presto-main/src/main/java/io/prestosql/operator/scalar/JsonExtract.java/JsonValueJsonExtractor/extract
class JsonValueJsonExtractor:        @Override
        public Slice extract(JsonParser jsonParser)
                throws IOException
        {
            if (!jsonParser.hasCurrentToken()) {
                throw new JsonParseException(jsonParser, "Unexpected end of value");
            }

            DynamicSliceOutput dynamicSliceOutput = new DynamicSliceOutput(ESTIMATED_JSON_OUTPUT_SIZE);
            try (JsonGenerator jsonGenerator = createJsonGenerator(JSON_FACTORY, dynamicSliceOutput)) {
                jsonGenerator.copyCurrentStructure(jsonParser);
            }
            return dynamicSliceOutput.slice();
        }