MariaDB [zabbix]> describe  select distinct i2.key_ from items i join item_discovery id on id.itemid=i.itemid j oin 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 | Extra                              |
+------+-------------+-------+--------+-----------------------------------+------------------+---------+------- ---------------------------+------+------------------------------------+
|    1 | SIMPLE      | i     | ALL    | PRIMARY                           | NULL             | NULL    | NULL                              | 1    | Using where; Using temporary       |
|    1 | SIMPLE      | i2    | ALL    | PRIMARY                           | NULL             | NULL    | NULL                              | 1    | Using join buffer (flat, BNL join) |
|    1 | SIMPLE      | f     | ref    | functions_1,functions_2           | functions_1      | 8       | const                             | 1    | 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    | Using index; Distinct              |
+------+-------------+-------+--------+-----------------------------------+------------------+---------+------- ---------------------------+------+------------------------------------+
4 rows in set (0.002 sec)


MariaDB [zabbix]> explain  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 | Extra                              |
+------+-------------+-------+--------+-----------------------------------+------------------+---------+----------------------------------+------+------------------------------------+
|    1 | SIMPLE      | i     | ALL    | PRIMARY                           | NULL             | NULL    | NULL                             | 1    | Using where; Using temporary       |
|    1 | SIMPLE      | i2    | ALL    | PRIMARY                           | NULL             | NULL    | NULL                             | 1    | Using join buffer (flat, BNL join) |
|    1 | SIMPLE      | f     | ref    | functions_1,functions_2           | functions_1      | 8       | const                            | 1    | 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    | Using index; Distinct              |
+------+-------------+-------+--------+-----------------------------------+------------------+---------+----------------------------------+------+------------------------------------+
4 rows in set (0.001 sec)