How to efficiently hit an index in a query that uses IN (mysql)
Reverse the 'subscription' index. As it is, MySQL will probably use the PRIMARY KEY for lookups by a single subscription_id.

Once MySQL hits a RANGE query, which an IN clause is, it doesn't use the rest of the index.

CREATE TABLE `table` (
  `by` varchar(20) NOT NULL,
  `subscription_id` int(11) NOT NULL,
  `subscription_type` int(11) NOT NULL,
  `feed_created_time` datetime NOT NULL,
  `content` text NOT NULL,
  PRIMARY KEY (`subscription_id`,`subscription_type`,`feed_created_time`),
  KEY `subscription` (`subscription_type`,`subscription_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;