lidavidm commented on a change in pull request #10887: URL: https://github.com/apache/arrow/pull/10887#discussion_r691360131
########## 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 | ++------------------+-------------------+ Review comment: Thanks, I wish reST didn't try to be so smart about this…or I wish there was something easier to control -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
