mongocxx统计集合中的文档

在shell中,计算集合中的文档数既简单又快速(显然是常数时间)。

> db.my_collection.count()
12345678
>

mongocxx::client client;
mongocxx::database db = MongoInit(client, ...);
vector<string> collection_names;
mongocxx::cursor cursor = db.list_collections();
for (const bsoncxx::document::view& doc : cursor) {
    string collection_name = doc["name"].get_utf8().value.to_string();
    collection_names.push_back(collection_name);
}

bsoncxx::document::view empty_filter;
for (const string& collection_name : collection_names) {
    LOG_INFO << collection_name;
    mongocxx::collection collection = db[collection_name];
    int64_t collection_count = collection.count_documents(empty_filter);
    LOG_INFO << collection_name << "    " << collection_count;
}

这段代码可以工作,但速度慢得出奇。我做错什么了吗?