mysql> explain event;
+-------------+-------------+------+-----+---------+----------------+
| Field       | Type        | Null | Key | Default | Extra          |
+-------------+-------------+------+-----+---------+----------------+
| xid         | int(11)     |      | PRI | NULL    | auto_increment |
| type        | varchar(32) | YES  | MUL | NULL    |                |
| subsystem   | tinytext    | YES  |     | NULL    |                |
| source_ipa  | tinytext    | YES  |     | NULL    |                |
| source_host | text        | YES  |     | NULL    |                |
| source_user | tinytext    | YES  |     | NULL    |                |
| target_host | tinytext    | YES  |     | NULL    |                |
| target_ipa  | tinytext    | YES  |     | NULL    |                |
| target_user | tinytext    | YES  |     | NULL    |                |
+-------------+-------------+------+-----+---------+----------------+

mysql> alter table event add unique(type,subsystem(16),source_ipa(32),source_host(32),source_user(16),target_host(32),target_ipa(32),target_user(16));