fix: issue introduced in #6833 - less than equal check for scale in d…

…ecimal conversion (#7070)

* fix <= check for scale in decimal conversion

* Update arrow-cast/src/cast/mod.rs

name change

Co-authored-by: Arttu <Blizzara@users.noreply.github.com>

* remove incorrect comment

---------

Co-authored-by: Arttu <Blizzara@users.noreply.github.com>

arrow-cast/src/cast/decimal.rs

@@ -167,8 +167,7 @@ where
     let array: PrimitiveArray<T> =
         if input_scale == output_scale && input_precision <= output_precision {
             array.clone()
-        } else if input_scale < output_scale {
-            // the scale doesn't change, but precision may change and cause overflow
+        } else if input_scale <= output_scale {
             convert_to_bigger_or_equal_scale_decimal::<T, T>(
                 array,
                 input_scale,

arrow-cast/src/cast/mod.rs

@@ -9980,7 +9980,26 @@ mod tests {
         };
         let result = cast_with_options(&array, &output_type, &options);
         assert_eq!(result.unwrap_err().to_string(),
-                   "Invalid argument error: 123456790 is too large to store in a Decimal128 of precision 6. Max is 999999");
+                   "Invalid argument error: 123456789 is too large to store in a Decimal128 of precision 6. Max is 999999");
+    }
+
+    #[test]
+    fn test_decimal_to_decimal_same_scale() {
+        let array = vec![Some(520)];
+        let array = create_decimal_array(array, 4, 2).unwrap();
+        let input_type = DataType::Decimal128(4, 2);
+        let output_type = DataType::Decimal128(3, 2);
+        assert!(can_cast_types(&input_type, &output_type));
+
+        let options = CastOptions {
+            safe: false,
+            ..Default::default()
+        };
+        let result = cast_with_options(&array, &output_type, &options);
+        assert_eq!(
+            result.unwrap().as_primitive::<Decimal128Type>().value(0),
+            520
+        );
     }
 
     #[test]