MariaDB [test]>  create or replace table t(i int primary key);
Query OK, 0 rows affected (0.001 sec)

MariaDB [test]> insert into t select seq from seq_1_to_10000000;
Query OK, 10000000 rows affected (12.952 sec)
Records: 10000000  Duplicates: 0  Warnings: 0

MariaDB [test]> 
MariaDB [test]> explain extended select count(*) from t ;
+------+-------------+-------+-------+---------------+---------+---------+------+---------+----------+-------------+
| id   | select_type | table | type  | possible_keys | key     | key_len | ref  | rows    | filtered | Extra       |
+------+-------------+-------+-------+---------------+---------+---------+------+---------+----------+-------------+
|    1 | SIMPLE      | t     | index | NULL          | PRIMARY | 4       | NULL | 9985950 |   100.00 | Using index |
+------+-------------+-------+-------+---------------+---------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.000 sec)

MariaDB [test]> select count(*) from t;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.842 sec)

MariaDB [test]> select count(*) from t;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.846 sec)

MariaDB [test]> select count(*) from t;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.844 sec)

MariaDB [test]> 
MariaDB [test]> explain extended select count(*) from t force index (primary);
+------+-------------+-------+-------+---------------+---------+---------+------+---------+----------+-------------+
| id   | select_type | table | type  | possible_keys | key     | key_len | ref  | rows    | filtered | Extra       |
+------+-------------+-------+-------+---------------+---------+---------+------+---------+----------+-------------+
|    1 | SIMPLE      | t     | index | NULL          | PRIMARY | 4       | NULL | 9985950 |   100.00 | Using index |
+------+-------------+-------+-------+---------------+---------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.000 sec)

MariaDB [test]> select count(*) from t force index (primary) ;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.843 sec)

MariaDB [test]> select count(*) from t force index (primary) ;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.846 sec)

MariaDB [test]> select count(*) from t force index (primary) ;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.843 sec)

MariaDB [test]> select count(*) from t force index (primary) ;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.844 sec)

MariaDB [test]> select version();
+---------------------+
| version()           |
+---------------------+
| 10.11.8-MariaDB-log |
+---------------------+
1 row in set (0.000 sec)