Aggregation Pipeline and Sharded Collections

Behavior

If the pipeline starts with an exact $match on a shard key, and the pipeline does not contain $out or $lookup stages, the entire pipeline runs on the matching shard only.

When aggregation operations run on multiple shards, the results are routed to the mongos to be merged, except in the following cases:

• If the pipeline includes the $out stage, the merge runs on the shard where the output collection lives.

• If the pipeline includes the $lookup stage that references an unsharded collection, the merge runs on the shard where the unsharded collection lives.

• If the pipeline includes a sorting or grouping stage, and the allowDiskUse setting is enabled, the merge runs on a randomly-selected 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.