Aggregation Pipeline and Sharded Collections

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The aggregation pipeline supports operations on sharded collections. This section describes behaviors specific to the aggregation pipeline and sharded collections.


Changed in version 3.2.

If the pipeline starts with an exact $match on a shard key, the entire pipeline runs on the matching shard only. Previously, the pipeline would have been split, and the work of merging it would have to be done on the primary shard.

Changed in version 3.6.

When aggregation operations run on multiple shards, the results are routed to the mongos to be merged. However, $out and $lookup must run on the primary shard and are aggregated there.


When splitting the aggregation pipeline into two parts, the pipeline is split to ensure that the shards perform as many stages as possible with consideration for optimization.

To see how the pipeline was split, include the explain option in the db.collection.aggregate() method.

Optimizations are subject to change between releases.