private void addPrefetchAttribute(Attribute attribute, SDField field, Schema schema) {
        if (attribute.getPrefetchValue() == null) { // Prefetch by default - unless any summary makes this dynamic
            // Check if there is an implicit dynamic definition
            SummaryField fieldSummaryField = field.getSummaryField(attribute.getName());
            if (fieldSummaryField != null && fieldSummaryField.getTransform().isDynamic()) return;

            // Check if an explicit class makes it dynamic (first is enough, as all must be the same, checked later)
            SummaryField explicitSummaryField = schema.getExplicitSummaryField(attribute.getName());
            if (explicitSummaryField != null && explicitSummaryField.getTransform().isDynamic()) return;
        }

        DocumentSummary summary = getOrCreateAttributePrefetchSummary(schema);
        SummaryField attributeSummaryField = new SummaryField(attribute.getName(), attribute.getDataType(), summary);
        attributeSummaryField.addSource(attribute.getName());
        attributeSummaryField.addDestination(SORTABLE_ATTRIBUTES_SUMMARY_CLASS);
        attributeSummaryField.setTransform(SummaryTransform.ATTRIBUTE);
        summary.add(attributeSummaryField);
    }
Makes implicitly defined summaries into explicit summaries

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