MySQL 用 limit 为什么会影响性能？

Java技术栈www.javastack.cn优秀的Java技术公众号一，前言 首先说明一下MySQL的版本：mysql> select version();+-----------+| version() |+-----------+| 5.7.17 |+-----------+1 row in set (0.00 sec)表结构：mysql> desc test;+--------+---------------------+------+-----+---------+----------------+| Field  | Type                | Null | Key | Default | Extra          |+--------+---------------------+------+-----+---------+----------------+| id     | bigint(20) unsigned | NO   | PRI | NULL    | auto_increment || val    | int(10) unsigned    | NO   | MUL | 0       |                || source | int(10) unsigned    | NO   |     | 0       |                |+--------+---------------------+------+-----+---------+----------------+3 rows in set (0.00 sec)id为自增主键，val为非唯一索引。灌入大量数据， ………………………………