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Supported Aggregation Pipeline Stages and Operators

This page describes the MongoDB aggregation pipeline stages and operators that Atlas Data Federation supports.

Note

By default, Atlas Data Federation does not return documents in any specific order for queries on Data Federations for S3 data stores. Atlas Data Federation reads the partitions concurrently and the underlying storage response order determines which documents Atlas Data Federation returns first, unless you define order using $sort in your query. For example, if you run the same findOne() query twice, you could see different documents, and if you use $skip, different documents might be skipped if $sort is not used in the query.

Supported and Unsupported Aggregation Pipeline Stages

Atlas Data Federation supports all the aggregation pipeline stages except the following:

For the following stages in Atlas Data Federation queries, Atlas Data Federation introduces an alternate syntax, includes a caveat, or deviates from server. See the Description column for details.

Pipeline Stage

Description

$group

Groups input documents by the specified _id expression and for each distinct grouping, outputs a document. Atlas Data Federation doesn't support empty string keys for the accumulator fields.

Example

The following is not supported:

{
              "$group" : {
                      "_id" : "$representationType",
                      "" : {
                              "$sum" : NumberInt(1)
                      }
              }
      }

$lookup

Performs a left outer join to a collection in the same database. Atlas Data Federation provides syntax for joining collections from different databases also. See $lookup for more information.

$match

Filters the documents to pass only the documents that match the specified conditions to the next pipeline stage. Atlas Data Federation supports $match. Note that the partition attributes for selecting specific files on S3 are only optimized for the following aggregation pipeline operators: $eq, $gt, $lt, $gte, $lte, $ne, $and, $or, $in.

$merge

Writes the results of the aggregation pipeline to a specified collection. Atlas Data Federation provides alternate syntax for the required into field to allow writes to an Atlas cluster. To learn more, see $merge.

$out

Takes the documents returned by the aggregation pipeline and writes them to a specified collection. Atlas Data Federation provides alternate syntax for writing to S3 and Atlas cluster.

To use $out to write to a collection in a different database on the same Atlas cluster, your Atlas cluster must be on MongoDB version 5.0 or later.

See $out for more information.

$sample

Randomly selects the specified number of documents from its input. Atlas Data Federation supports $sample, but does not provide a truly random sample and returns the first set of documents that it finds.

$skip

Skips over the specified number of documents that pass into the stage and passes the remaining documents to the next stage in the pipeline. Atlas Data Federation supports $skip, but this does not reduce data scan because Data Federation accesses all partitions that correspond to your query.

Supported Aggregation Pipeline Operators

Atlas Data Federation supports all the aggregation pipeline operators. However, some operators are only available in queries on collections that are mapped to an Atlas cluster data store. The geospatial query operators are only available in such queries. The following evaluation query operators are also restriced to queries on collections mapped to an Atlas cluster data store:

Pipeline Stage	Description
`$geoNear`	Outputs documents in order of nearest to farthest from a specified point. Atlas Data Federation supports `$geoNear` in queries on virtual collections that are mapped to one or more Atlas collections. Atlas Data Federation doesn't support `$geoNear` for S3 or HTTP federated database instance stores. See Querying Data in Your Atlas Cluster for more information.
`$graphLookup`	Performs a recursive search on a collection. Atlas Data Federation supports `$graphLookup` in queries on virtual collections that are mapped to one Atlas collection only. Atlas Data Federation doesn't support `$graphLookup` for: S3 or HTTP stores. Queries on virtual collections that are mapped to multiple Atlas collections. See Querying Data in Your Atlas Cluster for more information.
`$search`	Performs a full-text search on the content of the fields covered by an Atlas Search index.
$text	Performs a text search on the content of the fields indexed with a text index.
$where	Passes either a string containing a JavaScript expression or a full JavaScript function to the query system.

Note

Atlas Data Federation doesn't include a server-side JavaScript engine. So, Atlas Data Federation doesn't support operators such as $where, $function, and $accumulator that require server-side scripting enabled.

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