Be explicit when id column is ambiguous

classes/ccache.php

@@ -141,7 +141,7 @@ class CCache {
 
 			$sth = $pdo->prepare("SELECT SUM(value) AS sv
 				FROM ttrss_counters_cache, ttrss_feeds
-				WHERE id = feed_id AND
+				WHERE ttrss_feeds.id = feed_id AND
 				(cat_id = :cat OR (:cat = 0 AND cat_id IS NULL)) AND
 				ttrss_counters_cache.owner_uid = :uid AND
 				ttrss_feeds.owner_uid = :uid");