Do certain string patterns affect query performance on a VARCHAR column? I am a beginner on MySQL Administration and performance problems. Now this is confusing me.

I have a VARCHAR column identity, the string in it is consisted of a Class type and an id, like note:123, user:4 to identify a row in one type of entity.

There are 5 class names for now.

Since the string could be constructed as note:123 or 123:note, either holds the meaning of it, I am wondering does 123:note will have better query performance if I do select * from table where identity='123:note' than the other? I am thinking that 123:note will have more variety from the beginning of string, because the id is at the front.

Or it just doesn't matter?

UPDATE:
the identity field is just a redundant field for query, the type and id are actually stored in other two fields in the same table.

So:

1st, where type='note' and id=123' is definitely faster than where identity in ('note:123', 'user:456'). I think it's definitely faster, right?

2nd, if I have to have this redundant column, note:123 or 123:note which pattern is faster when I do where identity in ('note:123', 'user:456'), and the length of identity is 20.

And here is create info: likeable_identity is the identity field I am talking about.

CREATE TABLE `likes` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) DEFAULT NULL,
  `created_at` int(11) NOT NULL,
  `likeable_id` int(11) DEFAULT NULL,
  `likeable_type` varchar(255) DEFAULT NULL,
  `likeable_identity` varchar(255) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `index_likes_on_user_id_and_likeable_id_and_likeable_type` (`user_id`,`likeable_id`,`likeable_type`),
  KEY `index_likes_on_user_id` (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=8814 DEFAULT CHARSET=utf8;


Thanks.
Several points:

1) Store the data in two columns.

2) Normalize 'type' to a separate table, so you'll only put an INT into your index rather than a varchar(255).

3) Switch to UNSIGNED INT for the ids, and maybe TINYINT for the types.

4) Tell the app people to learn about multiple WHERE clauses for their SQL and give them a good index.

3) Once you do those, your question might become, "should my composite index be (id,type) or (type,id)?"