count¶

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Definition
Behavior
Examples

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:

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{
  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: copy readConcern: { level: <value> } Possible read concern levels are: `"local"`. This is the default read concern level. `"available"`. This is the default for reads against secondaries when Read Operations and Causally Consistent Sessions and “level” are unspecified. The query returns the instance’s most recent data. `"majority"`. Available for replica sets that use WiredTiger storage engine. `"linearizable"`. Available for read operations on the `primary` only. 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.

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 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:

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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 } }
] )

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.

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:

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

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

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{ "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'):

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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:

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{ "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:

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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:

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{ "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":

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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:

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{ "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 and election protocol version 1.

Starting in MongoDB 3.6, support for read concern "majority" is enabled by default. For MongoDB 3.6.1 - 3.6.x, you can disable read concern "majority". For more information, see Disable Read Concern Majority.
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.

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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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