cursor.count()¶

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Definition
Behavior
Examples

Definition¶

cursor.count()¶

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.

Changed in version 2.6: MongoDB supports the use of hint() with count(). See Specify the Index to Use for an example.

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.

Behavior¶

Sharded Clusters¶

On a sharded cluster, count() 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 $group stage of the db.collection.aggregate() method to $sum the documents. For example, the following operation counts the documents in a collection:

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

To get a count of documents that match a query condition, include the $match stage as well:

db.collection.aggregate(
   [
      { $match: <query condition> },
      { $group: { _id: null, count: { $sum: 1 } } }
   ]
)

See Perform a Count for an example.

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()