Count does not filter out unowned (orphaned) documents and can therefore report larger values than one will find via a normal query, or using itcount() in the shell.
The following conditions can lead to counts being off:
- Active migrations
- Orphaned documents (left from failed migrations)
- Non-Primary read preferences (see
A workaround to get accurate counts is to ensure all migrations have been cleaned up and no migrations are active. To query non-primaries you must also ensure that there is no replication lag including any migration data, in addition to the above requirements.
For issues with counts/reads from non-primaries please see
A "fast count" is a count run without a predicate. It is "fast" because the implementation only reads the metadata, without fetching any documents.
The problem of count() reporting inaccurate results has been fixed for non-"fast counts," that is, starting in 4.0, counts which are run with a predicate are accurate when run on sharded clusters. "Fast counts" (count() run without a predicate) may still report too many documents (see SERVER-33753).
In general, if one needs an accurate count of how many documents are in a collection, we do not recommend using the count command. Instead, we suggest using the $count aggregation stage, like this:
See the docs.
For users who need the performance of "fast count", and are okay with approximate results, we suggest using $collStats instead of the count command: