cursor.count()

Inaccurate Counts Without Query Predicate

When you call count() on a find() operation which does not specify a query predicate, the count() method can return inaccurate document counts. These count() methods return 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 or file copy based initial sync, the count may be incorrect.

For counts based on collection metadata, see also collStats pipeline stage with the count option.

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 } }
] )

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.

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()