public Mappings diffMappings(String tableName, Mappings mappings) {
        if (!mappingStructures.containsKey(tableName)) {
            return new Mappings();
        }
        Map<String, Object> diffProperties =
            mappingStructures.get(tableName).diffFields(new Fields(mappings));
        return Mappings.builder()
                       .type(ElasticSearchClient.TYPE)
                       .properties(diffProperties)
                       .build();
    }
Returns mappings with fields that not exist in the input mappings.
The input mappings should be history mapping from current index.
Do not return _source config to avoid current index update conflict.
oap-server/server-storage-plugin/storage-elasticsearch-plugin/src/main/java/org/apache/skywalking/oap/server/storage/plugin/elasticsearch/base/IndexStructures.java