"limit 1" on a primary key - any point?

[Geoff]

I suspect I know the answer to this question already but I'd just like to check ;)

I have a web application where speed is critical. Given a table that is indexed thus:

CREATE TABLE `counts` (
`uid` varchar(20) NOT NULL default '',
`iid` bigint(10) NOT NULL default '0',
`count` int(10) NOT NULL default '0',
[...],
PRIMARY KEY (`uid`,`iid`) ) ENGINE=MyISAM DEFAULT CHARSET=latin1

my usual select statement is of the form:
SELECT count from counts WHERE uid='xxxxxxxx' AND iid='xxxxxxxxxx';

Is there any advantage in specifying "limit 1" or is that implicit in MySQL's analysis of the statement? i.e. does MySQL know that the primary key must be unique so it automatically stops after the first hit and doesn't go on to search for any other rows (even if "limit 1" is not specified)?

Geoff