variables in the aggregation pipeline, see Variables in Aggregation Expressions.

The operation returns the following result:操作返回以下结果：

{ "_id" : null, "combined_object_size" : 789 }

Return Document with Largest Specified Field¶

The following pipeline returns the document with the largest current_task field in bytes:

db.employees.aggregate([
   // First Stage
   { $project: { name: "$name", task_object_size: { $bsonSize: "$current_task" } }  },
   // Second Stage
   { $sort: { "task_object_size" : -1 } },
   // Third Stage
   { $limit: 1 }
])

First Stage

The first stage of the pipeline projects:

• The name field
• The task_object_size field, which uses $bsonSize to return the size of the document’s current_task field in bytes.

This stage outputs the following documents to the next stage:

{ "_id" : 1, "name" : "Alice", "task_object_size" : 109 }
{ "_id" : 2, "name" : "Bob", "task_object_size" : 152 }
{ "_id" : 3, "name" : "Charlie", "task_object_size" : null }
{ "_id" : 4, "name" : "Dianne", "task_object_size" : 99 }

Second Stage

The second stage sorts the documents by task_object_size in descending order.

This stage outputs the following documents to the next stage:

{ "_id" : 2, "name" : "Bob", "task_object_size" : 152 }
{ "_id" : 1, "name" : "Alice", "task_object_size" : 109 }
{ "_id" : 4, "name" : "Dianne", "task_object_size" : 99 }
{ "_id" : 3, "name" : "Charlie", "task_object_size" : null }

Third Stage

The third stage limits the output documents to only return the document appearing first in the sort order:

{ "_id" : 2, "name" : "Bob", "task_object_size" : 152 }