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- db.collection.count()
db.collection.count()¶
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Definition¶
-
db.collection.
count
(query, options)¶ Note
MongoDB drivers compatible with the 4.0 features deprecate their respective cursor and collection
count()
APIs in favor of new APIs forcountDocuments()
andestimatedDocumentCount()
. For the specific API names for a given driver, see the driver documentation.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.Important
- Avoid using the
db.collection.count()
method without a query predicate since without the query predicate, the method returns results based on the collection’s metadata, which may result in an approximate count. In particular,- On a sharded cluster, the resulting count will not correctly filter out orphaned documents.
- After an unclean shutdown, the count may be incorrect.
- For counts based on collection metadata, see also collStats pipeline stage with the count option.
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.For MongoDB 4.0.3+ (and 3.6.1+), 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.- Avoid using the
Behavior¶
Count and Transactions¶
You cannot use count
and shell helpers
count()
and db.collection.count()
in
transactions.
For details, see Transactions and Count Operations.
Sharded Clusters¶
On a sharded cluster, db.collection.count()
without a query predicate 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:
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.
Examples¶
Count all Documents in a Collection¶
To count the number of all documents in the orders
collection, use
the following operation:
This operation is equivalent to the following:
Count all Documents that Match a Query¶
Count the number of the documents in the orders
collection with the field ord_dt
greater than new
Date('01/01/2012')
:
The query is equivalent to the following: