EmilyMatt opened a new pull request, #18812:
URL: https://github.com/apache/datafusion/pull/18812

   ## Which issue does this PR close?
   
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   - Closes #18811 .
   
   ## Rationale for this change
   
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   The main issue I was trying to solve is that despite the unreachable!() 
block in the avro-to-arrow schema conversion, when providing an arrow schema to 
the reader, the writer_schema from the file could still contain Ref types, and 
we'd never know about it, there is no error handling there, the values would 
simply return as Null.
   This is much more severe than just crashing due to a not-yet-implemented 
feature.
   However I used this opportunity to implement this for the schema conversion 
as well.
   
   ## What changes are included in this PR?
   
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   A new lookup method for the schema, both when resolving positions, and when 
actually converting avro schemas to arrow schemas, this will let us resolve Ref 
types under some constraints (Namely, arrow does not allow circular 
dependencies in schema, I'm not sure how this could ever be accomplished 
without some drastic changes in the way schemas are composed)
   
   ## Are these changes tested?
   
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   Yes, added as many tests as I could think of.
   
   ## Are there any user-facing changes?
   
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   Not really, should just work out of the box.


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