presto-main/src/main/java/io/prestosql/operator/scalar/JsonExtract.java/ScalarValueJsonExtractor/extract
class ScalarValueJsonExtractor:        @Override
        public Slice extract(JsonParser jsonParser)
                throws IOException
        {
            JsonToken token = jsonParser.getCurrentToken();
            if (token == null) {
                throw new JsonParseException(jsonParser, "Unexpected end of value");
            }
            if (!token.isScalarValue() || token == VALUE_NULL) {
                return null;
            }
            return utf8Slice(jsonParser.getText());
        }