索引覆盖
Extra字段显示Using index，索引处取得的数据即是要求的数据，则不会再回数据文件再查询，直接返回了。
explain select id from account where id = 1;
输出：
mysql> explain select id from account where id = 1;
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | account | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
回表查询
取得索引上保存的数据，再回数据文件中查询所要求的的全部的属性值。
explain select * from account where id = 1;
输出：
mysql> explain select * from account where id = 1;
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | account | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
原理
B+树在叶子节点保存节点索引和节点数据，这里的节点数据保存的只是创建索引时指定的字段，其他字段在数据文件中。
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