db.collection.count()

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

When you call count() without a query predicate, you may receive inaccurate document counts. 如果在没有查询谓词的情况下调用count()，则可能会收到不准确的文档计数。Without a query predicate, count() methods return results based on the collection's metadata, which may result in an approximate count. In particular,如果没有查询谓词，count()方法会根据集合的元数据返回结果，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, db.collection.count() without a query predicate 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，则没有查询谓词的db.collection.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。

Accuracy after Unexpected Shutdown意外停机后的准确性

After an unclean shutdown of a mongod using the Wired Tiger storage engine, count statistics reported by count() may be inaccurate.在使用Wired Tiger存储引擎不干净地关闭mongod之后，count()报告的计数统计数据可能不准确。

The amount of drift depends on the number of insert, update, or delete operations performed between the last checkpoint and the unclean shutdown. 漂移量取决于在最后一个检查点和非干净关闭之间执行的插入、更新或删除操作的数量。Checkpoints usually occur every 60 seconds. 检查点通常每60秒出现一次。However, mongod instances running with non-default --syncdelay settings may have more or less frequent checkpoints.然而，使用非默认--syncdelay设置运行的mongod实例可能或多或少有频繁的检查点。

Run validate on each collection on the mongod to restore statistics after an unclean shutdown.对mongod上的每个集合运行validate，以在不干净的关闭后恢复统计信息。

After an unclean shutdown:不干净关机后：

• validate updates the count statistic in the collStats output with the latest value.使用最新值更新collStats输出中的计数统计信息。
• Other statistics like the number of documents inserted or removed in the collStats output are estimates.其他统计数据，如collStats输出中插入或删除的文档数量，都是估计值。

Client Disconnection客户端断开连接

Starting in MongoDB 4.2, if the client that issued db.collection.count() disconnects before the operation completes, MongoDB marks db.collection.count() for termination using killOp.从MongoDB 4.2开始，如果发出db.collection.count()的客户端在操作完成之前断开连接，MongoDB会使用killOp标记db.collection.count()终止。

Count all Documents in a Collection统计集合中的所有文档

To count the number of all documents in the orders collection, use the following operation:要计算orders集合中所有文档的数量，请使用以下操作：

db.orders.count()

This operation is equivalent to the following:此操作等效于以下操作：

db.orders.find().count()

Count all Documents that Match a Query统计与查询匹配的所有文档

Count the number of the documents in the orders collection with the field ord_dt greater than new Date('01/01/2012'):统计orders集合中字段ord_dt大于new Date('01/01/2012')的文档数：

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

The query is equivalent to the following:该查询等效于以下内容：

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