[GitHub] [arrow] alamb commented on pull request #8784: WIP: ARROW-10674: [Rust] Fix BigDecimal to be little endian; Add IPC Reader/Writer for Decimal type to allow integration tests

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alamb commented on pull request #8784:
URL: https://github.com/apache/arrow/pull/8784#issuecomment-737356705


   @sweb  -- it seems from the arrow definition that the endianness *may* be big or little endiain:
   https://github.com/apache/arrow/blob/master/format/Schema.fbs#L175-L178
   
   ```
   /// Exact decimal value represented as an integer value in two's
   /// complement. Currently only 128-bit (16-byte) and 256-bit (32-byte) integers
   /// are used. The representation uses the endianness indicated
   /// in the Schema.
   ```
   
   
   So I suggest at least validating the schema in the IPC message and error'ing with a "unimplemented" type error if it the schema is Big Endian. 
   
   The endianness can be checked:
   https://docs.rs/arrow/2.0.0/arrow/ipc/gen/Schema/struct.Schema.html#method.endianness
   
   Note that I still think this implementation (that now passes a new test) is still ok to merge in as is (as it is *more* correct for one case) but gracefully handling an unimplemented endianness would be better. 


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