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$collStats (aggregation)

Definition

$collStats

New in version 3.4.

Returns statistics regarding a collection or view.

The $collStats stage has the following prototype form:

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{
  $collStats:
    {
      latencyStats: { histograms: <boolean> },
      storageStats: { scale: <number> },
      count: {},
      queryExecStats: {}
    }
}

The $collStats stage accepts an argument document with the following optional fields:

Field Name	Description
latencyStats	Adds latency statistics to the return document.
latencyStats.histograms	Adds latency histogram information to the embedded documents in latencyStats if true.
storageStats	Adds storage statistics to the return document. Specify an empty document (i.e. storageStats: {}) to use the default scale factor of 1 for the various size data. Scale factor of 1 displays the returned sizes in bytes. Specify the scale factor (i.e. storageStats: { scale: <number> }) to use the specified scale factor for the various size data. For example, to display kilobytes rather than bytes, specify a scale value of 1024. If you specify a non-integer scale factor, MongoDB uses the integer part of the specified factor. For example, if you specify a scale factor of 1023.999, MongoDB uses 1023 as the scale factor. The scale factor does not affect those sizes that specify the unit of measurement in the field name, such as "bytes currently in the cache".
count	Adds the total number of documents in the collection to the return document. Note The count is based on the collection’s metadata, which provides a fast but sometimes inaccurate count for sharded clusters. See count Field New in version 3.6.
queryExecStats	Adds query execution statistics to the return document. New in version 4.4.

For a collection in a replica set or a non-sharded collection in a cluster, $collStats outputs a single document. For a sharded collection, $collStats outputs one document per shard. The output document includes the following fields:

Field Name	Description
ns	The namespace of the requested collection or view.
shard	The name of the shard the output document corresponds to. Only present when $collStats runs on a sharded cluster. Both sharded and non-sharded collections will produce this field. New in version 3.6.
host	The hostname and port of the mongod process which produced the output document. New in version 3.6.
localTime	The current time on the MongoDB server, expressed as UTC milliseconds since the Unix epoch.
latencyStats	Statistics related to request latency for a collection or view. See latencyStats Document for details on this document. Only present when the latencyStats: {} option is specified.
storageStats	Statistics related to a collection’s storage engine. See storageStats Document for details on this document. The various size data is scaled by the specified factor (with the exception of those sizes that specify the unit of measurement in the field name). Only present when the storageStats option is specified. Returns an error if applied to a view.
count	The total number of documents in the collection. This data is also available in storageStats.count. Note The count is based on the collection’s metadata, which provides a fast but sometimes inaccurate count for sharded clusters. Only present when the count: {} option is specified. Returns an error if applied to a view.
queryExecStats	Statistics related to query execution for the collection. Only present when the queryExecStats: {} option is specified. Returns an error if applied to a view.

Behavior

$collStats must be the first stage in an aggregation pipeline, or else the pipeline returns an error.

Transactions

$collStats is not allowed in transactions.

latencyStats Document

The latencyStats embedded document only exists in the output if you specify the latencyStats option.

Field Name	Description
reads	Latency statistics for read requests.
writes	Latency statistics for write requests.
commands	Latency statistics for database commands.

Each of these fields contains an embedded document bearing the following fields:

Field Name	Description
latency	A 64-bit integer giving the total combined latency in microseconds.
ops	A 64-bit integer giving the total number of operations performed on the collection since startup.
histogram	An array of embedded documents, each representing a latency range. Each document covers twice the previous document’s range. For upper values between 2048 microseconds and roughly 1 second, the histogram includes half-steps. This field only exists given the latencyStats: { histograms: true } option. Empty ranges with a zero count are omitted from the output. Each document bears the following fields: Field Name Description micros A 64-bit integer giving the inclusive upper time bound of the current latency range in microseconds. The document’s range spans between the previous document’s micros value, exclusive, and this document’s micros value, inclusive. count A 64-bit integer giving the number of operations with latency less than or equal to micros. For example, if collStats returns the following histogram: copy histogram: [ { micros: NumberLong(1), count: NumberLong(10) }, { micros: NumberLong(2), count: NumberLong(1) }, { micros: NumberLong(4096), count: NumberLong(1) }, { micros: NumberLong(16384), count: NumberLong(1000) }, { micros: NumberLong(49152), count: NumberLong(100) } ] This indicates that there were: 10 operations taking 1 microsecond or less, 1 operation in the range (1, 2] microseconds, 1 operation in the range (3072, 4096] microseconds, 1000 operations in the range (12288, 16384], and 100 operations in the range (32768, 49152].

For example, if you run $collStats with the latencyStats: {} option on a matrices collection:

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db.matrices.aggregate( [ { $collStats: { latencyStats: { histograms: true } } } ] )

This query returns a result similar to the following:

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{ "ns" : "test.matrices",
  "host" : mongo.example.net:27017",
  "localTime" : ISODate("2017-10-06T19:43:56.599Z"),
  "latencyStats" :
    { "reads" :
        { "histogram" : [
            { "micros" : NumberLong(16),
              "count" : NumberLong(3) },
            { "micros" : NumberLong(32),
              "count" : NumberLong(1) },
            { "micros" : NumberLong(128),
              "count" : NumberLong(1) } ],
          "latency" : NumberLong(264),
          "ops" : NumberLong(5) },
      "writes" :
        { "histogram" : [
            { "micros" : NumberLong(32),
              "count" : NumberLong(1) },
            { "micros" : NumberLong(64),
              "count" : NumberLong(3) },
            { "micros" : NumberLong(24576),
              "count" : NumberLong(1) } ],
          "latency" : NumberLong(27659),
          "ops" : NumberLong(5) },
      "commands" :
        { "histogram" : [
            {
               "micros" : NumberLong(196608),
               "count" : NumberLong(1)
            }
          ],
          "latency" : NumberLong(0),
          "ops" : NumberLong(0) },
      "transactions" : {
         "histogram" : [ ],
         "latency" : NumberLong(0),
         "ops" : NumberLong(0)
      }
    }
}

storageStats Document

The storageStats embedded document only exists in the output if you specify the storageStats option.

The contents of this document are dependent on the storage engine in use. See Output for a reference on this document.

For example, if you run $collStats with the storageStats: {} option on a matrices collection using the WiredTiger Storage Engine:

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db.matrices.aggregate( [ { $collStats: { storageStats: { } } } ] )

This query returns a result similar to the following:

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{
  "ns" : "test.matrices",
  "host" : mongo.example.net:27017",
  "localTime" : ISODate("2020-03-06T01:44:57.437Z"),
  "storageStats" : {
    "size" : 608500363,
    "count" : 1104369,
    "avgObjSize" : 550,
    "storageSize" : 352878592,
    "freeStorageSize" : 2490380,  // Starting in MongoDB 4.4
    "capped" : false,
    "wiredTiger" : {
      ...
    },
    "nindexes" : 2,
    "indexDetails" : {
      ...
    },
    "indexBuilds" : [    // Starting in MongoDB 4.2
       "_id_1_abc_1"
    ],
    "totalIndexSize" : 260337664,
    "totalSize" : 613216256,    // Starting in MongoDB 4.4
    "indexSizes" : {
      "_id_" : 9891840,
      "_id_1_abc_1" : 250445824
    },
    "scaleFactor" : 1    // Starting in MongoDB 4.2
  }
}

See Output for a reference on this document.

In-progress Indexes

Starting in MongoDB 4.2, the returned storageStats includes information on indexes being built. For details, see:

• collStats.nindexes
• collStats.indexDetails
• collStats.indexBuilds
• collStats.totalIndexSize
• collStats.indexSizes

Performing $collStats with the storageStats option on a view results in an error.

count Field

New in version 3.6.

The count field only exists in the output if you specify the count option.

For example, if you run $collStats with the count: {} option on a matrices collection:

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db.matrices.aggregate( [ { $collStats: { count: { } } } ] )

The query returns a result similar to the following:

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{
  "ns" : "test.matrices",
  "host" : mongo.example.net:27017",
  "localTime" : ISODate("2017-10-06T19:43:56.599Z"),
  "count" : 1103869
}

Note

The count is based on the collection’s metadata, which provides a fast but sometimes inaccurate count for sharded clusters.

The total number of documents in the collection is also available as storageStats.count when storageStats: {} is specified. For more information, see storageStats Document.

queryExecStats Document

New in version 4.4.

The queryExecStats embedded document only exists in the output if you specify the queryExecStats option.

The collectionScans field contains an embedded document bearing the following fields:

Field Name	Description
total	A 64-bit integer giving the total number of queries that performed a collection scan. The total consists of queries that did and did not use a tailable cursor.
nonTailable	A 64-bit integer giving the number of queries that performed a collection scan that did not use a tailable cursor.

For example, if you run $collStats with the queryExecStats: {} option on a matrices collection:

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db.matrices.aggregate( [ { $collStats: { queryExecStats: { } } } ] )

The query returns a result similar to the following:

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{
  "ns": "test.matrices",
  "host": "mongo.example.net:27017",
  "localTime": ISODate("2020-06-03T14:23:29.711Z"),
  "queryExecStats": {
      "collectionScans": {
          "total": NumberLong(33),
          "nonTailable": NumberLong(31)
      }
  }
}

$collStats on Sharded Collections

$collStats outputs one document per shard when run on sharded collections. Each output document contains a shard field with the name of the shard the document corresponds to.

For example, if you run $collStats on a sharded collection with the count: {} option on a collection named matrices:

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db.matrices.aggregate( [ { $collStats: { count: { } } } ] )

The query returns a result similar to the following:

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{
  "ns" : "test.matrices",
  "shard" : "s1",
  "host" : "s1-mongo1.example.net:27017",
  "localTime" : ISODate("2017-10-06T15:14:21.258Z"),
  "count" : 661705
}
{
  "ns" : "test.matrices",
  "shard" : "s2",
  "host" : "s2-mongo1.example.net:27017",
  "localTime" : ISODate("2017-10-06T15:14:21.258Z"),
  "count" : 442164
}

See also

collStats, db.collection.stats()

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