[ARROW-16757] [C++] Remove "scalar" output modality from array kernels

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 9.0.0
• Component/s: C++
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/32092
• Assignee: Wes McKinney
• Reporter: Wes McKinney
• Created: 06/Jun/22 16:11
• Updated: 11/Jan/23 11:46
• Resolved: 08/Jul/22 02:06

Description

Supporting scalar outputs from array kernels (where all the inputs are scalars) introduces needless complexity into the kernel implementations, causing duplication of effort and excess code generation for paltry benefit. In the scenario where all inputs are scalars, it would be better to promote them all to arrays of length 1 (either by creating the arrays or constructing an appropriate ArraySpan per ARROW-16756) and invoking the array code path. This would enable us to delete thousands of lines of code and ease the ongoing development and maintenance of the array kernels codebase

Issue Links

• relates to: ARROW-16755 [C++] Improve array expression and kernel evaluation performance on small inputs
• supercedes: ARROW-16288 [C++] ValueDescr::SCALAR nearly unused and does not work for projection
• links to: GitHub Pull Request #13521
• links to: GitHub Pull Request #13557