public static String enoughFields(String summaryClass, Result result) {
        var summaryFields = result.getQuery().getPresentation().getSummaryFields();
        if (isDefaultRequest(summaryClass) && ! summaryFields.isEmpty()) {
            // it's enough to have the string we would use as fillMarker
            return syntheticName(summaryFields);
        } else {
            // this is 'always' enough:
            return ALL_FIELDS_CLASS;
        }
    }
resolve summary class to use when none provided
container-search/src/main/java/com/yahoo/prelude/fastsearch/PartialSummaryHandler.java