(root@node01) > select count(*) from customer where year(c_since)=2020;
+----------+
| count(*) |
+----------+
|      702 |
+----------+
1 row in set (0.46 sec)

(root@node01) > explain select count(*) from customer where year(c_since)=2020;
+----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | customer | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 287382 |   100.00 | Using where |
+----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

(root@node01) > alter table customer add key idx_customer_since((year(c_since)));          
Query OK, 0 rows affected (6.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

(root@node01) > select count(*) from customer where year(c_since)=2020;
+----------+
| count(*) |
+----------+
|      702 |
+----------+
1 row in set (0.01 sec)

(root@node01) > explain select count(*) from customer where year(c_since)=2020;
+----+-------------+----------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
| id | select_type | table    | partitions | type | possible_keys      | key                | key_len | ref   | rows | filtered | Extra |
+----+-------------+----------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | customer | NULL       | ref  | idx_customer_since | idx_customer_since | 5       | const |  702 |   100.00 | NULL  |
+----+-------------+----------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)