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- db.collection.count()
db.collection.count()¶
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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"
,- you must start the
mongod
instances with the--enableMajorityReadConcern
command line option (or thereplication.enableMajorityReadConcern
set totrue
if using a configuration file). - replica sets must use WiredTiger storage engine and election
protocol version 1
.
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.- you must start the
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
$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: <query condition> },
{ $group: { _id: null, count: { $sum: 1 } } }
]
)
See Perform a Count for an example.
Index Use¶
Consider a collection with the following index:
{ a: 1, b: 1 }
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:
db.collection.find( { a: 5, b: 5 } ).count()
db.collection.find( { a: { $gt: 5 } } ).count()
db.collection.find( { a: 5, b: { $gt: 10 } } ).count()
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.
db.collection.find( { a: 5, b: { $in: [ 1, 2, 3 ] } } ).count()
db.collection.find( { a: { $gt: 5 }, b: 5 } ).count()
db.collection.find( { a: 5, b: 5, c: 5 } ).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
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:
db.orders.count()
This operation is equivalent to the following:
db.orders.find().count()
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')
:
db.orders.count( { ord_dt: { $gt: new Date('01/01/2012') } } )
The query is equivalent to the following:
db.orders.find( { ord_dt: { $gt: new Date('01/01/2012') } } ).count()