presto-record-decoder/src/main/java/io/prestosql/decoder/raw/RawColumnDecoder.java/RawValueProvider/getSlice
class RawValueProvider:        @Override
        public Slice getSlice()
        {
            Slice slice = Slices.wrappedBuffer(value.slice());
            return Varchars.truncateToLength(slice, columnType);
        }