Document Size in MongoDb

Question

Thinking of switching to DynamoDB in future, but want to make sure my documents are under the 1KB, as they charge per KB. Is there a quick way to know how large a document is in a collection?

Solution

As a general guide you can check the average size of documents in a collection collname using the avgObjSize value reported by collection stats():

db.collname.stats()

To find and count large documents you can use something similar to:

var maxSize = 1024;
var bigDocs = 0;
db.collname.find().forEach(
    function (doc) {
        var docSize = Object.bsonsize(doc);
        if (docSize >= maxSize) {
            bigDocs++;
            print(doc._id + ' is ' + docSize + ' bytes');
        }
    }
)
print("Found " + bigDocs + " documents bigger than " + maxSize + " bytes")

Note that both these examples are using the MongoDB BSON representation, which will vary from the size required to represent the same data in other databases.