Aggregation Pipeline and Sharded Collections
On this page
The aggregation pipeline supports operations on sharded collections. This section describes behaviors specific to the aggregation pipeline and sharded collections.
If the pipeline starts with an exact
$match on a
shard key, and the pipeline does not contain
$lookup stages, the entire pipeline runs on the matching
When aggregation operations run on multiple shards, the results are
routed to the
mongos to be merged, except in the
If the pipeline includes the
$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.
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
Optimizations are subject to change between releases.