$listSampledQueries (aggregation stage)

Definition

$listSampledQueries: Returns sampled queries for all collections or a specific collection. Sampled queries are used by the analyzeShardKey command to calculate metrics about the read and write distribution of a shard key.

Syntax

$listSampledQueries has this syntax:

{
   $listSampledQueries: { namespace: <namespace> }
}

Behavior

To list sampled queries for a single collection, specify the collection in the namespace argument.
To list sampled queries for all collections, omit the namespace argument.

Access Control

$listSampledQueries requires the clusterMonitor role on the cluster.

Limitations

You cannot use $listSampledQueries on Atlas multi-tenant configurations.
You cannot use $listSampledQueries on standalone deployments.
You cannot use $listSampledQueries directly against a --shardsvr replica set. When running on a sharded cluster, $listSampledQueries must run against a mongos.

Examples

List Sampled Queries for All Collections

The following aggregation operation lists all sampled queries for all collections in the replica set:

db.aggregate( [  { $listSampledQueries: {  } } ] )

List Sampled Queries for A Specific Collection

The following aggregation operation lists all sampled queries for a post collection in the social database:

db.aggregate( [  { $listSampledQueries: { namespace: "social.post" } } ] )

To use the MongoDB Node.js driver to add a $listSampledQueries stage to an aggregation pipeline, use the $listSampledQueries operator in a pipeline object.

List Sampled Queries for All Collections

The following aggregation operation lists all sampled queries for all collections in the replica set:

const pipeline = [{ $listSampledQueries: {} }];
const cursor = db.aggregate(pipeline);
return cursor;

List Sampled Queries for A Specific Collection

The following aggregation operation lists all sampled queries for the movies collection in the sample_mflix database:

const pipeline = [{ $listSampledQueries: { namespace: "sample_mflix.movies" } }];
const cursor = db.aggregate(pipeline);
return cursor;

Output

The output fields differ for read and write queries.

Read Queries

{
   _id: <uuid>,
   ns: "<database>.<collection>",
   collectionUuid: <collUUID>,
   cmdName: <find|aggregate|count|distinct>,
   cmd: {
     filter: <object>,
     collation: <object>,
     let: <object>
   },
   expireAt: <date>
}

Field Name	Type	Description
`_id`	UUID	Sample ID for the query.
`ns`	string	Namespace of the sampled collection.
`collectionUuid`	UUID	ID of the sampled collection.
`cmdName`	string	Name of the sampled command. Can be one of: `"find"` `"aggregate"` `"count"` `"distinct"`
`cmd.filter`	object	Filter the command ran with, if applicable.
`cmd.collation`	object	Collation the command ran with, if applicable.
`cmd.let`	object	Custom variables the command ran with, if applicable.
`expireAt`	date	Date that the sample expires.

Write Queries

{
   _id: <uuid>,
   ns: "<database>.<collection>",
   collectionUuid: <collUUID>,
   cmdName: <update|delete|findAndModify>,
   cmd: <object>,
   expireAt: <date>
}

Field Name	Type	Description
`_id`	UUID	Sample ID for the query.
`ns`	string	Namespace of the sampled collection.
`collectionUuid`	UUID	ID of the sampled collection.
`cmdName`	string	Name of the sampled command. Can be one of: `"update"` `"delete"` `"findAndModify"`
`cmd`	object	Command object
`expireAt`	date	Date that the sample expires.

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