librenms: MySQL index for table notifications_attribs

MySQL slowlog (Server version: 5.7.26 MySQL Community Server):
# User@Host: librenms[librenms] @ localhost []  Id: 19228
# Query_time: 6.830290
# Lock_time: 0.000043
# Rows_sent: 1
# Rows_examined: 23752917
select count(*) as aggregate from `notifications` left join `notifications_attribs` on `notifications_attribs`.`notifications_id` = `notifications`.`notifications_id` where (`notifications_attribs`.`key` = 'sticky' and `notifications_attribs`.`value` = 1) or (not exists (select 1 from `notifications_attribs` where notifications.notifications_id = notifications_attribs.notifications_id and `notifications_attribs`.`user_id` = 123));

mysql> select count(*) from notifications;
+----------+
| COUNT(*) |
+----------+
|       52 |
+----------+
1 row in set (0.00 sec)

mysql> select count(*) from notifications_attribs;
+----------+
| COUNT(*) |
+----------+
|      893 |
+----------+
1 row in set (0.00 sec)

mysql> select count(1) as aggregate from `notifications` left join `notifications_attribs` on `notifications_attribs`.`notifications_id` = `notifications`.`notifications_id` where (`notifications_attribs`.`key` = 'sticky' and `notifications_attribs`.`value` = 1) or (not exists     (select 1 from `notifications_attribs` where notifications.notifications_id = notifications_attribs.notifications_id and `notifications_attribs`.`user_id` = 123) );
+-----------+
| aggregate |
+-----------+
|         0 |
+-----------+
1 row in set (7.30 sec)

mysql> explain extended select count(*) as aggregate from `notifications` left join `notifications_attribs` on `notifications_attribs`.`notifications_id` = `notifications`.`notifications_id` where (`notifications_attribs`.`key` = 'sticky' and `notifications_attribs`.`value` = 1) or (not exists (select 1 from `notifications_attribs` where notifications.notifications_id = notifications_attribs.notifications_id and `notifications_attribs`.`user_id` = 123));
+----+--------------------+-----------------------+------------+-------+---------------+------------------------------+---------+------+------+----------+----------------------------------------------------+
| id | select_type        | table                 | partitions | type  | possible_keys | key                          | key_len | ref  | rows | filtered | Extra                                              |
+----+--------------------+-----------------------+------------+-------+---------------+------------------------------+---------+------+------+----------+----------------------------------------------------+
|  1 | PRIMARY            | notifications         | NULL       | index | NULL          | notifications_severity_index | 5       | NULL |   51 |   100.00 | Using index                                        |
|  1 | PRIMARY            | notifications_attribs | NULL       | ALL   | NULL          | NULL                         | NULL    | NULL |  790 |   100.00 | Using where; Using join buffer (Block Nested Loop) |
|  2 | DEPENDENT SUBQUERY | notifications_attribs | NULL       | ALL   | NULL          | NULL                         | NULL    | NULL |  790 |     1.00 | Using where                                        |
+----+--------------------+-----------------------+------------+-------+---------------+------------------------------+---------+------+------+----------+----------------------------------------------------+
3 rows in set, 3 warnings (0.00 sec)

mysql> show indexes from notifications_attribs;
+-----------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table                 | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| notifications_attribs |          0 | PRIMARY  |            1 | attrib_id   | A         |         790 |     NULL | NULL   |      | BTREE      |         |               |
+-----------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

mysql> CREATE INDEX `notifications_attribs_user_idx` ON `notifications_attribs` (`notifications_id`,`user_id`);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show indexes from notifications_attribs;
+-----------------------+------------+--------------------------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table                 | Non_unique | Key_name                       | Seq_in_index | Column_name      | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------------------+------------+--------------------------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| notifications_attribs |          0 | PRIMARY                        |            1 | attrib_id        | A         |         790 |     NULL | NULL   |      | BTREE      |         |               |
| notifications_attribs |          1 | notifications_attribs_user_idx |            1 | notifications_id | A         |          51 |     NULL | NULL   |      | BTREE      |         |               |
| notifications_attribs |          1 | notifications_attribs_user_idx |            2 | user_id          | A         |         790 |     NULL | NULL   |      | BTREE      |         |               |
+-----------------------+------------+--------------------------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)

mysql> explain extended select count(*) as aggregate from `notifications` left join `notifications_attribs` on `notifications_attribs`.`notifications_id` = `notifications`.`notifications_id` where (`notifications_attribs`.`key` = 'sticky' and `notifications_attribs`.`value` = 1) or (not exists (select 1 from `notifications_attribs` where notifications.notifications_id = notifications_attribs.notifications_id and `notifications_attribs`.`user_id` = 123));
+----+--------------------+-----------------------+------------+-------+--------------------------------+--------------------------------+---------+-----------------------------------------------+------+----------+-------------+
| id | select_type        | table                 | partitions | type  | possible_keys                  | key                            | key_len | ref                                           | rows | filtered | Extra       |
+----+--------------------+-----------------------+------------+-------+--------------------------------+--------------------------------+---------+-----------------------------------------------+------+----------+-------------+
|  1 | PRIMARY            | notifications         | NULL       | index | NULL                           | notifications_severity_index   | 5       | NULL                                          |   51 |   100.00 | Using index |
|  1 | PRIMARY            | notifications_attribs | NULL       | ref   | notifications_attribs_user_idx | notifications_attribs_user_idx | 4       | librenms.notifications.notifications_id       |   15 |   100.00 | Using where |
|  2 | DEPENDENT SUBQUERY | notifications_attribs | NULL       | ref   | notifications_attribs_user_idx | notifications_attribs_user_idx | 8       | librenms.notifications.notifications_id,const |    1 |   100.00 | Using index |
+----+--------------------+-----------------------+------------+-------+--------------------------------+--------------------------------+---------+-----------------------------------------------+------+----------+-------------+
3 rows in set, 3 warnings (0.00 sec)

mysql> select count(1) as aggregate from `notifications` left join `notifications_attribs` on `notifications_attribs`.`notifications_id` = `notifications`.`notifications_id` where (`notifications_attribs`.`key` = 'sticky' and `notifications_attribs`.`value` = 1) or (not exists     (select 1 from `notifications_attribs` where notifications.notifications_id = notifications_attribs.notifications_id and `notifications_attribs`.`user_id` = 123) );
+-----------+
| aggregate |
+-----------+
|         0 |
+-----------+
1 row in set (0.00 sec)