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Aggregation Pipeline

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An aggregation pipeline consists of one or more stages that process documents:

  • Each stage performs an operation on the input documents. For example, a stage can filter documents, group documents, and calculate values.
  • The documents that are output from one stage are input to the next stage.
  • An aggregation pipeline can return results for groups of documents. For example, return the total, average, maximum, and minimum values.

Create the following collection that contains orders for products:

db.orders.insertMany( [
{ _id: 0, productName: "Steel beam", status: "new", quantity: 10 },
{ _id: 1, productName: "Steel beam", status: "urgent", quantity: 20 },
{ _id: 2, productName: "Steel beam", status: "urgent", quantity: 30 },
{ _id: 3, productName: "Iron rod", status: "new", quantity: 15 },
{ _id: 4, productName: "Iron rod", status: "urgent", quantity: 50 },
{ _id: 5, productName: "Iron rod", status: "urgent", quantity: 10 }
] )

The following aggregation pipeline example contains two stages and returns the total quantity of urgent orders for each product:

db.orders.aggregate( [
{ $match: { status: "urgent" } },
{ $group: { _id: "$productName", sumQuantity: { $sum: "$quantity" } } }
] )

The $match stage:

  • Filters the documents to those with a status of urgent.
  • Outputs the filtered documents to the $group stage.

The $group stage:

  • Groups the input documents by productName.
  • Uses $sum to calculate the total quantity for each productName, which is stored in the sumQuantity field returned by the aggregation pipeline.

Example output:

[
{ _id: 'Steel beam', sumQuantity: 50 },
{ _id: 'Iron rod', sumQuantity: 60 }
]
Tip
See also:

An aggregation pipeline consists of one or more stages that process documents:

  • Each stage transforms the documents as they pass through the pipeline.
  • A stage does not have to output one document for every input document. For example, some stages may produce new documents or filter out documents.
  • The same stage can appear multiple times in the pipeline with these stage exceptions: $out, $merge, and $geoNear.
  • For all available stages, see Aggregation Pipeline Stages.

To run an aggregation pipeline, use:

Starting in MongoDB 4.2, you can use the aggregation pipeline to update documents using these methods:

Command
mongosh Methods

Some pipeline stages accept a pipeline expression as the operand. Pipeline expressions specify the transformation to apply to the input documents. Expressions have a document structure and can contain other expression.

Pipeline expressions can only operate on the current document in the pipeline and cannot refer to data from other documents: expression operations provide in-memory transformation of documents.

Generally, expressions are stateless and are only evaluated when seen by the aggregation process with one exception: accumulator expressions.

The accumulators, used in the $group stage, maintain their state (for example, totals, maximums, minimums, and related data) as documents progress through the pipeline. Some accumulators are available in the $project stage; however, when used in the $project stage, the accumulators do not maintain their state across documents.

Starting in version 4.4, MongoDB provides the $accumulator and $function aggregation operators. These operators provide users with the ability to define custom aggregation expressions in JavaScript.

For more information on expressions, see Expressions.

In MongoDB, the aggregate command operates on a single collection, logically passing the entire collection into the aggregation pipeline. To optimize the operation, wherever possible, use the following strategies to avoid scanning the entire collection.

MongoDB's query planner analyzes an aggregation pipeline to determine whether indexes can be used to improve pipeline performance. For example, the following pipeline stages can take advantage of indexes:

Note

The following pipeline stages do not represent a complete list of all stages which can use an index.

$match
The $match stage can use an index to filter documents if it occurs at the beginning of a pipeline.
$sort
The $sort stage can use an index as long as it is not preceded by a $project, $unwind, or $group stage.
$group

The $group stage can sometimes use an index to find the first document in each group if all of the following criteria are met:

  • The $group stage is preceded by a $sort stage that sorts the field to group by,
  • There is an index on the grouped field which matches the sort order and
  • The only accumulator used in the $group stage is $first.

See Optimization to Return the First Document of Each Group for an example.

$geoNear
The $geoNear pipeline operator takes advantage of a geospatial index. When using $geoNear, the $geoNear pipeline operation must appear as the first stage in an aggregation pipeline.

Changed in version 3.2: Starting in MongoDB 3.2, indexes can cover an aggregation pipeline. In MongoDB 2.6 and 3.0, indexes could not cover an aggregation pipeline since even when the pipeline uses an index, aggregation still requires access to the actual documents.

If your aggregation operation requires only a subset of the data in a collection, use the $match, $limit, and $skip stages to restrict the documents that enter at the beginning of the pipeline. When placed at the beginning of a pipeline, $match operations use suitable indexes to scan only the matching documents in a collection.

Placing a $match pipeline stage followed by a $sort stage at the start of the pipeline is logically equivalent to a single query with a sort and can use an index. When possible, place $match operators at the beginning of the pipeline.

An aggregation pipeline has some limitations on the value types and the result size. See Aggregation Pipeline Limits.

An aggregation pipeline has an internal optimization phase that provides improved performance for certain sequences of operators. See Aggregation Pipeline Optimization.

An aggregation pipeline supports operations on sharded collections. See Aggregation Pipeline and Sharded Collections.

As of MongoDB 5.0 the map-reduce operation is deprecated.

An aggregation pipeline provides better performance and usability than a map-reduce operation.

Map-reduce operations can be rewritten using aggregation pipeline operators, such as $group, $merge, and others.

For map-reduce operations that require custom functionality, MongoDB provides the $accumulator and $function aggregation operators starting in version 4.4. Use these operators to define custom aggregation expressions in JavaScript.

For examples of aggregation pipeline alternatives to map-reduce operations, see Map-Reduce to Aggregation Pipeline and Map-Reduce Examples.

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