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(DOCS-14913): locf operator (#107) (#118)
* (DOCS-14913): locf operator * updates per review feedback * Add context to example * more tweaks * adjustments * formatting fix * grammar * updates per second round of review * tweaks * typo fix * line break * (DOCS-14913): Address review comments * change word * updates per review * tweaks * remove extra digit from hour field * fix example output * adjust example format * fix release notes after rebase
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‎source/includes/extracts-agg-operators.yaml

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expressions. If the two values are a date and a number,
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specify the date argument first as it is not meaningful to
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subtract a date from a number.
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---
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ref: agg-operators-miscellaneous
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content: |
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Available in :pipeline:`$setWindowFields` stage.
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* - :group:`$locf`
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- .. include:: /includes/fact-locf-description.rst
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Available in :pipeline:`$setWindowFields` stage.
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.. versionadded:: 5.2
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* - :group:`$max`
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- Returns the maximum value that results from applying an

‎source/includes/fact-locf-description.rst

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Last observation carried forward. Sets values for ``null`` and missing
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fields in a :ref:`window <setWindowFields-window>` to the last non-null
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value for the field.

‎source/includes/setWindowFields-operators.rst

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:group:`$max`, :group:`$min`, :group:`$push`, :group:`$stdDevSamp`,
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:group:`$stdDevPop`, and :group:`$sum`.
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.. _setWindowFields-gap-filling-operators:
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- Gap filling operators: :group:`$locf`.
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.. _setWindowFields-order-operators:
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- Order operators: :group:`$first`, :group:`$last`, and :group:`$shift`.
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.. _setWindowFields-rank-operators:
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- Rank operators: :group:`$denseRank`, :group:`$documentNumber`, and
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:group:`$rank`.
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:group:`$rank`.

‎source/reference/operator/aggregation.txt

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- Calculates the natural log of a number.
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* - :group:`$locf`
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- .. include:: /includes/fact-locf-description.rst
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.. versionadded:: 5.2
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* - :expression:`$log`
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- Calculates the log of a number in the specified base.
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/reference/operator/aggregation/let
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/reference/operator/aggregation/literal
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/reference/operator/aggregation/ln
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/reference/operator/aggregation/locf
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/reference/operator/aggregation/log
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/reference/operator/aggregation/log10
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/reference/operator/aggregation/lt

‎source/reference/operator/aggregation/locf.txt

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===================
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$locf (aggregation)
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===================
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.. default-domain:: mongodb
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.. contents:: On this page
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:local:
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:backlinks: none
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:depth: 1
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:class: singlecol
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Definition
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----------
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.. group:: $locf
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.. versionadded:: 5.2
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.. include:: /includes/fact-locf-description.rst
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:group:`$locf` is only available in the :pipeline:`$setWindowFields`
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stage.
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Syntax
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------
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The :group:`$locf` expression has this syntax:
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.. code-block:: none
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{ $locf: <expression> }
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For more information on expressions, see
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:ref:`aggregation-expressions`.
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Behavior
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--------
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If a field being filled contains both ``null`` and non-null values,
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:group:`$locf` sets the ``null`` and missing values to the field's
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last known non-null value according to the sort order specified in
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:pipeline:`$setWindowFields`.
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``null`` and missing field values that appear before non-null values
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in the sort order remain ``null``.
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If a field being filled contains only ``null`` or missing values in a
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:ref:`partition <setWindowFields-partitionBy>`, :group:`$locf` sets the
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field value to ``null`` for that partition.
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Example
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-------
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Create a ``stock`` collection that contains data for two different
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company stocks trading in the financial market. The collection contains
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the following properties of the stocks, recorded at hourly intervals:
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.. list-table::
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:header-rows: 1
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:stub-columns: 1
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:widths: 10 10 20
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* - Field
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- Type
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- Description
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* - ``time``
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- Date
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- Time when the stock ``price`` and ``volume`` were observed.
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* - ``company``
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- String
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- Name of the company.
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* - ``price``
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- Decimal
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- Price of the stock at the ``time`` observed.
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* - ``volume``
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- Integer
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- Number of shares traded over one day.
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Not all documents in the dataset contain all of the previous fields.
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.. code-block:: javascript
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db.stock.insertMany( [
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{
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time: "2021-03-08T09:00:00.000Z",
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company: "CompanyA",
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price: 500,
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volume: 200000
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},
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{
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time: "2021-03-08T09:00:00.000Z",
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company: "CompanyB",
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price: 20,
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volume: 100000
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},
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{
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time: "2021-03-08T10:00:00.000Z",
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company: "CompanyA",
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price: 490,
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volume: 205000
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},
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{
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time: "2021-03-08T10:00:00.000Z",
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company: "CompanyB",
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price: 22,
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volume: 105000
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},
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{
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time: "2021-03-08T11:00:00.000Z",
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company: "CompanyA"
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},
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{
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time: "2021-03-08T11:00:00.000Z",
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company: "CompanyB",
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price: 24,
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volume: null
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},
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{
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time: "2021-03-08T12:00:00.000Z",
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company: "CompanyA",
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price: 510,
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volume: 220000
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},
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{
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time: "2021-03-08T12:00:00.000Z",
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company: "CompanyB"
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},
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{
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time: "2021-03-08T13:00:00.000Z",
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company: "CompanyA",
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price: 505,
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volume: 225000
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},
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{
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time: "2021-03-08T13:00:00.000Z",
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company: "CompanyB",
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price: 28,
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volume: 120000
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}
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] )
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The following example uses the :group:`$locf` operator to set ``null``
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and missing fields to the values from the last document with a non-null
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value in the corresponding partition:
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.. code-block:: javascript
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:emphasize-lines: 7-8
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db.stock.aggregate( [
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{
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$setWindowFields: {
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partitionBy: "$company",
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sortBy: { time: 1 },
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output: {
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price: { $locf: "$price" },
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volume: { $locf: "$volume" }
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}
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}
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}
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] )
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In the example:
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- ``partitionBy: "$company"`` :ref:`partitions
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<setWindowFields-partitionBy>` the documents by ``company``. There
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are partitions for ``CompanyA`` and ``CompanyB``.
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- ``sortBy: { time: 1 }`` :ref:`sorts
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<setWindowFields-sortBy>` the documents in each partition by
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``time`` in ascending order (``1``), so the earliest
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``time`` is first.
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- For documents where ``price`` or ``volume`` is ``null`` or missing,
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the :group:`$locf` operator sets the missing field to the previous
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value for that field in the partition.
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Example output:
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.. code-block:: javascript
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:copyable: false
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[
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{
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_id: ObjectId("61b11166b412e1948a407dc6"),
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time: '2021-03-08T09:00:00.000Z',
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company: 'CompanyA',
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price: 500,
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volume: 200000
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},
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{
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_id: ObjectId("61b11166b412e1948a407dc8"),
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time: '2021-03-08T10:00:00.000Z',
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company: 'CompanyA',
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price: 490,
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volume: 205000
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},
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{
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_id: ObjectId("61b11166b412e1948a407dca"),
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time: '2021-03-08T11:00:00.000Z',
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company: 'CompanyA',
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price: 490,
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volume: 205000
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},
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{
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_id: ObjectId("61b11166b412e1948a407dcc"),
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time: '2021-03-08T12:00:00.000Z',
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company: 'CompanyA',
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price: 510,
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volume: 220000
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},
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{
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_id: ObjectId("61b11166b412e1948a407dce"),
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time: '2021-03-08T13:00:00.000Z',
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company: 'CompanyA',
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price: 505,
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volume: 225000
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},
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{
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_id: ObjectId("61b11166b412e1948a407dc7"),
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time: '2021-03-08T09:00:00.000Z',
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company: 'CompanyB',
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price: 20,
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volume: 100000
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},
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{
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_id: ObjectId("61b11166b412e1948a407dc9"),
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time: '2021-03-08T10:00:00.000Z',
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company: 'CompanyB',
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price: 22,
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volume: 105000
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},
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{
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_id: ObjectId("61b11166b412e1948a407dcb"),
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time: '2021-03-08T11:00:00.000Z',
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company: 'CompanyB',
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price: 24,
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volume: 105000
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},
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{
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_id: ObjectId("61b11166b412e1948a407dcd"),
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time: '2021-03-08T12:00:00.000Z',
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company: 'CompanyB',
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price: 24,
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volume: 105000
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},
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{
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_id: ObjectId("61b11166b412e1948a407dcf"),
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time: '2021-03-08T13:00:00.000Z',
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company: 'CompanyB',
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price: 28,
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volume: 120000
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}
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]

‎source/release-notes/5.2.txt

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.. include:: /includes/in-dev.rst
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.. _5.2-rel-notes-aggregation:
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Aggregation
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-----------
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.. _5.2-rel-notes-new-agg-operators:
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New Aggregation Operators
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~~~~~~~~~~~~~~~~~~~~~~~~~
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MongoDB 5.2 introduces the following aggregation operators:
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.. list-table::
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:header-rows: 1
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:widths: 20 80
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* - Operator
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- Description
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* - :group:`$locf`
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- .. include:: /includes/fact-locf-description.rst
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.. _5.2-rel-notes-general:
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General Improvements

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