unary_dyn returns PrimitiveArray for DictionaryArray

[tustvold]

Which part is this question about

The implementation for unary_dyn currently iterates the dictionary keys, looking up the corresponding value for each non-null key, and storing the result in a PrimitiveArray.

Aside from being grossly inefficient, it seems counter-intuitive at least to me that calling dyn_unary would change the type of the array.

Describe your question

Should this kernel instead just apply the unary operation to the dictionary values, leaving the keys unchanged?

Additional context

Tagging @viirya @sunchao who implemented and reviewed this implementation initially in #1990

[tustvold]

Nvm brain fart, too early in the morning 😅