This is a design deficiency in CallFunction. If a function is nullary (zero input arguments), the execution machinery has no way to determine the output length from an empty vector of datums. We should add CallFunction variants based on ExecBatch and ExecSpan (from ARROW-16755) and figure out how to deal with the split between scalar (where inputs must all be the same length or scalar) and vector functions (whose input arguments can have different length)

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• [C++] Improve array expression and kernel evaluation performance on small inputs (relates to)
• [C++] Introduce initial ArraySpan, ExecSpan non-owning / shared_ptr-free data structures for kernel execution, refactor scalar kernels (relates to)

_{Note: This issue was originally created as ARROW-16819. Please see the migration documentation for further details.}