[I] Support `Union` data type coercion [datafusion]

["friendlymatthew (via GitHub)" <gi...@apache.org>]

friendlymatthew opened a new issue, #18825:
URL: https://github.com/apache/datafusion/issues/18825

   ### Is your feature request related to a problem or challenge?
   
   Union arrays currently cannot be used in comparison operations with scalar values or other types. When attempting to filter or compare a union column against a constant value (e.g., `where union_column = 67`), datafusion fails during type coercion
   
   This prevents common filtering and selection ops on union-typed data. At work, we interact with a lot of JSON and accessing data with `->` returns Union types. Currently the following query will err:
   
   ```sql
   select * from records
   where json_val->'id' = 123
   ```
   
   ### Describe the solution you'd like
   
   - Write a `union_coercion` function that handles union-to-scalar and union-to-union type coercion, following the pattern established by other composite types like `struct_coercion` and `map_coercion`
   - Extend `comparison_coercion` to chain to `union_coercion` as part of its coercion attempts
   
   On the arrow-side, comparison kernel support for unions are being developed:
   - https://github.com/apache/arrow-rs/issues/8837
   - https://github.com/apache/arrow-rs/issues/8881
   
   
   
   ### Describe alternatives you've considered
   
   _No response_
   
   ### Additional context
   
   _No response_


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Re: [I] Support `Union` data type coercion [datafusion]

["adriangb (via GitHub)" <gi...@apache.org>]

adriangb closed issue #18825: Support `Union` data type coercion
URL: https://github.com/apache/datafusion/issues/18825


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