cursor.count()

Inaccurate Counts Without Query Predicate没有查询谓词的计数不准确

When you call count() on a find() operation which does not specify a query predicate, the count() method can return inaccurate document counts. 当对未指定查询谓词的find()操作调用count()时，count()方法可能会返回不准确的文档计数。These count() methods return results based on the collection's metadata, which may result in an approximate count. In particular,这些count()方法返回基于集合元数据的结果，这可能会导致近似计数。特别地，

• On a sharded cluster, the resulting count will not correctly filter out orphaned documents.在分片集群上，生成的计数将无法正确筛选出孤立文档。
• After an unclean shutdown or file copy based initial sync, the count may be incorrect.在不干净的关闭或基于文件副本的初始同步之后，计数可能不正确。

For counts based on collection metadata, see also collStats pipeline stage with the count option.有关基于集合元数据的计数，请参阅带有count选项的collStats管道阶段。

Count and Transactions计数和事务

You cannot use count and shell helpers count() and db.collection.count() in transactions.不能在事务中使用count和shell助手count()以及db.collection.count()。

For details, see Transactions and Count Operations.有关详细信息，请参阅事务和计数操作。

Sharded Clusters分片集群

On a sharded cluster, count() without a query predicate in the find can result in an inaccurate count if orphaned documents exist or if a chunk migration is in progress.在分片集群上，如果存在孤立文档或正在进行块迁移，则find中没有查询谓词的count()可能会导致计数不准确。

To avoid these situations, on a sharded cluster, use the db.collection.aggregate() method:为了避免这些情况，在分片集群上，使用db.collection.aggregate()方法：

You can use the $count stage to count the documents. For example, the following operation counts the documents in a collection:您可以使用$count阶段对文档进行计数。例如，以下操作对集合中的文档进行计数：

db.collection.aggregate( [
   { $count: "myCount" }
])

The $count stage is equivalent to the following $group + $project sequence:$count阶段相当于以下$group + $project序列：

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

Index Use索引使用

Consider a collection with the following index:考虑具有以下索引的集合：

{ a: 1, b: 1 }

When performing a count, MongoDB can return the count using only the index if:在执行计数时，MongoDB可以仅使用索引返回计数，如果：

• the query can use an index,查询可以使用索引，
• the query only contains conditions on the keys of the index, and查询仅包含索引键上的条件，并且
• the query predicates access a single contiguous range of index keys.查询谓词访问单个连续范围的索引键。

For example, the following operations can return the count using only the index:例如，以下操作可以仅使用索引返回计数：

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.但是，如果查询可以使用索引，但查询谓词不访问单个连续范围的索引键，或者查询还包含索引外字段的条件，那么除了使用索引之外，MongoDB还必须读取文档以返回计数。

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.在这种情况下，在最初读取文档的过程中，MongoDB会将文档分页到内存中，这样同一计数操作的后续调用将具有更好的性能。

Count All Documents统计所有文档

The following operation counts the number of all documents in the orders collection:以下操作统计orders集合中所有文档的数量：

db.orders.find().count()

Count Documents That Match a Query计数与查询匹配的文档

The following operation counts the number of the documents in the orders collection with the field ord_dt greater than new Date('01/01/2012'):以下操作统计orders集合中字段ord_dt大于new Date('01/01/2012')的文档数：

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

Limit Documents in Count限制计数中的文档

The following operation counts the number of the documents in the orders collection with the field ord_dt greater than new Date('01/01/2012') taking into account the effect of the limit(5):以下操作统计orders集合中字段ord_dt大于new Date('01/01/2012')的文档数，同时考虑limit(5)的影响：

db.orders.find( { ord_dt: { $gt: new Date('01/01/2012') } } ).limit(5).count(true)

Specify the Index to Use指定要使用的索引

The following operation uses the index named "status_1", which has the index key specification of { status: 1 }, to return a count of the documents in the orders collection with the field ord_dt greater than new Date('01/01/2012') and the status field is equal to "D":以下操作使用名为"status_1"的索引（其索引键规范为{ status: 1 }）返回订单集合中字段ord_dt大于new Date('01/01/2012')且status字段等于"D"的文档计数：

db.orders.find(
   { ord_dt: { $gt: new Date('01/01/2012') }, status: "D" }
).hint( "status_1" ).count()