[ARROW-17339] [C++] Simplify IPC writer for dense unions

• Type: Improvement
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Component/s: C++
• Labels: good-first-issue good-second-issue
• External issue URL: https://github.com/apache/arrow/issues/32613
• Assignee: Unassigned
• Reporter: Antoine Pitrou
• Created: 08/Aug/22 12:48
• Updated: 11/Jan/23 11:50

Description

In ARROW-10580 we fixed the Arrow C++ implementation so that dense union offsets are always (non-strictly) monotonic for a given child, as mandated by the spec.

The IPC writer implementation, however, still assumes that dense union offsets may be in any order:
https://github.com/apache/arrow/blob/5719576c611929dd790f7f8a1ae3169a8f96f7f1/cpp/src/arrow/ipc/writer.cc#L476-L485

This can probably be simplified, making it slightly less costly to emit a sliced union array.

Issue Links

• relates to: ARROW-10580 [C++] When Validating, ensure DenseUnionArray offsets are increasing