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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.

Behavior

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.

For aggregation operations that must run on multiple shards, if the operations do not require running on the database’s primary shard, these operations will route the results to a random shard to merge the results to avoid overloading the primary shard for that database. The $out stage and the $lookup stage require running on the database’s primary shard.

Optimization

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.