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Extend 2.6 release notes for $out #1246

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16 changes: 14 additions & 2 deletions source/release-notes/2.6.txt
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Expand Up @@ -88,8 +88,20 @@ Aggregation Pipeline Changes
:pipeline:`$out`.

:pipeline:`$out` will create a new collection if one does not
already exist in the current database.

already exist in the current database. This collection will not be
visible at that name until the aggregation completes. If the aggregation
fails part-way through, no collection will be created.

If the output collection already exists, the behavior is the same
as MapReduce's replace mode. To be specific if the aggregation completes
successfully, the output collection will be atomically replaced by a newer
version. Any indexes that existed at the start of the aggregation on the
original output collection will also exist on the new collection.
Importantly, this means that the pipeline will error out if the output
documents would violate any unique indexes (including _id) that existed
on the original output collection.


You may *only* specify :pipeline:`$out` at the end of a pipeline.

.. example::
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