Weston Pace created ARROW-16288:
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Summary: [C++] ValueDescr::SCALAR nearly unused and does not work
for projection
Key: ARROW-16288
URL: https://issues.apache.org/jira/browse/ARROW-16288
Project: Apache Arrow
Issue Type: Improvement
Components: C++
Reporter: Weston Pace
First, there are almost no kernels that actually use this shape. Only the
functions "all", "any", "list_element", "mean", "product", "struct_field", and
"sum" have kernels with this shape. Most kernels that have special logic for
scalars handle it by using {{ValueDescr::ANY}}
Second, when passing an expression to the project node, the expression must be
bound based on the dataset schema. Since the binding happens based on a schema
(and not a batch) the function is bound to ValueDescr::ARRAY
(https://github.com/apache/arrow/blob/a16be6b7b6c8271202ff766b99c199b2e29bdfa8/cpp/src/arrow/compute/exec/expression.cc#L461)
This results in an error if the function has only ValueDescr::SCALAR kernels
and would likely be a problem even if the function had both types of kernels
because it would get bound to the wrong kernel.
This simplest fix may be to just get rid of ValueDescr and change all kernels
to ValueDescr::ANY behavior. If we choose to keep it we will need to figure
out how to handle this kind of binding.
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