$collStats (aggregation)

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Definition

Behavior

Definition

$collStats

Returns statistics regarding a collection or view.

The $collStats stage has the following prototype form:

{
  $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. The count is based on the collection's metadata, which provides a fast but sometimes inaccurate count for sharded clusters. See `count` Field
`queryExecStats`	Adds query execution statistics to the return document.

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.
`host`	The hostname and port of the `mongod` process which produced the output document.
`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`. 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.

Accuracy After Unexpected Shutdown

After an unclean shutdown of a mongod using the Wired Tiger storage engine, size and count statistics reported by $collStats 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 statistics after an unclean shutdown.

After an unclean shutdown:

validate updates the count statistic in the collStats output with the latest value.
Other statistics like the number of documents inserted or removed in the collStats output are estimates.

Redaction

When using Queryable Encryption, $collStats output redacts certain information for encrypted collections:

The output omits "queryExecStats"
The output omits "latencyStats"
The output redacts "WiredTiger", if present, to include only the url field.

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.
`transactions`	Latency statistics for database transactions.

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 lower 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 lower 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:

histogram: [
  { micros: NumberLong(0), 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 [1]:

10 operations taking 2 microsecond or less
1 operation in the range [2, 4) microseconds
1 operation in the range [4096, 6144) microseconds
1000 operations in the range [16384, 24576) microseconds
100 operations in the range [49152, 65536) microseconds

[1]	The `(` symbol notation on this page means the value is exclusive. The `]` symbol notation on this page means the value is inclusive.

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

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

This query returns a result similar to the following:

{ "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:

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

This query returns a result similar to the following:

{
  "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,
    "capped" : false,
    "wiredTiger" : {
      ...
    },
    "nindexes" : 2,
    "indexDetails" : {
      ...
    },
    "indexBuilds" : [
       "_id_1_abc_1"
    ],
    "totalIndexSize" : 260337664,
    "totalSize" : 613216256,
    "indexSizes" : {
      "_id_" : 9891840,
      "_id_1_abc_1" : 250445824
    },
    "scaleFactor" : 1
  }
}

See Output for a reference on this document.

Note

In-progress Indexes

The returned storageStats includes information on indexes being built. For details, see:

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

`count` Field

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:

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

The query returns a result similar to the following:

{
  "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

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:

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

The query returns a result similar to the following:

{
  "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:

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

The query returns a result similar to the following:

{
  "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
}

$collStats (aggregation)

Definition

Behavior

Accuracy After Unexpected Shutdown

Redaction

Transactions

`latencyStats` Document

`storageStats` Document

Note

In-progress Indexes

`count` Field

Note

`queryExecStats` Document

`$collStats` on Sharded Collections

Tip

See also:

Definition

Behavior

Accuracy After Unexpected Shutdown

Redaction

Transactions

latencyStats Document

storageStats Document

Note

In-progress Indexes

count Field

Note

queryExecStats Document

$collStats on Sharded Collections

Tip

See also:

`latencyStats` Document

`storageStats` Document

`count` Field

`queryExecStats` Document

`$collStats` on Sharded Collections