explain select hostname from database_instance where cluster_name='our.cluster.name:3306';
+----+-------------+-------------------+------+------------------+------------------+---------+-------+------+-------------+
| id | select_type | table             | type | possible_keys    | key              | key_len | ref   | rows | Extra       |
+----+-------------+-------------------+------+------------------+------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | database_instance | ref  | cluster_name_idx | cluster_name_idx | 130     | const |    4 | Using where |
+----+-------------+-------------------+------+------------------+------------------+---------+-------+------+-------------+

explain select hostname from database_instance where 'our.cluster.name:3306' in(cluster_name, '') ;
+----+-------------+-------------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table             | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------------------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | database_instance | ALL  | NULL          | NULL | NULL    | NULL | X000 | Using where |
+----+-------------+-------------------+------+---------------+------+---------+------+------+-------------+

This may, at first glance, make you want to avoid this trick altogether, but it really becomes more viable in your common website “filter by multiple options” type of queries (where multiple columns are involved, and there’s no way for you to index any combination of them), or when you know you’re already filtering by some other column that is indexed.

X000 above is to obscure production number of servers.