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 (13.018 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     | ALL  | NULL          | NULL | NULL    | NULL | 9985950 |   100.00 |       |
+------+-------------+-------+------+---------------+------+---------+------+---------+----------+-------+
1 row in set, 1 warning (0.000 sec)

MariaDB [test]> select count(*) from t;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.756 sec)

MariaDB [test]> select count(*) from t;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.754 sec)

MariaDB [test]> select count(*) from t;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.754 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     | ALL  | NULL          | NULL | NULL    | NULL | 9985950 |   100.00 |       |
+------+-------------+-------+------+---------------+------+---------+------+---------+----------+-------+
1 row in set, 1 warning (0.000 sec)

MariaDB [test]> select count(*) from t force index (primary) ;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.760 sec)

MariaDB [test]> select count(*) from t force index (primary) ;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.758 sec)

MariaDB [test]> select count(*) from t force index (primary) ;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.759 sec)

MariaDB [test]> select count(*) from t force index (primary) ;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.762 sec)

MariaDB [test]> select version();
+----------------+
| version()      |
+----------------+
| 11.0.5-MariaDB |
+----------------+
1 row in set (0.000 sec)

MariaDB [test]>