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DOCS-13229 correct merge behavior sharded cluster #5021

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13 changes: 10 additions & 3 deletions source/core/aggregation-pipeline-sharded-collections.txt
Original file line number Diff line number Diff line change
Expand Up @@ -30,9 +30,16 @@ 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 in the
following cases:

- If the pipeline includes the :pipeline:`$out` or
:pipeline:`$lookup` stages, the merge runs on the
:ref:`primary shard <primary-shard>`.

- If the pipeline includes a sorting or grouping stage, and the
:ref:`allowDiskUse <aggregate-cmd-allowDiskUse>` setting is enabled,
the merge runs on a randomly-selected shard.

Optimization
------------
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4 changes: 3 additions & 1 deletion source/reference/command/aggregate.txt
Original file line number Diff line number Diff line change
Expand Up @@ -99,7 +99,9 @@ arguments:



* - ``allowDiskUse``
* - .. _aggregate-cmd-allowDiskUse:

``allowDiskUse``

- boolean

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