dimakuz opened a new pull request, #710:
URL: https://github.com/apache/arrow-go/pull/710

   …imal256
   
   bigEndianToDecimal256 in column_readers.go has a bug in the partial-word 
sign extension path: it shifts by wordLen (byte count) instead of wordLen*8 
(bit count). This is a copy-paste error from bigEndianToDecimal128 which 
correctly uses `* 8`.
   
   The bug corrupts negative Decimal256 values when the Parquet byte width is 
not a multiple of 8. For example, Decimal256 with precision 40 uses 17 bytes 
(DecimalSize(40) = 17). Reading such values back splits into iterations of 8 + 
8 + 1 bytes, and the 1-byte partial word triggers the broken shift, mangling 
the sign extension.
   
   ### Rationale for this change
   
   
   ### What changes are included in this PR?
   
   
   ### Are these changes tested?
   
   
   ### Are there any user-facing changes?
   
   


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