[ARROW-5588] [C++] Better support for building UnionArrays

• Type: New Feature
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 0.15.0
• Component/s: C++
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/22029
• Assignee: Ben Kietzman
• Reporter: Ben Kietzman
• Created: 13/Jun/19 13:03
• Updated: 11/Jan/23 07:41
• Resolved: 12/Jul/19 14:40

Description

UnionBuilders (for both sparse and dense mode unions) are not currently supported by MakeBuilder or ArrayFromJSON. This increases friction when working with and testing against union arrays, and support should be added to both. For ArrayFromJSON each entry must be specified with a (type code, value) pair:

ArrayFromJSON(union_({field("lint", list(int32())), field("str", utf8())}), R"([
  [0, null],
  [1, "hello"],
  [0, [1, 2]],
  [1, "world"]
])");

DenseUnionBuilder currently requires the user to explicitly input offsets, but if it were modified to hold pointers to child builders (as ListBuilder, for example) then those offsets could be derived from the lengths of child builders (which is much more user friendly).

Issue Links

• blocks: ARROW-5594 [C++] add support for UnionArrays to Take and Filter
• links to: GitHub Pull Request #4781