private DocumentSummary getOrCreateAttributePrefetchSummary(Schema schema) {
        DocumentSummary summary = schema.getSummariesInThis().get(SORTABLE_ATTRIBUTES_SUMMARY_CLASS);
        if (summary == null) {
            summary = new DocumentSummary(SORTABLE_ATTRIBUTES_SUMMARY_CLASS, schema);
            schema.addSummary(summary);
        }
        return summary;
    }
Makes implicitly defined summaries into explicit summaries

@author bratseth
config-model/src/main/java/com/yahoo/schema/processing/ImplicitSummaries.java