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count

Definition

count

Counts the number of documents in a collection or a view. Returns a document that contains this count and as well as the command status.

count has the following form:

{
  count: <collection or view>,
  query: <document>,
  limit: <integer>,
  skip: <integer>,
  hint: <hint>,
  readConcern: <document>
}

count has the following fields:

Field Type Description
count string The name of the collection or view to count.
query document Optional. A query that selects which documents to count in the collection or view.
limit integer Optional. The maximum number of matching documents to return.
skip integer Optional. The number of matching documents to skip before returning results.
hint string or document

Optional. The index to use. Specify either the index name as a string or the index specification document.

New in version 2.6.

readConcern document

Optional. Specifies the read concern. The option has the following syntax:

readConcern: { level: <value> }

Possible read concern levels are:

For more formation on the read concern levels, see Read Concern Levels.

For "local" (default) or "majority" read concern level, you can specify the afterClusterTime option to have the read operation return data that meets the level requirement and the specified after cluster time requirement. For more information, see Read Operations and Causally Consistent Sessions.

MongoDB also provides the count() and db.collection.count() wrapper methods in the mongo shell.

Important

  • Avoid using the count and its wrapper methods without a query predicate since without the query predicate, these operations return results based on the collection’s metadata, which may result in an approximate count. In particular, the resulting count will not correctly filter out orphaned documents in a sharded cluster.
  • For counts based on collection metadata, the collStats pipeline stage with the count option is preferred over count and its wrapper methods.

Behavior

On a sharded cluster, the count command when run without a query predicate 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" }
   ]
)
  • You can use the $group stage to $sum the documents. For example, the following operation counts the documents in a collection:
db.collection.aggregate(
   [
      { $group: { _id: null, count: { $sum: 1 } } }
   ]
)

See also

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

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 to restore the correct statistics after an unclean shutdown.

Note

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

Examples

The following sections provide examples of the count command.

Count All Documents

The following operation counts the number of all documents in the orders collection:

db.runCommand( { count: 'orders' } )

In the result, the n, which represents the count, is 26, and the command status ok is 1:

{ "n" : 26, "ok" : 1 }

Count Documents That Match a Query

The following operation returns a count of the documents in the orders collection where the value of the ord_dt field is greater than Date('01/01/2012'):

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

In the result, the n, which represents the count, is 13 and the command status ok is 1:

{ "n" : 13, "ok" : 1 }

Skip Documents in Count

The following operation returns a count of the documents in the orders collection where the value of the ord_dt field is greater than Date('01/01/2012') and skip the first 10 matching documents:

db.runCommand( { count:'orders',
                 query: { ord_dt: { $gt: new Date('01/01/2012') } },
                 skip: 10 }  )

In the result, the n, which represents the count, is 3 and the command status ok is 1:

{ "n" : 3, "ok" : 1 }

Specify the Index to Use

The following operation uses the index { status: 1 } to return a count of the documents in the orders collection where the value of the ord_dt field is greater than Date('01/01/2012') and the status field is equal to "D":

db.runCommand(
   {
     count:'orders',
     query: {
              ord_dt: { $gt: new Date('01/01/2012') },
              status: "D"
            },
     hint: { status: 1 }
   }
)

In the result, the n, which represents the count, is 1 and the command status ok is 1:

{ "n" : 1, "ok" : 1 }

Override Default Read Concern

To override the default read concern level of "local", use the readConcern option.

The following operation on a replica set specifies a Read Concern of "majority" to read the most recent copy of the data confirmed as having been written to a majority of the nodes.

Important

  • To use read concern level of "majority", replica sets must use WiredTiger storage engine.
  • To use the readConcern level of "majority", you must specify a nonempty query condition.
  • Regardless of the read concern level, the most recent data on a node may not reflect the most recent version of the data in the system.
db.runCommand(
   {
     count: "restaurants",
     query: { rating: { $gte: 4 } },
     readConcern: { level: "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.

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