CREATE TABLE t1 (a int, b int, INDEX idx(a));
INSERT INTO t1 VALUES (9,0), (7,1), (1,9), (7,3), (2,1);

CREATE TABLE t2 (a int, b int, INDEX idx(a));
INSERT INTO t2 VALUES (2,1), (6,4), (7,6), (9,4);

CREATE TABLE t3 (a int, b int);
INSERT INTO t3 VALUES (1,0), (1,1), (1,3);

SELECT * FROM t3
  WHERE a = (SELECT COUNT(DISTINCT t2.b) FROM t1, t2
                WHERE t1.a = t2.a AND t2.a BETWEEN 7 AND 9
                      AND t3.b = t1.b
              GROUP BY t1.b);

MariaDB [test]> SELECT * FROM t3
    ->   WHERE a = (SELECT COUNT(DISTINCT t2.b) FROM t1, t2
    ->                 WHERE t1.a = t2.a AND t2.a BETWEEN 7 AND 9
    ->                       AND t3.b = t1.b
    ->               GROUP BY t1.b);
+------+------+
| a    | b    |
+------+------+
|    1 |    0 |
|    1 |    3 |
+------+------+

 
MariaDB [test]> EXPLAIN
    -> SELECT * FROM t3
    ->   WHERE a = (SELECT COUNT(DISTINCT t2.b) FROM t1, t2
    ->                 WHERE t1.a = t2.a AND t2.a BETWEEN 7 AND 9
    ->                       AND t3.b = t1.b
    ->               GROUP BY t1.b);
+----+--------------------+-------+-------+---------------+------+---------+-----------+------+----------------------------------------------------+
| id | select_type        | table | type  | possible_keys | key  | key_len | ref       | rows | Extra                                              |
+----+--------------------+-------+-------+---------------+------+---------+-----------+------+----------------------------------------------------+
|  1 | PRIMARY            | t3    | ALL   | NULL          | NULL | NULL    | NULL      |    3 | Using where                                        |
|  2 | DEPENDENT SUBQUERY | t1    | range | idx           | idx  | 5       | NULL      |    2 | Using index condition; Using where; Using filesort |
|  2 | DEPENDENT SUBQUERY | t2    | ref   | idx           | idx  | 5       | test.t1.a |    2 | Using index condition                              |
+----+--------------------+-------+-------+---------------+------+---------+-----------+------+----------------------------------------------------+
3 rows in set (0.00 sec)

MariaDB [test]> set optimizer_switch='index_condition_pushdown=off';
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> SELECT * FROM t3
    ->   WHERE a = (SELECT COUNT(DISTINCT t2.b) FROM t1, t2
    ->                 WHERE t1.a = t2.a AND t2.a BETWEEN 7 AND 9
    ->                       AND t3.b = t1.b
    ->               GROUP BY t1.b);
+------+------+
| a    | b    |
+------+------+
|    1 |    0 |
|    1 |    1 |
|    1 |    3 |
+------+------+
3 rows in set (0.00 sec)