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DOCS-13063 Clarify $group order behaviour #5211

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Apr 19, 2021
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DOCS-13063 Clarify $group order behaviour #5211

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5 changes: 0 additions & 5 deletions source/includes/note-group-pipeline-sort-order.rst
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9 changes: 4 additions & 5 deletions source/reference/operator/aggregation/first.txt
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Expand Up @@ -43,11 +43,10 @@ For more information on expressions, see :ref:`aggregation-expressions`.
Behavior
--------

When using :group:`$first` in a :pipeline:`$group` stage, the
:pipeline:`$group` stage should follow a :pipeline:`$sort` stage to
have the input documents in a defined order.

.. include:: /includes/note-group-pipeline-sort-order.rst
When using :group:`$first` in a :pipeline:`$group` stage, the output
value depends on the order of the documents coming into pipeline. To
guarantee a defined order, the :pipeline:`$group` pipeline stage should
follow a :pipeline:`$sort` stage.

Example
-------
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9 changes: 4 additions & 5 deletions source/reference/operator/aggregation/last.txt
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Expand Up @@ -43,11 +43,10 @@ For more information on expressions, see :ref:`aggregation-expressions`.
Behavior
--------

When using :group:`$last` in a :pipeline:`$group` stage, the
:pipeline:`$group` stage should follow a :pipeline:`$sort` stage to
have the input documents in a defined order.

.. include:: /includes/note-group-pipeline-sort-order.rst
When using :group:`$last` in a :pipeline:`$group` stage, the output
value depends on the order of the documents coming into pipeline. To
guarantee a defined order, the :pipeline:`$group` pipeline stage should
follow a :pipeline:`$sort` stage.

Example
-------
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12 changes: 11 additions & 1 deletion source/reference/operator/aggregation/push.txt
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Expand Up @@ -29,6 +29,15 @@ Definition

For more information on expressions, see :ref:`aggregation-expressions`.

Behavior
--------

When using :group:`$push` in a :pipeline:`$group` stage, the order of
the documents in the output array is determined by the order of the
documents coming into pipeline. To guarantee a defined order, the
:pipeline:`$group` pipeline stage should follow a :pipeline:`$sort`
stage.

Example
-------

Expand All @@ -52,7 +61,8 @@ compute the list of items and quantities sold for each group:

db.sales.aggregate(
[
{
{ $sort: { date: 1, item: 1 } },
{
$group:
{
_id: { day: { $dayOfYear: "$date"}, year: { $year: "$date" } },
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