presto-record-decoder/src/main/java/io/prestosql/decoder/avro/AvroColumnDecoder.java/AvroColumnDecoder/locateNode
class AvroColumnDecoder:    private static Object locateNode(GenericRecord element, String columnMapping)
    {
        Object value = element;
        for (String pathElement : Splitter.on('/').omitEmptyStrings().split(columnMapping)) {
            if (value == null) {
                return null;
            }
            value = ((GenericRecord) value).get(pathElement);
        }
        return value;
    }