MongoDB在分片集群上count统计出的数量可能会不正确

On a sharded cluster, count can result in an inaccurate count if orphaned documents exist or if a chunk migration is in progress.

To avoid these situations, on a sharded cluster, use the $group stage of the db.collection.aggregate() method to $sum the documents. For example, the following operation counts the documents in a collection:

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

To get a count of documents that match a query condition, include the $match stage as well:

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

参见文档：https://docs.mongodb.com/manual/reference/command/count/