MariaDB [zabbix]> analyze  select distinct i2.key_ from items i join item_discovery id on id.itemid=i.itemid join items i2 on id.parent_itemid=i2.itemid join functions f on i.itemid=f.itemid and f.triggerid=13272 where i.flags=2;
+------+-------------+-------+--------+-----------------------------------+------------------+---------+----------------------------------+------+-----------+----------+------------+------------------------------------+
| id   | select_type | table | type   | possible_keys                     | key              | key_len | ref                              | rows | r_rows    | filtered | r_filtered | Extra                              |
+------+-------------+-------+--------+-----------------------------------+------------------+---------+----------------------------------+------+-----------+----------+------------+------------------------------------+
|    1 | SIMPLE      | i     | ALL    | PRIMARY                           | NULL             | NULL    | NULL                             | 1    | 188622.00 |   100.00 |      13.01 | Using where; Using temporary       |
|    1 | SIMPLE      | i2    | ALL    | PRIMARY                           | NULL             | NULL    | NULL                             | 1    | 188622.00 |   100.00 |     100.00 | Using join buffer (flat, BNL join) |
|    1 | SIMPLE      | f     | ref    | functions_1,functions_2           | functions_1      | 8       | const                            | 1    | 1.00      |   100.00 |       0.00 | Using where; Distinct              |
|    1 | SIMPLE      | id    | eq_ref | item_discovery_1,item_discovery_2 | item_discovery_1 | 16      | zabbix.i.itemid,zabbix.i2.itemid | 1    | 0.00      |   100.00 |     100.00 | Using index; Distinct              |
+------+-------------+-------+--------+-----------------------------------+------------------+---------+----------------------------------+------+-----------+----------+------------+------------------------------------+
4 rows in set (10 hours 22 min 12.916 sec)