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
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
$outor$lookupstages, the merge runs on the primary shard.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.