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$bsonSize (aggregation)![](/docs/v5.2/assets/link.svg)
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Definition![](/docs/v5.2/assets/link.svg)
$bsonSize
New in version 4.4.
Returns the size in bytes of a given document (i.e. bsontype
Object
) when encoded as BSON. You can use$bsonSize
as an alternative to the Object.bsonSize() method.$bsonSize
has the following syntax:{ $bsonSize: <object> } The argument can be any valid expression as long as it resolves to either an object or
null
. For more information on expressions, see Expressions.
Behavior![](/docs/v5.2/assets/link.svg)
If the argument is an object, the expression returns the size of the object in bytes when the object is encoded as BSON.
If the argument is null
, the expression returns null
.
If the argument resolves to a data type other than an object or
null
, $bsonSize
errors.
Examples![](/docs/v5.2/assets/link.svg)
Return Sizes of Documents![](/docs/v5.2/assets/link.svg)
In mongosh
, create a sample collection named
employees
with the following documents:
db.employees.insertMany([ { "_id": 1, "name": "Alice", "email": "alice@company.com", "position": "Software Developer", "current_task": { "project_id": 1, "project_name": "Aggregation Improvements", "project_duration": 5, "hours": 20 } }, { "_id": 2, "name": "Bob", "email": "bob@company.com", "position": "Sales", "current_task": { "project_id": 2, "project_name": "Write Blog Posts", "project_duration": 2, "hours": 10, "notes": "Progress is slow. Waiting for feedback." } }, { "_id": 3, "name": "Charlie", "email": "charlie@company.com", "position": "HR (On Leave)", "current_task": null }, { "_id": 4, "name": "Dianne", "email": "diane@company.com", "position": "Web Designer", "current_task": { "project_id": 3, "project_name": "Update Home Page", "notes": "Need to scope this project." } } ]);
The following aggregation projects
:
The
name
fieldThe
object_size
field, which uses$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:
{ "_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![](/docs/v5.2/assets/link.svg)
The following pipeline returns the combined size of all of the
documents in the employees
collection:
db.employees.aggregate([ { "$group": { "_id": null, "combined_object_size": { $sum: { $bsonSize: "$$ROOT" } } } } ])
When you specify an $group
_id value of null
, or any
other constant value, the $group
stage calculates
accumulated values for all the input documents as a whole.
The operation uses the $sum
operator to calculate the combined
$bsonSize
of each document in the collection. The
$$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:
{ "_id" : null, "combined_object_size" : 789 }
Return Document with Largest Specified Field![](/docs/v5.2/assets/link.svg)
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 { $limit: 1 } ])
- First Stage
The first stage of the pipeline
projects
:The
name
fieldThe
task_object_size
field, which uses$bsonSize
to return the size of the document'scurrent_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 bytask_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 }