mongodb - mongo的sort排序问题

Question

最近用了mongo，发现个问题，某个集合里，文档对象里有个ctime字段，是MongoDate类型的，我做这样的查询db.find().sort({'ctime':-1}).limit(10)，发现是全表扫描了一遍，要想快除非建个{'ctime':-1}的索引

但是用mysql的时候，ctime不建索引，扫描数量也不会很高，因为对数据库底层不是很了解，希望有大神可以指点一下，或者也许是我mongo使用错误，下面是mysql的详情

Answer 1

In order not to scan the entire table, indexes are of course necessary. Otherwise, how do we know that the ctime in other data must not be greater than the found pieces of data. As for MySQL, how do you know it has not scanned the entire table? MySQL's data storage is compact, and its scanning speed is much faster than Mongo. Therefore, MySQL has an advantage in the absence of indexes.

One of the advantages of Mongo is its rich query methods, and to demonstrate this feature of Mongo, reasonable and necessary indexes are indispensable.