presto-main/src/main/java/io/prestosql/block/BlockJsonSerde.java/Serializer/serialize
class Serializer:        @Override
        public void serialize(Block block, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
                throws IOException
        {
            SliceOutput output = new DynamicSliceOutput(64);
            writeBlock(blockEncodingSerde, output, block);
            String encoded = Base64.getEncoder().encodeToString(output.slice().getBytes());
            jsonGenerator.writeString(encoded);
        }