parthchandra opened a new pull request, #3939:
URL: https://github.com/apache/datafusion-comet/pull/3939

   ## Which issue does this PR close?
   
   Part of https://github.com/apache/datafusion-comet/issues/286
   
   ## Rationale for this change
   
   `CometCast.canCastToString` had previously marked DecimalType -> StringType 
as Compatible with a caveat. However,  the native Decimal128 -> String cast was 
not explicitly handled in cast_array and it was falling through to DataFusion's 
built-in cast, which was incompatible with Spark legacy mode for large scale 
values.
   
   For example, zero with a large scale: Decimal(38,18) zero exponent = -18 
which produced "0E-18" in Spark LEGACY but "0.000000000000000000" from Comet
   
     TRY and ANSI modes are compatible
   
   
   ## What changes are included in this PR?
   
   Adds explicit handling for legacy mode
   
   ## How are these changes tested?
   
   unit tests


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