MariaDB [_]> SHOW CREATE TABLE t1;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                     |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `a` bigint(20) unsigned NOT NULL,
  `b` bigint(20) DEFAULT NULL,
  PRIMARY KEY (`a`),
  KEY `b` (`b`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [_]> SELECT COUNT(*), COUNT(DISTINCT b) FROM t1;
+----------+-------------------+
| COUNT(*) | COUNT(DISTINCT b) |
+----------+-------------------+
|  2267612 |           1974979 |
+----------+-------------------+
1 row in set (1.64 sec)

MariaDB [_]> SELECT COUNT(DISTINCT b) FROM t1;
+-------------------+
| COUNT(DISTINCT b) |
+-------------------+
|           1974979 |
+-------------------+
1 row in set (3.73 sec)

MariaDB [_]> EXPLAIN SELECT COUNT(DISTINCT b) FROM t1;
+------+-------------+-------+-------+---------------+------+---------+------+---------+--------------------------+
| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows    | Extra                    |
+------+-------------+-------+-------+---------------+------+---------+------+---------+--------------------------+
|    1 | SIMPLE      | t1    | range | NULL          | b    | 9       | NULL | 2267951 | Using index for group-by |
+------+-------------+-------+-------+---------------+------+---------+------+---------+--------------------------+
1 row in set (0.00 sec)

MariaDB [_]> SELECT COUNT(DISTINCT b+1) FROM t1;
+---------------------+
| COUNT(DISTINCT b+1) |
+---------------------+
|             1974979 |
+---------------------+
1 row in set (1.68 sec)

MariaDB [_]> EXPLAIN SELECT COUNT(DISTINCT b+1) FROM t1;
+------+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows    | Extra       |
+------+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
|    1 | SIMPLE      | t1    | index | NULL          | b    | 9       | NULL | 2267950 | Using index |
+------+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)