[GitHub] [arrow] jorisvandenbossche commented on a change in pull request #10887: ARROW-13311: [C++][Documentation] Document hash aggregate kernels

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jorisvandenbossche commented on a change in pull request #10887:
URL: https://github.com/apache/arrow/pull/10887#discussion_r691865972



##########
File path: docs/source/cpp/compute.rst
##########
@@ -234,6 +238,93 @@ Notes:
 
 * \(6) Output is Float64 or input type, depending on QuantileOptions.
 
+* \(7) tdigest/t-digest computes approximate quantiles, and so only needs a
+  fixed amount of memory. See the `reference implementation
+  <https://github.com/tdunning/t-digest>`_ for details.
+
+Grouped Aggregations ("group by")
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Grouped aggregations are not directly invokable, but are used as part of a
+SQL-style "group by" operation. Like scalar aggregations, grouped aggregations
+reduce multiple input values to a single output value. Instead of aggregating
+all values of the input, however, grouped aggregations partition the input
+values on some set of "key" columns, then aggregate each group individually,
+emitting one output value per input group.
+
+As an example, for the following table:
+
++------------------+-----------------+
+| Column ``key``   | Column ``x``    |
++==================+=================+
+| "a"              | 2               |
++------------------+-----------------+
+| "a"              | 5               |
++------------------+-----------------+
+| "b"              | null            |
++------------------+-----------------+
+| "b"              | null            |
++------------------+-----------------+
+| null             | null            |
++------------------+-----------------+
+| null             | 9               |
++------------------+-----------------+
+
+we can compute a sum of the column ``x``, grouped on the column ``key``.
+This gives us three groups, with the following results. Note that null is
+treated as a distinct key value.
+
++------------------+-----------------------+
+| Column ``key``   | Column ``sum(x)``     |
++==================+=======================+
+| "a"              | 7                     |
++------------------+-----------------------+
+| "b"              | null                  |
++------------------+-----------------------+
+| null             | 9                     |
++------------------+-----------------------+
+
+The supported aggregation functions are as follows. All function names are
+prefixed with ``hash_``, which differentiates them from their scalar
+equivalents above and reflects how they are implemented internally.
+
++---------------+-------+-------------+----------------+----------------------------------+-------+
+| Function name | Arity | Input types | Output type    | Options class                    | Notes |
++===============+=======+=============+================+==================================+=======+
+| hash_all      | Unary | Boolean     | Int64          | :struct:`ScalarAggregateOptions` | \(1)  |
++---------------+-------+-------------+----------------+----------------------------------+-------+
+| hash_any      | Unary | Any         | Int64          | :struct:`ScalarAggregateOptions` | \(1)  |

Review comment:
       ```suggestion
   | hash_all      | Unary | Boolean     | Boolean        | :struct:`ScalarAggregateOptions` | \(1)  |
   +---------------+-------+-------------+----------------+----------------------------------+-------+
   | hash_any      | Unary | Boolean     | Boolean        | :struct:`ScalarAggregateOptions` | \(1)  |
   ```
   
   ? 
   
   At least, the non-hash versions of any/all only work on boolean input, and return booleans (didn't check for the hashed version)




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