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cursor.count()

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  • Definition
  • Behavior
  • Examples

Definition

cursor.count()

Important

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:

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.

Tip

See also:

cursor.size()

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:

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

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

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

Tip

See also:

$collStats to return an approximate count based on the collection's metadata.

Index Use

Consider a collection with the following index:

{ 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:

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.

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:

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'):

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):

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":

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

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