オプティマイザさん的には `select count(0)` らしい

gistfile1.txt

mysql80 110> EXPLAIN SELECT COUNT(*);
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | No tables used |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

mysql80 110> SHOW WARNINGS;
+-------+------+----------------------------------------------+
| Level | Code | Message                                      |
+-------+------+----------------------------------------------+
| Note  | 1003 | /* select#1 */ select count(0) AS `COUNT(*)` |
+-------+------+----------------------------------------------+
1 row in set (0.00 sec)