cursor.count()¶

On this page

Definition
Behavior
Examples

Definition¶

cursor.count()¶

mongo Shell Method

This page documents the mongo shell method, and does not refer to the MongoDB Node.js driver (or any other driver) method. For corresponding MongoDB driver API, refer to your specific MongoDB driver documentation instead.

Note

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

Counts the number of documents referenced by a cursor. Append the count() method to a find() query to return the number of matching documents. The operation does not perform the query but instead counts the results that would be returned by the query.

Important

Avoid using count() if the find() operation is run without a query predicate since without the query predicate, these count() returns results based on the collection’s metadata, which may result in an approximate count. In particular,
- On a sharded cluster, the resulting count will not correctly filter out orphaned documents.
- After an unclean shutdown, the count may be incorrect.
For counts based on collection metadata, see also collStats pipeline stage with the count option.

The count() method has the following prototype form:

copy

db.collection.find(<query>).count()

The count() method has the following parameter:

Parameter	Type	Description
`applySkipLimit`	boolean	Optional. Specifies whether to consider the effects of the `cursor.skip()` and `cursor.limit()` methods in the count. By default, the `count()` method ignores the effects of the `cursor.skip()` and `cursor.limit()`. Set `applySkipLimit` to `true` to consider the effect of these methods.

MongoDB also provides an equivalent db.collection.count() as an alternative to the db.collection.find(<query>).count() construct.

MongoDB supports the use of hint() with count(). See Specify the Index to Use for an example.

Behavior¶

Count and Transactions¶

You cannot use count and shell helpers count() and db.collection.count() in transactions.

For details, see Transactions and Count Operations.

Sharded Clusters¶

On a sharded cluster, count() without a query predicate in the find can result in an inaccurate count if orphaned documents exist or if a chunk migration is in progress.

To avoid these situations, on a sharded cluster, use the db.collection.aggregate() method:

You can use the $count stage to count the documents. For example, the following operation counts the documents in a collection:

copy

db.collection.aggregate( [
   { $count: "myCount" }
])

The $count stage is equivalent to the following $group + $project sequence:

copy

db.collection.aggregate( [
   { $group: { _id: null, count: { $sum: 1 } } }
   { $project: { _id: 0 } }
] )

Index Use¶

Consider a collection with the following index:

copy

{ a: 1, b: 1 }

When performing a count, MongoDB can return the count using only the index if:

the query can use an index,
the query only contains conditions on the keys of the index, and
the query predicates access a single contiguous range of index keys.

For example, the following operations can return the count using only the index:

copy

db.collection.find( { a: 5, b: 5 } ).count()
db.collection.find( { a: { $gt: 5 } } ).count()
db.collection.find( { a: 5, b: { $gt: 10 } } ).count()

If, however, the query can use an index but the query predicates do not access a single contiguous range of index keys or the query also contains conditions on fields outside the index, then in addition to using the index, MongoDB must also read the documents to return the count.

copy

db.collection.find( { a: 5, b: { $in: [ 1, 2, 3 ] } } ).count()
db.collection.find( { a: { $gt: 5 }, b: 5 } ).count()
db.collection.find( { a: 5, b: 5, c: 5 } ).count()

In such cases, during the initial read of the documents, MongoDB pages the documents into memory such that subsequent calls of the same count operation will have better performance.

Examples¶

The following are examples of the count() method.

Count All Documents¶

The following operation counts the number of all documents in the orders collection:

copy

db.orders.find().count()

Count Documents That Match a Query¶

The following operation counts the number of the documents in the orders collection with the field ord_dt greater than new Date('01/01/2012'):

copy

db.orders.find( { ord_dt: { $gt: new Date('01/01/2012') } } ).count()

Limit Documents in Count¶

The following operation counts the number of the documents in the orders collection with the field ord_dt greater than new Date('01/01/2012') taking into account the effect of the limit(5):

copy

db.orders.find( { ord_dt: { $gt: new Date('01/01/2012') } } ).limit(5).count(true)

Specify the Index to Use¶

The following operation uses the index named "status_1", which has the index key specification of { status: 1 }, to return a count of the documents in the orders collection with the field ord_dt greater than new Date('01/01/2012') and the status field is equal to "D":

copy

db.orders.find(
   { ord_dt: { $gt: new Date('01/01/2012') }, status: "D" }
).hint( "status_1" ).count()

← cursor.comment() cursor.explain() →