Function visit_decimal 

fn visit_decimal<T: NativeDecimalType, V: FixedLenByteArrayVisitor>(
    array: &PrimitiveArray<T>,
    visitor: V,
) -> Result<V::Output>

Passes the big-endian two’s complement bytes of the values of array to visitor

The values are truncated to their low FixedLenByteArrayVisitor::decimal_length bytes, which must not be more than the length of the Arrow decimal type. A value too large for the length is truncated, as ArrowWriter has always done, since Arrow does not validate precision.