db.collection.count()

db.collection.count()

Definition

db.collection.count(query, options)

mongo Shell Method

This page documents the mongo shell method, and doesnot refer to the MongoDB Node.js driver (or any other driver)method. For corresponding MongoDB driver API, refer to your specificMongoDB driver documentation instead.

Note

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

Returns the count of documents that would match afind() query for the collection or view. Thedb.collection.count() method does not perform thefind() operation but instead counts andreturns the number of results that match a query.

Important

Avoid using the db.collection.count() method withouta query predicate since without the query predicate, the methodreturns results based on the collection’s metadata, which mayresult in an approximate count. In particular,
- On a sharded cluster, the resulting count will not correctlyfilter out orphaned documents.
- After an unclean shutdown, the count may beincorrect.
For counts based on collection metadata, see alsocollStats pipeline stage with the countoption.

ParameterTypeDescriptionquerydocumentThe query selection criteria.optionsdocumentOptional. Extra options for modifying the count.

The options document contains the following fields:

FieldTypeDescriptionlimitintegerOptional. The maximum number of documents to count.skipintegerOptional. The number of documents to skip before counting.hintstring or documentOptional. An index name hint or specification for the query.

New in version 2.6.

maxTimeMSintegerOptional. The maximum amount of time to allow the query to run.readConcernstringOptional. Specifies the read concern. The default level is"local".

To use read concern level of "majority", replicasets must use WiredTiger storage engine.

You can disable read concern "majority" for a deploymentwith a three-member primary-secondary-arbiter (PSA) architecture;however, this has implications for change streams (in MongoDB 4.0 andearlier only) and transactions on sharded clusters. For more information,see Disable Read Concern Majority.

To ensure that a single thread can read its own writes, use"majority" read concern and "majority"write concern against the primary of the replica set.

To use a read concern level of "majority", you mustspecify a nonempty query condition.

New in version 3.2.

collationdocumentOptional.

Specifies the collation to use for the operation.

Collation allows users to specifylanguage-specific rules for string comparison, such as rules forlettercase and accent marks.

The collation option has the following syntax:

collation: {
   locale: <string>,
   caseLevel: <boolean>,
   caseFirst: <string>,
   strength: <int>,
   numericOrdering: <boolean>,
   alternate: <string>,
   maxVariable: <string>,
   backwards: <boolean>
}

When specifying collation, the locale field is mandatory; allother collation fields are optional. For descriptions of the fields,see Collation Document.

If the collation is unspecified but the collection has adefault collation (see db.createCollection()), theoperation uses the collation specified for the collection.

If no collation is specified for the collection or for theoperations, MongoDB uses the simple binary comparison used in priorversions for string comparisons.

You cannot specify multiple collations for an operation. Forexample, you cannot specify different collations per field, or ifperforming a find with a sort, you cannot use one collation for thefind and another for the sort.

New in version 3.4.

count() is equivalent to thedb.collection.find(query).count() construct.

Behavior

Count and Transactions

You cannot use count and shell helperscount() and db.collection.count() intransactions.

For details, see Transactions and Count Operations.

Sharded Clusters

On a sharded cluster, db.collection.count() without a query predicate can result in an inaccurate count iforphaned documents exist or if achunk migration is in progress.

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

You can use the $count stage to count the documents. Forexample, 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 theindex 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 onlythe 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 notaccess a single contiguous range of index keys or the query alsocontains conditions on fields outside the index, then in addition tousing the index, MongoDB must also read the documents to return thecount.

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 pagesthe documents into memory such that subsequent calls of the same countoperation will have better performance.

Accuracy after Unexpected Shutdown

After an unclean shutdown of a mongod using the Wired Tiger storage engine, count statistics reported bycount() may be inaccurate.

The amount of drift depends on the number of insert, update, or deleteoperations performed between the last checkpoint and the unclean shutdown. Checkpointsusually occur every 60 seconds. However, mongod instances runningwith non-default —syncdelay settings may have more or less frequentcheckpoints.

Run validate on each collection on the mongodto restore the correct statistics after an unclean shutdown.

Note

This loss of accuracy only applies to count()operations that do not include a query predicate.

Client Disconnection

Starting in MongoDB 4.2, if the client that issued the db.collection.count()disconnects before the operation completes, MongoDB marksthe db.collection.count() for termination (i.e. killOp on theoperation).

Examples

Count all Documents in a Collection

To count the number of all documents in the orders collection, usethe following operation:

db.orders.count()

This operation is equivalent to the following:

db.orders.find().count()

Count all Documents that Match a Query

Count the number of the documents in the orderscollection with the field ord_dt greater than newDate('01/01/2012'):

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

The query is equivalent to the following:

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