Database: Rewrite system statistics query to perform better

[papagian]

What this PR does / why we need it:
This fix modifies the statistics queries for addressing efficiency issues following to this suggestion.

Which issue(s) this PR fixes:

Fixes #19137

Special notes for your reviewer:
Opening this as a draft since I'm not still able to evaluate the performance; I need to populate my database with more users and roles.

[papagian]

The execution plans for the old and the new query are:

mysql> EXPLAIN SELECT COUNT(*) FROM `user` as u WHERE (SELECT COUNT(*) FROM org_user  WHERE org_user.user_id=u.id AND org_user.role='Admin')>0;
+----+--------------------+----------+-------+---------------+----------------+---------+------+------+--------------------------+
| id | select_type        | table    | type  | possible_keys | key            | key_len | ref  | rows | Extra                    |
+----+--------------------+----------+-------+---------------+----------------+---------+------+------+--------------------------+
|  1 | PRIMARY            | u        | index | NULL          | UQE_user_login | 762     | NULL |    1 | Using where; Using index |
|  2 | DEPENDENT SUBQUERY | org_user | ALL   | NULL          | NULL           | NULL    | NULL |    1 | Using where              |
+----+--------------------+----------+-------+---------------+----------------+---------+------+------+--------------------------+
2 rows in set (0.00 sec)

mysql> EXPLAIN SELECT COUNT(*) FROM `user` as u, org_user WHERE ( org_user.user_id=u.id AND org_user.role='Admin' );
+----+-------------+----------+-------+---------------+----------------+---------+------+------+----------------------------------------------------+
| id | select_type | table    | type  | possible_keys | key            | key_len | ref  | rows | Extra                                              |
+----+-------------+----------+-------+---------------+----------------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | u        | index | PRIMARY       | UQE_user_login | 762     | NULL |    1 | Using index                                        |
|  1 | SIMPLE      | org_user | ALL   | NULL          | NULL           | NULL    | NULL |    1 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+----------+-------+---------------+----------------+---------+------+------+----------------------------------------------------+
2 rows in set (0.00 sec)

[marefr]

Great 👍