In MongoDB the db.collection.estimatedDocumentCount() method returns the count of all documents in a collection or view.

The collection part is the name of the collection or view to perform the count operation on.

The db.collection.estimatedDocumentCount() method wraps the count command.

Example

Suppose we have a collection called pets with the following documents:

{ "_id" : 1, "name" : "Wag", "type" : "Dog", "weight" : 20 }
{ "_id" : 2, "name" : "Bark", "type" : "Dog", "weight" : 10 }
{ "_id" : 3, "name" : "Meow", "type" : "Cat", "weight" : 7 }
{ "_id" : 4, "name" : "Scratch", "type" : "Cat", "weight" : 8 }
{ "_id" : 5, "name" : "Bruce", "type" : "Bat", "weight" : 3 }
{ "_id" : 6, "name" : "Fetch", "type" : "Dog", "weight" : 17 }
{ "_id" : 7, "name" : "Jake", "type" : "Dog", "weight" : 30 }

We can use the following query to return the number of documents in the collection:

db.pets.estimatedDocumentCount()

Result:

7

The db.collection.estimatedDocumentCount() method doesn’t take a query filter. It instead uses metadata to return the document count for the whole collection.

However, it still works when provided with an empty document.

db.pets.estimatedDocumentCount({})

Result:

7

The maxTimeMS Parameter

The db.collection.estimatedDocumentCount() method accepts just one (optional) parameter: the maxTimeMS parameter. This allows you to set the maximum amount of time that the count operation can run.

Example:

db.pets.estimatedDocumentCount({}, { maxTimeMS: 5000 })

Result:

7

Deprecation of count() in Favour of estimatedDocumentCount()

Note that the MongoDB documentation states the following:

MongoDB drivers compatible with the 4.0 features deprecate their respective cursor and collection count() APIs in favor of new APIs for countDocuments() and estimatedDocumentCount(). For the specific API names for a given driver, see the driver documentation.

More Information

On a sharded cluster, the resulting count will not correctly filter out orphaned documents.

Also, after an unclean shutdown, the count may be incorrect.

See the MongoDB documentation for more information.