db.collection.estimatedDocumentCount()

Mechanics机械师

db.collection.estimatedDocumentCount() does not take a query filter and instead uses metadata to return the count for a collection.不采用查询筛选器，而是使用元数据返回集合的计数。

For a view:对于视图：

• There is no metadata.没有元数据。
• The document count is calculated by executing the aggregation pipeline in the view definition.文档计数是通过执行视图定义中的聚合管道来计算的。
• There is no fast estimated document count.没有快速估计的文档计数。

Sharded Clusters分片集群

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。

Unclean Shutdown不清洁关闭

This section only applies to collections.本节仅适用于集合。

After an unclean shutdown, the count may be incorrect.不干净关机后，计数可能不正确。

After an unclean shutdown of a mongod using the Wired Tiger storage engine, count statistics reported by db.collection.estimatedDocumentCount() may be inaccurate.在使用Wired Tiger存储引擎不干净地关闭mongod之后，db.collection.estimatedDocumentCount()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 output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 output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.estimatedDocumentCount() disconnects before the operation completes, MongoDB marks db.collection.estimatedDocumentCount() for termination using killOp.从MongoDB 4.2开始，如果发出db.collection.estimatedDocumentCount()的客户端在操作完成前断开连接，MongoDB会使用killOp标记db.collection.estimatedDocumentCount()终止。

Count and Transactions计数和事务

When you use db.collection.estimatedDocumentCount() in a transaction, the resulting count will not filter out any uncommitted multi-document transactions.在事务中使用db.collection.estimatedDocumentCount()时，生成的计数将不会筛选掉任何未提交的多文档事务。