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Definition

db. aggregate ( )

Runs a specified admin/diagnostic pipeline which does not require an underlying collection. For aggregations on collection data, see db.collection.aggregate().

The db.aggregate() method has the following syntax:

copy

db.aggregate( [ <pipeline> ], { <options> } )

• The pipeline parameter is an array of stages to execute. It must start with a compatible stage that does not require an underlying collection, such as $currentOp or $listLocalSessions.

• The options document can contain the following fields and values:

  Field	Type	Description
  explain	boolean	Optional. Specifies to return the information on the processing of the pipeline. See Return Information on Aggregation Pipeline Operation for an example. New in version 2.6.
  allowDiskUse	boolean	Optional. Enables writing to temporary files. When set to true, most aggregation operations can write data to the _tmp subdirectory in the dbPath directory with the following exceptions: $graphLookup stage $addToSet accumulator expression used in the $group stage (Starting in version 3.6.17) $push accumulator expression used in the $group stage (Starting in version 3.6.17) For an example of allowDiskUse, see Perform Large Sort Operation with External Sort.
  cursor	document	Optional. Specifies the initial batch size for the cursor. The value of the cursor field is a document with the field batchSize. See Specify an Initial Batch Size for syntax and example. New in version 2.6.
  maxTimeMS	non-negative integer	Optional. Specifies a time limit in milliseconds for processing operations on a cursor. If you do not specify a value for maxTimeMS, operations will not time out. A value of 0 explicitly specifies the default unbounded behavior. MongoDB terminates operations that exceed their allotted time limit using the same mechanism as db.killOp(). MongoDB only terminates an operation at one of its designated interrupt points.
  bypassDocumentValidation	boolean	Optional. Available only if you specify the $out aggregation operator. Enables db.collection.aggregate to bypass document validation during the operation. This lets you insert documents that do not meet the validation requirements. New in version 3.2.
  readConcern	document	Optional. Specifies the read concern. The readConcern option has the following syntax: Changed in version 3.6. copy readConcern: { level: <value> } Possible read concern levels are: "local". This is the default read concern level. "available". This is the default for reads against secondaries when Read Operations and Causally Consistent Sessions and “level” are unspecified. The query returns the instance’s most recent data. "majority". Available for replica sets that use WiredTiger storage engine. "linearizable". Available for read operations on the primary only. For more formation on the read concern levels, see Read Concern Levels. For "local" (default) or "majority" read concern level, you can specify the afterClusterTime option to have the read operation return data that meets the level requirement and the specified after cluster time requirement. For more information, see Read Operations and Causally Consistent Sessions.
  collation	document	Optional. Specifies the collation to use for the operation. Collation allows users to specify language-specific rules for string comparison, such as rules for lettercase and accent marks. The collation option has the following syntax: copy collation: { locale: <string>, caseLevel: <boolean>, caseFirst: <string>, strength: <int>, numericOrdering: <boolean>, alternate: <string>, maxVariable: <string>, backwards: <boolean> } When specifying collation, the locale field is mandatory; all other collation fields are optional. For descriptions of the fields, see Collation Document. If the collation is unspecified but the collection has a default collation (see db.createCollection()), the operation uses the collation specified for the collection. If no collation is specified for the collection or for the operations, MongoDB uses the simple binary comparison used in prior versions for string comparisons. You cannot specify multiple collations for an operation. For example, you cannot specify different collations per field, or if performing a find with a sort, you cannot use one collation for the find and another for the sort. New in version 3.4.
  hint	string or document	Optional. The index to use for the aggregation. The index is on the initial collection/view against which the aggregation is run. Specify the index either by the index name or by the index specification document. Note The hint does not apply to $lookup and $graphLookup stages. New in version 3.6.
  comment	string	Optional. Users can specify an arbitrary string to help trace the operation through the database profiler, currentOp, and logs. New in version 3.6.

Example

use admin
db.aggregate( [ {
   $currentOp : { allUsers: true, idleConnections: true } }, {
   $match : { shard: "shard01" }
   }
] )