Index: wp-admin/includes/schema.php
===================================================================
--- wp-admin/includes/schema.php	(revision 21378)
+++ wp-admin/includes/schema.php	(working copy)
@@ -106,6 +106,7 @@
   KEY comment_post_ID (comment_post_ID),
   KEY comment_approved_date_gmt (comment_approved,comment_date_gmt),
   KEY comment_date_gmt (comment_date_gmt),
+  KEY comment_author (wcomment_author(2)),
   KEY comment_parent (comment_parent)
 ) $charset_collate;
 CREATE TABLE $wpdb->links (