coercion of input types in coalesce leads to type unsupported arrow cast

[davidhewitt]

Describe the bug

Tested on datafusion main at 5239d1a.

It looks like coercion of input types in coalesce (and probably other similar expressions) it's possible to end up with a common type of Dictionary(..., Utf8View), but arrow does not support casting to that type.

To Reproduce

select coalesce(arrow_cast('', 'Utf8View'), arrow_cast('', 'Dictionary(UInt32, Utf8)'));

leads to

Arrow error: Cast error: Unsupported output type for dictionary packing: Utf8View

Expected behavior

I would expect either:

• datafusion's coercion rules to not select a type which is not supported to go inside a dictionary (Utf8View), OR
• dictionary packing a Utf8View should be supported

Additional context

No response

[davidhewitt]

I have a workaround in pydantic@ba9fa86, which might be the basis for a fix within datafusion if dict-packing string views is seen as unnecessary?

[alamb]

I believe @jayzhan211 is working on coercion

• Signature::Coercible with user defined implicit casting #14440

[tustvold]

dict-packing string views is seen as unnecessar

I agree that a dictionary of a view type is a somewhat redundant construction