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DOCS-13229 Correcting merging in sharded cluster statement #4923

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DOCS-13229 Correcting merging in sharded cluster statement #4923

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8 changes: 5 additions & 3 deletions source/core/aggregation-pipeline-sharded-collections.txt
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Expand Up @@ -30,9 +30,11 @@ to be done on the primary shard.
.. versionchanged:: 3.6

When aggregation operations run on multiple shards, the results are
routed to the :binary:`~bin.mongos` to be merged. However, :pipeline:`$out`
and :pipeline:`$lookup` must run on the :ref:`primary shard <primary-shard>`
and are aggregated there.
routed to the :binary:`~bin.mongos` to be merged except when :pipeline:`$out`
and :pipeline:`$lookup` are present, and must merge on the :ref:`primary
shard <primary-shard>` or when :ref:`allowDiskUse` is true and there is a
sorting or grouping stage present in which case one of the shards will be
randomly selected to perform the merge.

Optimization
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