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aggregation $sort clarification #2107

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aggregation $sort clarification #2107

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9 changes: 6 additions & 3 deletions source/reference/operator/aggregation/sort.txt
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Expand Up @@ -117,8 +117,11 @@ option in :method:`db.collection.aggregate()` method and the

:pipeline:`$sort` operator can take advantage of an index when
placed at the **beginning** of the pipeline or placed **before**
the following aggregation operators: :pipeline:`$project`,
:pipeline:`$unwind`, and :pipeline:`$group`.

the :pipeline:`$project`,
:pipeline:`$unwind`, and :pipeline:`$group` aggregation operators.
If :pipeline:`$project`, :pipeline:`$unwind`, or :pipeline:`$group`
occur prior to the :pipeline:`$sort` operation, :pipeline:`$sort`
cannot use any indexes.

.. todo:: if a sort precedes the first $group in a sharded system,
all documents must go to the mongos for sorting.