Optimize this SQL!

User access source_slug (say, the_source_slug). System tries to get other distinct source_slugs that other users (who have used the_source_slug) have accessed earlier, sorted by most frequently used to least frequently used. Basically, system is trying to find users with similar interest and show other pages they have accessed earlier.

Need help with optimizing this query:

SELECT DISTINCT(SOURCE_SLUG), COUNT(SOURCE_SLUG) CATCOUNT 
  FROM topsources 
 WHERE SOURCE_SLUG <> ? 
   AND USER_ID IN (SELECT DISTINCT(USER_ID) 
                     FROM topsources WHERE SOURCE_SLUG = ?) 
GROUP BY SOURCE_SLUG ORDER BY CATCOUNT DESC

Table Structure:

`topsources` (
  `USER_ID` varchar(255) NOT NULL,
  `DATE_AND_HOUR` varchar(255) NOT NULL,
  `UPDATED_TIME` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `ITEM_ID` int(11) NOT NULL,
  `SOURCE_SLUG` varchar(100) NOT NULL,
  `FEED_PAGE_URL` varchar(255) NOT NULL,
  `CATEGORY_SLUG` varchar(100) NOT NULL,
  `REFERRER` varchar(2048) DEFAULT NULL,
  PRIMARY KEY (`USER_ID`,`DATE_AND_HOUR`(30),`ITEM_ID`),
  KEY `USER_ID` (`USER_ID`),
  KEY `FEED_PAGE_URL` (`FEED_PAGE_URL`),
  KEY `SOURCE_SLUG` (`SOUR开发者_C百科CE_SLUG`),
  KEY `CATEGORY_SLUG` (`CATEGORY_SLUG`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

• Use inner join instead of a subselect
• add an index on SOURCE_SLUG