private void populateIndexToFields(Schema schema) {
        for (SDField field : schema.allConcreteFields() ) {
            if ( ! field.doesIndexing()) continue;
            addIndexField(field.getName(), field);
        }
    }
A map from index names to a List of fields going to that index
config-model/src/main/java/com/yahoo/schema/processing/MultifieldIndexHarmonizer.java