Get the size of all the documents in a MongoDB query?

To get the size of all the documents in a MongoDB query, you need to loop through the documents and calculate their cumulative BSON size using Object.bsonsize() method.

Syntax

var cursor = db.collection.find();
var totalSize = 0;
cursor.forEach(function(doc) {
    totalSize += Object.bsonsize(doc);
});
print(totalSize);

Sample Data

db.sizeOfAllDocumentsDemo.insertMany([
    {
        "StudentFirstName": "John",
        "StudentSubject": ["MongoDB", "Java"]
    },
    {
        "StudentFirstName": "Larry",
        "StudentSubject": ["MySQL", "PHP"],
        "StudentAge": 21
    },
    {
        "StudentFirstName": "Chris",
        "StudentSubject": ["SQL Server", "C#"],
        "StudentAge": 23,
        "StudentCountryName": "US"
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cd3c0c1edc6604c74817cd7"),
        ObjectId("5cd3c0d9edc6604c74817cd8"),
        ObjectId("5cd3c0fbedc6604c74817cd9")
    ]
}

Example: Calculate Total Documents Size

Let us calculate the cumulative size of all documents in the collection ?

var allDocument = db.sizeOfAllDocumentsDemo.find();
var counter = 0;
allDocument.forEach(
    function(myDocument) {
        counter = counter + Object.bsonsize(myDocument)
    }
);
print(counter);

358

How It Works

• find() returns a cursor to all documents in the collection
• forEach() iterates through each document in the cursor
• Object.bsonsize() returns the BSON size of each document in bytes
• The sizes are accumulated in the counter variable

Conclusion

Use Object.bsonsize() with forEach() to calculate the total size of all documents returned by a MongoDB query. The result shows the cumulative BSON size in bytes.