count¶

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Definition
Behavior
Examples

Definition¶

count¶

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

count has the following form:

{
  count: <collection-name>,
  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 to count.
`query`	document	Optional. A query that selects which documents to count in a collection.
`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 default level is `"local"`. To use a read concern level of `"majority"`, you must use the WiredTiger storage engine and start the `mongod` instances with the --enableMajorityReadConcern command line option (or the `replication.enableMajorityReadConcern` setting if using a configuration file). Only replica sets using `protocol version 1` support `"majority"` read concern. Replica sets running protocol version 0 do not support `"majority"` read concern. 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 must specify a nonempty `query` condition. New in version 3.2.

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

Behavior¶

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.

For MongoDB instances using the WiredTiger storage engine, after an unclean shutdown, statistics on size and count may off by up to 1000 documents as reported by collStats, dbStats, count. To restore the correct statistics for the collection, run validate on the collection.

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 a read concern level of "majority", you must use the WiredTiger storage engine and start the mongod instances with the --enableMajorityReadConcern command line option (or the replication.enableMajorityReadConcern setting if using a configuration file).

Only replica sets using protocol version 1 support "majority" read concern. Replica sets running protocol version 0 do not support "majority" read concern.
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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