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Aggregation Pipeline Operators

$binarySize (aggregation)

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  • Definition
  • Behavior
  • Example

Definition

$binarySize

New in version 4.4.

Returns the size of a given string or binary data value's content in bytes.

$binarySize has the following syntax:

{ $binarySize: <string or binData> }

The argument can be any valid expression as long as it resolves to either a string or binary data value. For more information on expressions, see Expressions.

Behavior

The argument for $binarySize must resolve to either:

  • A string,

  • A binary data value, or

  • null.

If the argument is a string or binary data value, the expression returns the size of the argument in bytes.

If the argument is null, the expression returns null.

If the argument resolves to any other data type, $binarySize errors.

String Size Calculation

If the argument for $binarySize is a string, the operator counts the number of UTF-8 encoded bytes in a string where each character may use between one and four bytes.

For example, US-ASCII characters are encoded using one byte. Characters with diacritic markings and additional Latin alphabetical characters (i.e. Latin characters outside of the English alphabet) are encoded using two bytes. Chinese, Japanese and Korean characters typically require three bytes, and other planes of unicode (emoji, mathematical symbols, etc.) require four bytes.

Consider the following examples:

Example
Results
Notes
{ $binarySize: "abcde" }
5
Each character is encoded using one byte.
{ $binarySize: "Hello World!" }
12
Each character is encoded using one byte.
{ $binarySize: "cafeteria" }
9
Each character is encoded using one byte.
{ $binarySize: "cafétéria" }
11
é is encoded using two bytes.
{ $binarySize: "" }
0
Empty strings return 0.
{ $binarySize: "$€λG" }
7
€ is encoded using three bytes. λ is encoded using two bytes.
{ $binarySize: "寿司" }
6
Each character is encoded using three bytes.

Example

In mongosh, create a sample collection named images with the following documents:

db.images.insertMany([
  { _id: 1, name: "cat.jpg", binary: new BinData(0, "OEJTfmD8twzaj/LPKLIVkA==")},
  { _id: 2, name: "big_ben.jpg", binary: new BinData(0, "aGVsZmRqYWZqYmxhaGJsYXJnYWZkYXJlcTU1NDE1Z2FmZCBmZGFmZGE=")},
  { _id: 3, name: "tea_set.jpg", binary: new BinData(0, "MyIRAFVEd2aImaq7zN3u/w==")},
  { _id: 4, name: "concert.jpg", binary: new BinData(0, "TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlzIHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2YgdGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY29udGludWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdlLCBleGNlZWRzIHRoZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBwbGVhc3VyZS4=")},
  { _id: 5, name: "empty.jpg", binary: new BinData(0, "") }
])

The following aggregation projects:

  • The name field

  • The imageSize field, which uses $binarySize to return the size of the document's binary field in bytes.

db.images.aggregate([
  {
    $project: {
      "name": "$name",
      "imageSize": { $binarySize: "$binary" }
    }
  }
])

The operation returns the following result:

{ "_id" : 1, "name" : "cat.jpg", "imageSize" : 16 }
{ "_id" : 2, "name" : "big_ben.jpg", "imageSize" : 41 }
{ "_id" : 3, "name" : "teaset.jpg", "imageSize" : 16 }
{ "_id" : 4, "name" : "concert.jpg", "imageSize" : 269 }
{ "_id" : 5, "name" : "empty.jpg", "imageSize" : 0 }

Find Largest Binary Data

The following pipeline returns the image with the largest binary data size:

db.images.aggregate([
   // First Stage
   { $project: { name: "$name", imageSize: { $binarySize: "$binary" } }  },
   // Second Stage
   { $sort: { "imageSize" : -1 } },
   // Third Stage
   { $limit: 1 }
])
First Stage

The first stage of the pipeline projects:

  • The name field

  • The imageSize field, which uses $binarySize to return the size of the document's binary field in bytes.

This stage outputs the following documents to the next stage:

{ "_id" : 1, "name" : "cat.jpg", "imageSize" : 16 }
{ "_id" : 2, "name" : "big_ben.jpg", "imageSize" : 41 }
{ "_id" : 3, "name" : "teaset.jpg", "imageSize" : 16 }
{ "_id" : 4, "name" : "concert.jpg", "imageSize" : 269 }
{ "_id" : 5, "name" : "empty.jpg", "imageSize" : 0 }
Second Stage

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

This stage outputs the following documents to the next stage:

{ "_id" : 4, "name" : "concert.jpg", "imageSize" : 269 }
{ "_id" : 2, "name" : "big_ben.jpg", "imageSize" : 41 }
{ "_id" : 1, "name" : "cat.jpg", "imageSize" : 16 }
{ "_id" : 3, "name" : "teaset.jpg", "imageSize" : 16 }
{ "_id" : 5, "name" : "empty.jpg", "imageSize" : 0 }
Third Stage

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

{ "_id" : 4, "name" : "concert.jpg", "imageSize" : 269 }

Tip

See also:

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$avg (aggregation)