索引 - Mysql Limit查询优化问题

Question

1.《高性能MYSQL》上的，没看懂这是什么优化，film_id应该是主键吧，求解释

2.顺便再问一个问题，这个sql说明使用索引，不过file_name并不是索引列，具体执行的时候是怎么执行，扫描索引列再匹配file_name，看着是这样子吧，这个sql能改写下达到相同的想过吗

Answer 1

1. This rewriting is mainly based on the structural characteristics of the index. The original statement may cause cross-page when the field is long, and the limit statement itself is very inefficient. In the rewritten query, index coverage can be achieved by querying film_id alone.

2. I don’t understand what you mean.