db.collection.count()¶

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Definition
Behavior
Examples

Definition¶

db.collection.count(query, options)¶

Returns the count of documents that would match a find() query. The db.collection.count() method does not perform the find() operation but instead counts and returns the number of results that match a query.

Parameter	Type	Description
`query`	document	The query selection criteria.
`options`	document	Optional. Extra options for modifying the count.

The options document contains the following fields:

Field	Type	Description
`limit`	integer	Optional. The maximum number of documents to count.
`skip`	integer	Optional. The number of documents to skip before counting.
`hint`	string or document	Optional. An index name hint or specification for the query. 2.6 新版功能.
`maxTimeMS`	integer	Optional. The maximum amount of time to allow the query to run.
`readConcern`	string	Optional. Specifies the read concern. The default level is `"local"`. To use read concern level of `"majority"`, you must start the `mongod` instances with the --enableMajorityReadConcern command line option (or the `replication.enableMajorityReadConcern` set to `true` if using a configuration file). replica sets must use WiredTiger storage engine and election `protocol version 1`. 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. 3.2 新版功能.

count() is equivalent to the db.collection.find(query).count() construct.

参见

cursor.count()

Behavior¶

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 $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.

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 to restore the correct statistics after an unclean shutdown.

注解

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

Examples¶

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()

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