presto-main/src/main/java/io/prestosql/util/JsonUtil.java/JsonJsonGeneratorWriter/writeJsonValue
class JsonJsonGeneratorWriter:        @Override
        public void writeJsonValue(JsonGenerator jsonGenerator, Block block, int position, ConnectorSession session)
                throws IOException
        {
            if (block.isNull(position)) {
                jsonGenerator.writeNull();
            }
            else {
                Slice value = JSON.getSlice(block, position);
                jsonGenerator.writeRawValue(value.toStringUtf8());
            }
        }