edubraqd opened a new pull request, #24899:
URL: https://github.com/apache/datafusion/pull/24899

   ## Which issue does this PR close?
   
   - Closes #24898.
   
   ## Rationale for this change
   
   `DecimalAverager::try_new` computed `10^sum_scale` and `10^target_scale` 
separately with `pow_wrapping(scale as u32)`. For a negative scale the exponent 
wraps and the result is 0, so `avg` divided by zero and panicked for any 
decimal input with a negative scale.
   
   ## What changes are included in this PR?
   
   The two factors were only ever used as the ratio `10^(target_scale - 
sum_scale)`, so `DecimalAverager` now stores that single multiplier, computed 
from the scale difference. The existing `Arithmetic Overflow in AvgAccumulator` 
error for a target scale smaller than the sum scale is kept (it is now the 
`scale_diff < 0` check).
   
   ## Are these changes tested?
   
   Yes. `avg_decimal_negative_scale` averages `Decimal128(10, -2)` values and 
checks the `Decimal128(14, 2)` result; it used to panic. The existing `average` 
and `functions-aggregate-common` unit tests pass.
   
   ## Are there any user-facing changes?
   
   `avg` over decimals with a negative scale returns a result instead of 
panicking.
   


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