The so-called index extension refers to that in the secondary index, innodb adds the primary key to the key of the secondary index, which usually leads to good performance and query plan. Index extension can be used for index scanning of ref, range, index ﹣ merge, loose index scanning, join and sorting optimization,

 CREATE TABLE `t1` (
  `i1` int(11) NOT NULL DEFAULT '0',
  `i2` int(11) NOT NULL DEFAULT '0',
  `d` date DEFAULT NULL,
  PRIMARY KEY (`i1`,`i2`),
  KEY `k_d` (`d`)

//This is the execution plan with index extension
mysql> EXPLAIN SELECT COUNT(*) FROM t1 WHERE i1 = 3 AND d = '2000-01-01';
+----+-------------+-------+------------+------+---------------+------+---------+-------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref         | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+-------------+------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | ref  | PRIMARY,k_d   | k_d  | 8       | const,const |    1 |   100.00 | Using index |
+----+-------------+-------+------------+-
//This is the execution plan without index extension
mysql> SET optimizer_switch = 'use_index_extensions=off';
Query OK, 0 rows affected (0.00 sec)

mysql> EXPLAIN SELECT COUNT(*) FROM t1 WHERE i1 = 3 AND d = '2000-01-01';
+----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | ref  | PRIMARY,k_d   | PRIMARY | 4       | const |    5 |    20.00 | Using where |
+----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

It can be seen that in the execution plan where the extension is used, the key len is longer, the ref is const which has been used twice, and the rows are fewer. In the extra, the using index method is used, and the overall effect is better