[ARROW-12042] [C++] Change or rationalize output of array_sort_indices on ChunkedArray

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 4.0.0
• Fix Version/s: 7.0.0
• Component/s: C++
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/27871
• Assignee: Antoine Pitrou
• Reporter: Ian Cook
• Created: 22/Mar/21 16:47
• Updated: 11/Jan/23 08:23
• Resolved: 03/Jan/22 17:37

Description

Currently when the array_sort_indices compute function is called on a ChunkedArray of two or more Arrays, it returns a ChunkedArray of Arrays of local sort indices for each Array. Demonstrating this with the R bindings:

> x <- ChunkedArray$create(c(2L, 1L), c(4L, 3L))
> arrow:::call_function("array_sort_indices", x, options = list(order = FALSE))
ChunkedArray
[
  [
    1,
    0
  ],
  [
    1,
    0
  ]
]

Compare to the sort_indices compute function which returns an Array of global sort indices in this case:

> arrow:::call_function("sort_indices", x, options = list(names = "", orders = 0L))
Array
<uint64>
[
  1,
  0,
  3,
  2
]

Is this behavior deliberate? If so, we should document it clearly. If not, we should change it.

Note that the docs currently states that array_sort_indices only works on Arrays https://arrow.apache.org/docs/cpp/compute.html#sorts-and-partitions (see note (4)) but evidently that is not exactly correct.

Issue Links

• links to: GitHub Pull Request #12064