public boolean isAccessingDiskSummary(SummaryField field) {
        if (!field.getTransform().isInMemory()) return true;
        if (field.getSources().isEmpty()) return isAccessingDiskSummary(getName());
        for (SummaryField.Source source : field.getSources()) {
            if (isAccessingDiskSummary(source.getName()))
                return true;
        }
        return false;
    }
Returns the first occurrence of an attribute having this name, or null if none
config-model/src/main/java/com/yahoo/schema/Schema.java