presto-main/src/main/java/io/prestosql/util/JsonUtil.java/RowJsonGeneratorWriter/writeJsonValue
class RowJsonGeneratorWriter:        @Override
        public void writeJsonValue(JsonGenerator jsonGenerator, Block block, int position, ConnectorSession session)
                throws IOException
        {
            if (block.isNull(position)) {
                jsonGenerator.writeNull();
            }
            else {
                Block rowBlock = type.getObject(block, position);
                jsonGenerator.writeStartArray();
                for (int i = 0; i < rowBlock.getPositionCount(); i++) {
                    fieldWriters.get(i).writeJsonValue(jsonGenerator, rowBlock, i, session);
                }
                jsonGenerator.writeEndArray();
            }
        }