ianmcook commented on a change in pull request #10887: URL: https://github.com/apache/arrow/pull/10887#discussion_r690709164
########## File path: docs/source/cpp/compute.rst ########## @@ -234,6 +238,90 @@ 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 +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 of the input +values on some set of "key" columns, then aggregate each group individually, +and emit one output per input group. + +As an example, for the following table: + ++-----------------+--------------+ +| Column "x" | Column "key" | ++=================+==============+ +| 2 | "a" | ++-----------------+--------------+ +| 5 | "a" | ++-----------------+--------------+ +| null | "b" | ++-----------------+--------------+ +| null | "b" | ++-----------------+--------------+ +| null | null | ++-----------------+--------------+ +| 5 | null | ++-----------------+--------------+ + +We compute a sum of column "x", grouped on the key column "key". This gives us +three groups: + ++-----------------+--------------+ +| Column "sum(x)" | Column "key" | ++=================+==============+ +| 7 | "a" | ++-----------------+--------------+ +| null | "b" | ++-----------------+--------------+ +| 5 | null | ++-----------------+--------------+ + +The supported aggregation functions are as follows. Review comment: Could you add a sentence here explaining the meaning of the `hash_` prefix in these function names? -- 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: github-unsubscr...@arrow.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
