count

Command Fields命令字段

count has the following fields:具有以下字段：

readConcern: { level: <value> }

collation: {
   locale: <string>,
   caseLevel: <boolean>,
   caseFirst: <string>,
   strength: <int>,
   numericOrdering: <boolean>,
   alternate: <string>,
   maxVariable: <string>,
   backwards: <boolean>
}

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. Without a query predicate, count commands return results based on the collection's metadata, which may result in an approximate count. In particular,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。如果没有查询谓词，count命令将根据集合的元数据返回结果，这可能导致近似计数。具体来说，

• 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

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

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

Accuracy and Sharded Clusters准确性和分片群集

On a sharded cluster, the count command when run 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，则在没有查询谓词的情况下运行count命令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 } }
] )

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. However, mongod instances running with non-default --syncdelay settings may have more or less frequent checkpoints.检查点通常每60秒出现一次。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，以在非干净关闭后恢复统计信息

After an unclean shutdown:非干净关闭之后：

• 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 count disconnects before the operation completes, MongoDB marks count for termination using killOp.从MongoDB 4.2开始，如果在操作完成之前发出count的客户端断开连接，MongoDB会使用killOp标记count终止。

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.runCommand( { count: 'orders' } )

In the result, the n, which represents the count, is 26, and the command status ok is 1:结果，表示计数的n为26，命令状态ok为1：

{ "n" : 26, "ok" : 1 }

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

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'):以下操作返回orders集合中ord_dt字段的值大于Date('01/01/2012')的文档数：

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:结果，表示计数的n为13，命令状态ok为1：

{ "n" : 13, "ok" : 1 }

Skip Documents in Count跳过计数中的文档

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:以下操作返回orders集合中ord_dt字段的值大于Date('01/01/2012')的文档的计数，并跳过前10个匹配的文档：

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:结果，表示计数的n为3，命令状态ok为1：

{ "n" : 3, "ok" : 1 }

Specify the Index to Use指定要使用的索引

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":以下操作使用索引{ status: 1 }返回orders集合中ord_dt字段的值大于Date('01/01/2012')且status字段等于"D"的文档的计数：

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:结果，表示计数的n为1，命令状态ok为1：

{ "n" : 1, "ok" : 1 }

Override Default Read Concern覆盖默认读取关注

To override the default read concern level of "local", use the readConcern option.要覆盖默认的读取关注级别"local"，请使用readConcern选项。

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.以下对副本集的操作指定"majority"读取关注，以读取已确认写入大多数节点的数据的最新副本。

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.要确保单个线程可以读取自己的写入，请对副本集的主线程使用"majority"读取关注和"majority"写入关注。