要弄懂回表操作，先弄懂索引结构。

https://www.51cto.com/article/699015.html

介绍:
mysql的普通索引存储的是索引字段和主键值(节省空间), 当查询的字段不在普通索引上时, 就会根据普通索引记录的主键值"回表"去查询主键索引.然后得到目标字段数据.
注:主键索引记录了主键值和该行所有字段值(不含LOB), 主键索引也叫簇集索引(cluster index).

例子:
假设有如下表结构

CREATE TABLE `t20221122` (
  `id` int(11) NOT NULL,
  `name` varchar(30) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `dream` varchar(20) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_name_age` (`name`,`dream`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert into t20221122 values(1, 'u1', 22, 'eat');
insert into t20221122 values(2, 'u2', 22, 'sleep');
insert into t20221122 values(3, 'u3', 22, 'stander');

当执行如下SQL时就会回表, 因为"dream"字段不在索引"idx_name_age"上, 所以会先得到 name=‘u2’ 的主键值, 然后根据主键值去查询目标字段, 从而得到数据.
select dream from t20221122 where name='u2';

(root@127.0.0.1) [db1]> explain select dream from t20221122 where name='u2';
+----+-------------+-----------+------------+------+---------------+--------------+---------+-------+------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys | key          | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-----------+------------+------+---------------+--------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | t20221122 | NULL       | ref  | idx_name_age  | idx_name_age | 93      | const |    1 |   100.00 | Using index |
+----+-------------+-----------+------------+------+---------------+--------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)