db.collection.count()
On this page
Definition
-
db.collection.
count
( query, options ) -
Returns the count of documents that would match a
find()
query for the collection or view. Thedb.collection.count()
method does not perform thefind()
operation but instead counts and returns the number of results that match a query.Parameter Type Description query
document The query selection criteria. options
document Optional. Extra options for modifying the count. The
options
document contains the following fields:Field Type Description limit
integer Optional. The maximum number of documents to count. skip
integer Optional. The number of documents to skip before counting. hint
string or document Optional. An index name hint or specification for the query.
New in version 2.6.
maxTimeMS
integer Optional. The maximum amount of time to allow the query to run. readConcern
string Optional. Specifies the read concern. The default level is
"local"
.To use read concern level of
"majority"
, replica sets must use WiredTiger storage engine and electionprotocol version 1
.Starting in MongoDB 3.6, support for read concern
"majority"
is enabled by default. For MongoDB 3.6.1 - 3.6.x, you can disable read concern"majority"
. For more information, see Disable Read Concern 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.To use a read concern level of
"majority"
, you must specify a nonemptyquery
condition.New in version 3.2.
count()
is equivalent to thedb.collection.find(query).count()
construct.
See also
Behavior
Sharded Clusters
On a sharded cluster, db.collection.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 db.collection.aggregate()
method:
You can use the
$count
stage to count the documents. For example, the following operation counts the documents in a collection:The
$count
stage is equivalent to the following$group
+$project
sequence:To get a count of documents that match a query condition, include the
$match
stage as well:Or, if using the
$group + $project
equivalent:
See also
$collStats
to return an approximate count based on the collection’s metadata.
Index Use
Consider a collection with the following index:
When performing a count, MongoDB can return the count using only the index if:
- the query can use an index,
- the query only contains conditions on the keys of the index, and
- the query predicates access a single contiguous range of index keys.
For example, the following operations can return the count using only the index:
If, however, the query can use an index but the query predicates do not access a single contiguous range of index keys or the query also contains conditions on fields outside the index, then in addition to using the index, MongoDB must also read the documents to return the count.
In such cases, during the initial read of the documents, MongoDB pages the documents into memory such that subsequent calls of the same count operation will have better performance.
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 the correct statistics after an unclean shutdown.
Note
This loss of accuracy only applies to count()
operations that do not include a query predicate.