See also

cursor.count()

Sharded Clusters

On a sharded cluster, db.collection.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 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:

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

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

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  db.collection.aggregate( [
     { $match: <query condition> },
     { $count: "myCount" }
  ] )
  

  Or, if using the $group + $project equivalent:

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  db.collection.aggregate( [
     { $match: <query condition> },
     { $group: { _id: null, myCount: { $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.

Accuracy after Unexpected Shutdown

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

The amount of drift depends on the number of insert, update, or delete operations performed between the last checkpoint and the unclean shutdown. Checkpoints usually occur every 60 seconds. However, mongod instances running with non-default --syncdelay settings may have more or less frequent checkpoints.

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

Count all Documents in a Collection

To count the number of all documents in the orders collection, use the 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 orders collection with the field ord_dt greater than new Date('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()