count (database command)

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,当您在没有查询谓词的情况下调用count时，您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，产生的计数将无法正确筛选掉孤立的文档。
• After an unclean shutdown or file copy based initial sync, the count may be incorrect.在不干净的关机或基于文件副本的初始同步后，计数可能不正确。

For counts based on collection metadata, see also collStats pipeline stage with the count option.关于基于集合元数据的计数，请参阅带有计数选项的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，产生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精度和分片集群

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:

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

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 restore statistics after an unclean shutdown.

After an unclean shutdown:

• validate updates the count statistic in the collStats output with the latest value.
• Other statistics like the number of documents inserted or removed in the collStats output are estimates.

Client Disconnection

If the client that issued count disconnects before the operation completes, MongoDB marks count for termination using killOp.

Count All Documents

The following operation counts the number of all documents in the orders collection:

db.runCommand( { count: 'orders' } )

In the result, the n, which represents the count, is 26, and the command status ok is 1:

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

Count Documents That Match a Query

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字段值大于Date('01/01/2012')的文档计数：

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

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:

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:

{ "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":

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:

{ "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.

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.